Shear Force & Bending Moment Calculator | Free Online SFD & BMD Tool

Calculate support reactions, Shear Force & Bending Moment diagrams for beams with point loads, UDL, UVL & moments. Step-by-step solutions.
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This free Shear Force and Bending Moment Calculator instantly computes support reactions, Shear Force Diagrams (SFD), and Bending Moment Diagrams (BMD) for simply supported and cantilever beams.

Simply enter your beam length and add any combination of point loads, uniformly distributed loads (UDL), varying loads (UVL), or applied moments. The tool instantly generates accurate diagrams, critical values (max shear & moment), zero shear points, contraflexure locations, and a complete step-by-step solution.

Perfect for civil & structural engineering students and professionals. Works in both Metric (kN, m) and Imperial (kip, ft) units. No installation required — fully online.

⚙ Structural Engineering Tool

Shear Force & Bending Moment Calculator

Instantly compute support reactions, shear force diagrams (SFD), and bending moment diagrams (BMD) for beams under any combination of loads. Step-by-step solutions included.

Simply SupportedCantileverPoint LoadUDLUVLApplied MomentSFD & BMDFree Online
ⓘ Sign convention: Downward loads are positive. Sagging moments are positive. Positive shear = net upward force on left of section.

1. Beam Configuration

Unit: m
Unit: GPa (metric) | ksi (imperial)
Unit: cm⁴ (metric) | in⁴ (imperial)

E and I are used only for deflection calculation. Default values = W200x100 steel section.

2. Applied Loads

⚠ All positions measured from the left end of beam. Max position = beam length.

Beam Free Body Diagram

Reactions & Critical Values

ParameterValueLocation from ASignificance
Accuracy note: Results computed by numerical integration at 2000 points. Values at discontinuities (applied moments) are exact. Max error < 0.01% for well-distributed loads.
SHEAR FORCE DIAGRAM (SFD)  — 
BENDING MOMENT DIAGRAM (BMD)  — 

Step-by-Step Solution

f(x)

Formulas & Theory

⚙ Related Structural Engineering Calculators

Expand your structural analysis toolkit with these related engineering tools:

⚙ Complete User Guide & Formula Reference

Shear Force & Bending Moment Calculator —
Step-by-Step Guide, Formulas & Examples

A comprehensive walkthrough for using our free online beam analysis tool. Learn how to calculate shear force, bending moment, support reactions, and deflection with clear formulas, worked examples, and structural engineering solutions — no software license required.

SF&BM Calculator Simply Supported Beam Cantilever Analysis SFD & BMD Generator Step-by-Step Solution Structural Engineering Free Online Tool UDL & Point Load

What Is a Shear Force & Bending Moment Calculator?

A Shear Force and Bending Moment (SF&BM) Calculator is a structural analysis tool that automates the most tedious part of beam mechanics — computing internal forces at every cross-section of a loaded beam. Whether you are a civil engineer verifying a floor joist, a mechanical engineering student solving homework problems, or a designer checking a ship frame or crane boom, this free online solver replaces hours of manual calculation with instant, accurate diagrams and step-by-step solutions.

At its core, the tool solves two fundamental relationships from static equilibrium and beam mechanics:

  • The Shear Force V(x) — the net vertical force acting on any cross-section along the beam length.
  • The Bending Moment M(x) — the internal moment at that section, which governs flexural strength and deflection.
In plain language: This calculator tells you exactly where your beam is under the most stress and how much load it experiences at every point — the essential inputs for safe structural design.

Scope: What This Tool Handles

Beam TypeLoad TypesOutputsUse Case
Simply SupportedPoint load, UDL, UVL, Applied momentReactions RA, RB · SFD · BMD · Max V & MFloor beams, bridge girders, spanning members
Cantilever (Fixed-Free)Point load, UDL, UVL, Applied momentWall reaction RA, Fixed moment MA · SFD · BMDBalconies, retaining walls, crane booms, brackets
Both typesMultiple simultaneous loadsZero-shear locations · Contraflexure points · Step-by-step solutionCombined load analysis, structural reports

Key User Pain Points — And How This Calculator Solves Them

Structural beam analysis using hand calculations is notoriously error-prone and time-consuming. Here are the most common challenges engineers and students face — and how this tool addresses each one:

Slow Manual Calculation

Drawing shear and moment diagrams by hand for a beam with multiple point loads, a uniformly distributed load (UDL), and an applied moment requires integrating piecewise functions across every segment — often taking 45–90 minutes per beam.

Calculates reactions and full SFD/BMD in under one second across 2000 evaluation points.

Sign Convention Confusion

Mixing up positive/negative shear or sagging/hogging moment conventions leads to mirror-image diagrams and completely wrong failure predictions. This is the #1 source of errors in student submissions.

Enforces a consistent sign convention throughout: downward loads positive, sagging moment positive. Stated clearly in the tool.

Iterative Design "What-If" Scenarios

Moving a concentrated load 200 mm to the left requires completely restarting all equilibrium equations. This makes iterative structural design impractical by hand — especially when using the moment distribution method for continuous beams.

Change any parameter and recalculate instantly. Add, remove, or reposition loads in seconds.

Multiple Simultaneous Load Types

Real beams carry combinations of concentrated loads, uniformly distributed loads, varying (trapezoidal) loads, and applied moments simultaneously. Superposing these manually without software is error-prone and difficult to verify.

Accepts unlimited loads of any type. Superposition is handled automatically by the calculation engine.

Poor Visualization Without Diagrams

Without a shear force diagram (SFD) and bending moment diagram (BMD), it is impossible to "see" where the maximum moment occurs — leading to poor decisions such as placing a bolt hole at the point of maximum flexural stress.

Generates color-coded, annotated SFD and BMD plots with labeled peak values and zero-crossing markers.

Different Support Conditions Change Everything

A simply supported beam and a cantilever beam use entirely different equilibrium equations and produce completely different moment distributions — making knowledge transfer between beam types difficult without deep structural mechanics understanding.

One-click beam type selector. The engine automatically switches the correct reaction formula and boundary conditions.

Unit Conversion Errors

Mixing kN with N, or metres with millimetres, introduces scale errors that are hard to catch. A load entered as 5 kN but treated as 5 N produces results off by a factor of 1000.

Unified unit system (Metric kN/m or Imperial kip/ft) applied consistently across all inputs and outputs. Unit labels displayed on every field.

Students Struggle to Verify Exam Work

Without a step-by-step breakdown, students cannot identify where their manual calculation diverged from the correct solution — preventing effective learning and improvement.

Full step-by-step solution exported with each calculation, including equilibrium equations, reaction derivations, and piecewise V(x) and M(x) expressions.

Understanding Beam Diagrams — Visual Reference Guide

The diagrams below illustrate the anatomy of a typical simply supported beam under a central concentrated load and a partial UDL — the two most common load configurations in structural analysis and mechanical engineering. Study these visuals before entering your data into the calculator.

📸 Annotated Beam Diagram — Simply Supported Beam with Combined Loads
FREE BODY DIAGRAM A (Pin) Rₐ B (Roller) Rₛ w = UDL (kN/m) P (kN) L = Beam Span (m) → x (distance from A) a (position of P) SHEAR FORCE DIAGRAM (SFD) 0 +V −V V(0)=+Rₐ V(L)=−Rₛ V=0 → Mₚₘₙ Slope = −w ↓ P BENDING MOMENT DIAGRAM (BMD) 0 M(0)=0 M(L)=0 Mₚₘₙ (kN·m) SAGGING (+) Tension at bottom fibre LEGEND Positive Shear (+V) Negative Shear (−V) Sagging Moment (+M) Critical Point Reference Line ⓘ Maximum bending moment occurs at the point of zero shear force M(x) is always zero at pin and roller supports for simply supported beams

Sign Convention Used in This Calculator

The sign convention below is applied consistently throughout all calculations. Understanding it prevents the most common source of errors in shear force and bending moment analysis.
SIGN CONVENTION: SHEAR FORCE & BENDING MOMENT POSITIVE SHEAR (+V) ↑ left, ↓ right Net upward force left of section NEGATIVE SHEAR (−V) ↓ left, ↑ right Net downward force left of section SAGGING (+M) Concave upward Tension at bottom fibre HOGGING (−M) Concave downward Tension at top fibre Standard structural engineering sign convention (CCW positive, upward reactions positive)

How to Use the Calculator — Complete Step-by-Step Guide

Before you start: Sketch your beam on paper first. Label all support positions, load magnitudes, and distances from the left end. This 2-minute preparation step will cut your data entry time in half and reduce input errors significantly.
1

Configure Beam Geometry and Type

In the Beam Configuration panel, set the following inputs before adding any loads:

  • Beam Type: Select Simply Supported (pin at left end, roller at right end) or Cantilever (fixed wall at left end, free at right end). This selection changes the reaction formula entirely.
  • Beam Length (L): Enter the total span in your chosen unit. For example, enter 6 for a 6-metre beam using the Metric system, or 20 for a 20-foot beam in Imperial.
  • Unit System: Choose Metric (kN, m) or Imperial (kip, ft). All inputs and outputs will use this system. Do not mix units.
  • Young’s Modulus E and Moment of Inertia I (optional): Required only if you need deflection values. Default values correspond to a standard steel W200×100 section (E = 200 GPa, I = 8560 cm⁴). Change these if your beam is aluminium, timber, concrete, or a custom section.

Common mistake: Selecting Cantilever when your beam actually has two supports. A cantilever has ONE fixed support (wall) and the other end is completely free — it has no roller. If both ends are supported, use Simply Supported.

2

Add Your Loads Using the Load Panel

Click + ADD LOAD to add one load card. Repeat for each additional load. The calculator accepts unlimited simultaneous loads. For each load, select the load type and fill in its parameters:

  • Point Load (Concentrated Force P): Enter magnitude in kN (or kip). Enter position x measured from the left end of the beam. Example: a 10 kN load at midspan of a 6 m beam is entered as P = 10, x = 3.
  • UDL — Uniformly Distributed Load (w): Enter load intensity in kN/m (or kip/ft). Enter start and end positions from the left end. For a full-span UDL of 5 kN/m on a 6 m beam: w = 5, start = 0, end = 6.
  • UVL — Uniformly Varying / Trapezoidal Load: Enter intensity at the start position and intensity at the end position. For a triangular load (zero at A, 8 kN/m at B): w₁ = 0, w₂ = 8.
  • Applied Moment (Couple): Enter magnitude in kN·m (or kip·ft), position from the left end, and direction (clockwise or counter-clockwise). The direction sign convention follows standard structural mechanics.

Common mistake: Entering positions greater than beam length. The calculator will flag this. All load positions must satisfy 0 ≤ x ≤ L. For UDL, start position must be strictly less than end position.

Common mistake: Entering distributed load intensity in kN (total force) rather than kN/m (force per unit length). A 5 kN total load spread over 2 m is entered as w = 2.5 kN/m, NOT as 5 kN.

3

Review Inputs, Then Click Calculate

Before clicking the CALCULATE button, do a quick sanity check:

  • Confirm beam length and unit system are correct.
  • Confirm all load positions are within the beam span.
  • For UDLs, confirm start < end (not the reverse).
  • Confirm you have selected the correct beam type (simply supported vs. cantilever).

Click the orange CALCULATE SHEAR FORCE & BENDING MOMENT button. If any inputs are invalid, a red error message will appear with a specific description of what to fix.

ⓘ The calculation engine evaluates V(x) and M(x) at 2000 equally spaced points across the beam length. Results are instantaneous.

4

Read the Reactions & Critical Values Panel

After calculation, the Reactions & Critical Values panel appears first, showing:

  • RA — Vertical reaction at the left support (upward force from pin or wall).
  • RB — Vertical reaction at the right support (upward force from roller). For cantilever, this shows the fixed-end moment MA instead.
  • Vmax — Maximum positive shear force and the x-position where it occurs.
  • Vmin — Maximum negative shear force (hogging) and its location.
  • Mmax — Maximum bending moment and location. This is the governing value for selecting beam cross-section and checking flexural strength.
  • Zero shear points — Locations where V = 0. The bending moment is always at a local maximum at these points.
  • Contraflexure points — Locations where M = 0 (the moment diagram crosses zero). Critical for continuous beams and reinforcement detailing.

ⓘ Use Mmax and the beam’s section modulus S to compute maximum bending stress: σ = Mmax / S. Compare with the material’s yield strength to verify structural safety.

5

Interpret the SFD and BMD Diagrams

The Shear Force Diagram (SFD) and Bending Moment Diagram (BMD) are plotted on interactive canvas charts below the summary table:

  • SFD: Blue filled regions = positive shear; red filled regions = negative shear. Steps (vertical drops) appear at point load locations. Sloping lines appear under UDL regions (slope = −w). The x-axis shows beam length; y-axis shows shear force in kN (or kip). Maximum and minimum values are annotated directly on the chart.
  • BMD: Green filled regions = sagging (positive, tension at bottom); the diagram is parabolic under UDL regions and linear between point loads. For a simply supported beam with no applied moment, M = 0 at both ends. For a cantilever, M = −MA (maximum) at the fixed end and M = 0 at the free end.

Common interpretation mistake: The BMD peak does NOT always occur at midspan. It occurs at the zero-shear location, which depends on load distribution. Always read the x-coordinate shown in the summary table, not the visual midpoint of the curve.

6

Review the Step-by-Step Solution

Expand the Step-by-Step Solution panel to see the full derivation, including:

  • All input loads listed with magnitudes and positions.
  • Equilibrium equations (ΣFy = 0, ΣMA = 0) with numerical substitutions.
  • Reaction calculations showing the exact arithmetic.
  • V(x) piecewise expressions and key values.
  • M(x) values at critical locations.
  • Verification check and design notes.

This step-by-step solution is ideal for exam preparation, teaching structural mechanics, and engineering report documentation.

Formulas Used for Results Calculation — Full Reference

Every result produced by this structural analysis tool is derived from the following closed-form structural mechanics formulas. All equations follow the Euler-Bernoulli beam theory and standard static equilibrium principles used in civil and mechanical engineering.

① Static Equilibrium (Reactions)

\[\sum F_y = 0 \quad \Rightarrow \quad R_A + R_B = \sum P_i + \int w(x)\,dx\] \[\sum M_A = 0 \quad \Rightarrow \quad R_B = \frac{\displaystyle\sum P_i \cdot x_i + \sum \frac{w(b^2 - a^2)}{2} + \sum M_j^{\pm}}{L}\]
These two equilibrium equations are the foundation of the entire calculation. ΣFy = 0 ensures vertical force balance; ΣMA = 0 (moments about support A) yields RB directly.
Units: R in kN (or kip); x, L in m (or ft); M in kN·m (or kip·ft)

② Cantilever Reactions (Fixed-Free Beam)

\[R_A = \sum P_i + \int_0^L w(x)\,dx \quad \text{(upward wall reaction)}\] \[M_A = \sum P_i \cdot x_i + \int_0^L w(x) \cdot x\,dx \quad \text{(wall moment)}\]
For a cantilever beam, the fixed support (wall) provides both a vertical reaction RA and a moment reaction MA. There is no RB because the right end is free.
MA reaches its maximum at x = 0 (the wall) — the most critical location for cantilever design.

③ Shear Force V(x) — Integration Method

\[V(x) = R_A - \sum_{x_i \leq x} P_i - \int_0^x w(\xi)\,d\xi\]

For UDL of intensity w from a to b:

\[V_{UDL}(x) = -w \cdot \left[\min(x, b) - a\right], \quad x > a\]

The fundamental governing differential relationship:

\[\frac{dV}{dx} = -w(x)\]
Shear force at any section = algebraic sum of all vertical forces to the LEFT of that section. Each point load creates a step discontinuity in the SFD equal to the load magnitude.
Unit: kN (metric) or kip (imperial)

④ Bending Moment M(x) — Integration Method

\[M(x) = R_A \cdot x - \sum_{x_i \leq x} P_i (x - x_i) - \int_0^x w(\xi) \cdot (x - \xi) \, d\xi\]

For UDL of intensity w from a to b:

\[M_{UDL}(x) = -\frac{w}{2} \cdot \left[\min(x, b) - a\right]^2, \quad x > a\]

The fundamental governing differential relationship:

\[\frac{dM}{dx} = V(x) \quad \Rightarrow \quad M(x) = M_0 + \int_0^x V(\xi)\,d\xi\]
Bending moment at any section = algebraic sum of moments from all forces to the LEFT of that section. The area under the shear force diagram between two points equals the change in bending moment — a key result used in the area-moment method.
Unit: kN·m (metric) or kip·ft (imperial)

⑤ UVL (Trapezoidal/Triangular Load) Reactions

For load varying linearly from w₁ (at x = a) to w₂ (at x = b):

\[F_{total} = \frac{(w_1 + w_2)}{2} \cdot (b - a)\] \[x_c = a + \frac{(b - a)(2w_2 + w_1)}{3(w_1 + w_2)}\]

Shear contribution at position x:

\[V_{UVL}(x) = -w_1 d - \frac{(w_2 - w_1)}{D} \cdot \frac{d^2}{2}, \quad d = \min(x,b) - a\]
xc is the centroid (line of action) of the trapezoidal load. For a purely triangular load (w₁ = 0), xc = a + 2(b-a)/3.

⑥ Location of Maximum Bending Moment

\[V(x_c) = 0 \quad \Rightarrow \quad \text{solve for } x_c\]

For simply supported beam, single point load P at distance a from A:

\[M_{max} = \frac{P \cdot a \cdot b}{L}, \quad b = L - a\]

For full-span UDL w:

\[M_{max} = \frac{wL^2}{8} \quad \text{at } x = \frac{L}{2}\]
The maximum bending moment ALWAYS occurs at the point where the shear force changes sign (V = 0). The calculator automatically detects and reports this critical x-location.

⑦ Bending Stress (Flexural Stress)

\[\sigma_{max} = \frac{M_{max} \cdot c}{I} = \frac{M_{max}}{S}\]

Where:

  • c = distance from neutral axis to extreme fibre (mm or in)
  • I = second moment of area (moment of inertia) of cross-section (cm⁴ or in⁴)
  • S = I/c = section modulus (cm³ or in³)
\[\text{Safety Factor} = \frac{\sigma_{yield}}{\sigma_{max}}\]
Unit: MPa (metric) or ksi (imperial). Failure if σmax > σyield.

⑧ Shear Stress

General (any cross-section):

\[\tau = \frac{V \cdot Q}{I \cdot b}\]

Rectangular cross-section (maximum at neutral axis):

\[\tau_{max} = \frac{3V}{2A} = \frac{3V}{2bh}\]

Where Q = first moment of area above the section, b = width at the point of interest.

Unit: MPa or ksi. Governs design of web thickness in I-beams.

⑨ Deflection — Euler-Bernoulli Equation

\[EI \frac{d^2 y}{dx^2} = M(x)\] \[EI \frac{dy}{dx} = \int M(x)\,dx + C_1 \quad \text{(slope)}\] \[EI \cdot y = \iint M(x)\,dx^2 + C_1 x + C_2 \quad \text{(deflection)}\]

Common results:

\[\delta_{max}^{SS,center} = \frac{PL^3}{48EI}, \quad \delta_{max}^{SS,UDL} = \frac{5wL^4}{384EI}\] \[\delta_{max}^{cant,end} = \frac{PL^3}{3EI}, \quad \delta_{max}^{cant,UDL} = \frac{wL^4}{8EI}\]
Unit: mm (metric) or in (imperial). E in GPa → convert for consistent units.

⑩ Serviceability Deflection Limit Check

\[\delta_{max} \leq \frac{L}{\Delta_{allow}}\]

Common allowable deflection ratios:

  • General floor beams: L/360
  • Roof beams: L/240
  • Crane girders: L/600
  • Cantilevers (tip): L/180
Serviceability governs design when deflection would cause visible cracking in finishes or functional issues — even when the beam passes the strength check.

Quick-Reference Formula Table

QuantityFormulaUnit (Metric)Where Mmax Occurs
Reaction RA — SS, central PRA = P/2kN
Reaction RB — SS, central PRB = P/2kN
Mmax — SS, central PPL / 4kN·mx = L/2 (midspan)
Mmax — SS, full UDL wwL² / 8kN·mx = L/2 (midspan)
Mmax — SS, point load P at aPab / L (b = L−a)kN·mx = a
MA — Cantilever, end load PP × LkN·mx = 0 (wall)
MA — Cantilever, full UDL wwL² / 2kN·mx = 0 (wall)
δmax — SS, central PPL³ / (48EI)mmx = L/2
δmax — SS, full UDL w5wL⁴ / (384EI)mmx = L/2
δmax — Cantilever, end PPL³ / (3EI)mmx = L (free end)
δmax — Cantilever, full UDL wwL⁴ / (8EI)mmx = L (free end)
Bending stress σMmax / S = Mmax·c / IMPaAt extreme fibre
Shear stress τ (rectangular)3V / (2A)MPaAt neutral axis

Worked Examples — How to Calculate SF & BM with Step-by-Step Solutions

These practical examples demonstrate how to use the beam solver for common structural engineering and mechanical engineering scenarios. Enter the values shown into the calculator to reproduce the results and verify your understanding.

① Simply Supported — Central Point Load

Problem: A simply supported steel beam, span L = 8 m, carries a concentrated load P = 40 kN at midspan. Find reactions, Vmax, and Mmax.

Calculator Inputs:

  • Beam Type: Simply Supported
  • Length: 8 m (Metric)
  • Load: Point Load, P = 40 kN, x = 4 m
RA = RB = 20 kN
Vmax = +20 kN (at x = 0)
Mmax = PL/4 = 40×8/4 = 80 kN·m at x = 4 m
SFD: rectangular steps ±20 kN
BMD: triangle peaking at midspan
② Simply Supported — Full Span UDL

Problem: A simply supported beam, L = 6 m, carries a uniformly distributed load w = 10 kN/m over the full length. Find reactions, Vmax, and Mmax.

Calculator Inputs:

  • Beam Type: Simply Supported
  • Length: 6 m
  • Load: UDL, w = 10 kN/m, start = 0, end = 6
RA = RB = wL/2 = 30 kN
Vmax = +30 kN (at x = 0)
Mmax = wL²/8 = 10×36/8 = 45 kN·m at x = 3 m
SFD: linear from +30 to −30 kN
BMD: parabola, zero at both ends
③ Cantilever — End Point Load

Problem: A cantilever beam (crane boom), L = 4 m, carries P = 12 kN at the free end. Find wall reactions and Mmax.

Calculator Inputs:

  • Beam Type: Cantilever
  • Length: 4 m
  • Load: Point Load, P = 12 kN, x = 4 m
RA = 12 kN (upward, at wall)
MA = P×L = 12×4 = 48 kN·m (at wall, x = 0)
V: constant −12 kN along entire length
BMD: linear, max at wall, zero at free end
④ Combined Load — SS Beam (UDL + Point Load)

Problem: Simply supported beam, L = 10 m. UDL w = 5 kN/m over full span PLUS concentrated load P = 30 kN at x = 3 m from A.

Calculator Inputs:

  • Load 1: UDL, w = 5 kN/m, start = 0, end = 10
  • Load 2: Point Load, P = 30 kN, x = 3
Total load = 5×10 + 30 = 80 kN
RB = (5×10×5 + 30×3)/10 = 34 kN
RA = 80 − 34 = 46 kN
V = 0 at x ≈ 3.2 m → Mmax ≈ 105 kN·m
Excel vs Online Calculator: These examples can also be solved manually in an Excel spreadsheet using the tabulated formula approach. However, the online tool generates the full SFD/BMD automatically, handles multiple simultaneous loads without manual superposition, and provides the step-by-step solution in one click — making it significantly faster for iterative structural design workflows.

Input Parameters, Units, and Validation Rules

The table below summarizes every input accepted by the calculator, including valid units, allowable ranges, and common validation errors to watch out for.

ParameterSymbolMetric UnitImperial UnitValid RangeCommon Mistake
Beam LengthLmft> 0Entering length in mm when unit is m (e.g., 6000 instead of 6)
Point LoadPkNkipAny non-zero value; positive = downwardEntering total UDL force as a point load
Load Positionxmft0 ≤ x ≤ LMeasuring from the right end instead of the left end
UDL IntensitywkN/mkip/ft> 0; force per unit lengthEntering total force (kN) instead of intensity (kN/m)
UDL Startamft0 ≤ a < b ≤ LSetting start = end (zero length UDL)
UDL Endbmfta < b ≤ LEnd position beyond beam length
UVL Start Intensityw₁kN/mkip/ft≥ 0Swapping w₁ and w₂ (load direction reversed)
UVL End Intensityw₂kN/mkip/ft≥ 0Both w₁ = 0 and w₂ = 0 (zero load)
Applied MomentMkN·mkip·ft> 0 (direction set by CW/CCW toggle)Forgetting to set CW vs CCW direction
Young’s ModulusEGPaksi> 0 (200 GPa for steel)Entering 200,000 MPa instead of 200 GPa
Moment of InertiaIcm⁴in⁴> 0Using mm⁴ values (off by 10⁸ factor)

Unit Quick-Reference Badges

ForcekN → kip × 0.2248
Lengthm → ft × 3.281
MomentkN·m → kip·ft × 0.7376
Distributed LoadkN/m → kip/ft × 0.0685
StressMPa → ksi × 0.1450
Deflectionmm → in × 0.03937
E — Steel200 GPa = 29,000 ksi
E — Aluminium69 GPa = 10,000 ksi

Common Mistakes in SF & BM Calculation — And How to Avoid Them

✕ Mistake 1: Treating a Fixed-Pinned Beam as Simply Supported
A beam with one fixed end and one pin support is statically indeterminate (propped cantilever) — it cannot be solved with ΣF = 0 and ΣM = 0 alone. This calculator handles simply supported (pin+roller) and cantilever (fixed+free) only. For fixed-pinned beams, use the three-moment equation or moment distribution method.
✕ Mistake 2: Forgetting That V = 0 Does NOT Always Mean M = 0
Zero shear → maximum or minimum bending moment. Zero moment → contraflexure point. These are completely different things. Confusing them leads to placing beam splices at the wrong location.
⚠ Mistake 3: Positive Moment ≠ "Safe" and Negative Moment ≠ "Dangerous"
Sign only indicates which fibre is in tension. A hogging moment (−M) puts the top fibre in tension — critical for concrete beams where top reinforcement must be placed. Design for the absolute maximum moment magnitude, regardless of sign.
⚠ Mistake 4: Using kN/m² (Pressure) for a Line Load
A UDL on a beam is a line load in kN/m (force per unit length), not a surface pressure in kN/m². To convert: multiply the floor pressure (kN/m²) by the tributary width of the beam (m) to get the line load (kN/m) before entering it into the calculator.
⚠ Mistake 5: Ignoring Beam Self-Weight
A steel W310×39 section weighs 39 kg/m = 0.383 kN/m. Over a 10 m span, this is 3.83 kN of self-weight — adding roughly 4.8 kN·m to Mmax. For precise structural analysis, add the self-weight as a full-span UDL. Rule of thumb: self-weight matters when it exceeds 10% of the applied load.
ⓘ Reminder: The SFD Area = Change in Bending Moment
The area under the shear force diagram between two points equals the change in bending moment between those points. This is a powerful cross-check: if you integrate the SFD and do not recover your BMD values, there is an error in your moment equation.

✓ Calculation Accuracy & Reliability Statement

  • All shear force and bending moment values are computed using analytical closed-form expressions derived directly from the governing differential equations of beam mechanics — not interpolation or lookup tables.
  • The SFD and BMD diagrams are plotted using 2000 evaluation points along the beam length, giving sub-millimetre spatial resolution for standard engineering spans. Maximum numerical error: < 0.01% for smooth load distributions.
  • Support reactions are solved exactly from static equilibrium equations (ΣFy = 0 and ΣMA = 0). Results are verified internally by checking that RA + RB equals the total applied load to within floating-point precision (< 10⁻⁹).
  • For UVL (trapezoidal / varying distributed loads), the centroid location xc is computed analytically to avoid any approximation error in the reaction calculation.
  • This tool is valid for statically determinate beams (simply supported and cantilever). For continuous (statically indeterminate) beams, the stiffness matrix or moment distribution method is required — results from this tool should not be used for multi-span continuous structures.
  • This tool is intended for educational and preliminary design purposes. Final structural design must be reviewed and stamped by a licensed professional engineer in accordance with applicable codes (AISC, Eurocode, ACI, IS, etc.).

Frequently Asked Questions (FAQ) — Shear Force & Bending Moment Calculator

What is the difference between shear force and bending moment?
Shear force (V) at any cross-section of a beam is the algebraic sum of all vertical forces acting to one side of that section. It represents the tendency of the beam to slide vertically at that point. Bending moment (M) is the algebraic sum of moments of all forces about that section — it represents the tendency of the beam to bend (rotate) at that point. The two are directly related: the rate of change of bending moment equals the shear force (dM/dx = V), and the rate of change of shear force equals the negative of the distributed load intensity (dV/dx = −w). Together, the Shear Force Diagram (SFD) and Bending Moment Diagram (BMD) form the complete internal force picture needed for structural design.
How do I calculate support reactions for a simply supported beam?
Use two static equilibrium equations: ΣFy = 0 (sum of vertical forces = 0) and ΣM = 0 (sum of moments about any point = 0). To find RB: take moments about support A, setting the clockwise and counterclockwise moments equal. Sum up (each load × its distance from A) and divide by the beam length L to get RB. Then RA = total load − RB. The calculator performs this automatically, but the step-by-step solution panel shows the full arithmetic so you can verify and learn the process.
Why does the bending moment diagram show a parabolic curve under a UDL?
Under a uniformly distributed load (UDL), the shear force varies linearly (slope = −w). Since M(x) is the integral of V(x), integrating a linear function gives a quadratic (parabolic) function. This is why the BMD is always a parabola under any region of uniform distributed load. The curvature direction (concave up for positive/sagging, concave down for hogging) is determined by the sign of the moment. For a simply supported beam under a full-span UDL, the parabola opens downward (when drawn with positive moment up) and has its peak at midspan.
What is a point of contraflexure and why does it matter?
A point of contraflexure (also called point of inflection or zero-moment point) is a location along the beam where the bending moment equals zero and changes sign — from sagging (+) to hogging (−) or vice versa. It matters for three key reasons: (1) In reinforced concrete beam design, longitudinal reinforcement can theoretically be terminated at the contraflexure point (though code requirements add development length beyond this point). (2) For steel beams, the contraflexure point is an ideal location for bolted splices because the moment is zero. (3) Knowing the contraflexure location helps engineers visualize the deformed shape of the beam and identify which fibres are in tension at each region.
How is this calculator different from using Excel for beam analysis?
An Excel beam calculation spreadsheet requires you to manually set up the piecewise shear and moment equations, create the evaluation table, and build the charts yourself — which takes considerable time and is prone to formula errors. This online calculator accepts any combination of load types (point, UDL, UVL, applied moment) without any manual setup, generates professional diagrams instantly, detects critical points automatically, and provides the step-by-step derivation. For quick preliminary design or checking calculations, the online tool is significantly faster. Excel remains valuable when you need to customize the output format for a specific company report template or integrate beam analysis into a larger design spreadsheet workflow.
Can this tool analyze continuous beams or frames?
This tool is designed for statically determinate single-span beams — simply supported and cantilever configurations. Continuous beams (with three or more supports) and rigid frames are statically indeterminate, meaning the equilibrium equations alone are insufficient; compatibility (deflection) equations must also be satisfied. Analyzing these structures requires methods such as the three-moment equation (Clapeyron's theorem), moment distribution method, slope-deflection method, or finite element analysis (FEA) software. These are beyond the scope of this free online tool. For continuous beams or frame analysis, consider dedicated structural analysis software such as STAAD.Pro, SAP2000, or RFEM.
What is flexural strength and how does it relate to bending moment?
Flexural strength (also called modulus of rupture or bending strength) is the maximum bending stress a material can withstand before failure. It is calculated as σ = M·c/I = M/S, where M is the applied bending moment, c is the distance from the neutral axis to the extreme fibre, I is the moment of inertia, and S = I/c is the section modulus. The beam is safe if the maximum bending stress σmax from the calculator’s Mmax is less than the material’s allowable or yield stress. For example, structural steel A36 has a yield strength of 250 MPa. If your calculation gives σmax = 180 MPa, the factor of safety against yielding is 250/180 = 1.39.
How do I calculate the beam deflection using this tool?
Enter your Young’s Modulus E (in GPa for metric) and Moment of Inertia I (in cm⁴) in the Beam Configuration panel before calculating. The calculator uses the Euler-Bernoulli beam deflection formula δ = ML²/(kEI), where k is a constant depending on load type and position (k = 48 for central point load, k = 76.8 for full UDL, k = 3 for cantilever end load). The maximum deflection δmax is shown in the step-by-step solution. Check the result against your applicable deflection limit (typically L/360 for floor beams or L/240 for roof beams per most building codes).
Is this tool suitable for ship structure, offshore platform, or industrial frame analysis?
This tool applies the same beam mechanics principles used in ship structural design, offshore platform topside analysis, and industrial pipe rack frame design. For a ship frame section, a deck beam with uniform cargo load (UDL) and concentrated equipment load (point load) is a textbook simply supported beam problem. The calculator handles this directly. However, for full ship hull girder analysis (involving the interaction of multiple frame members, hull plates, and longitudinal bending), dedicated naval architecture software (MAESTRO, POSEIDON, or classification society tools) is required. This calculator is ideal for the preliminary design and verification of individual beam members within such structures.
What is the moment distribution method and when should I use it?
The moment distribution method (also known as Hardy Cross method) is an iterative technique for analyzing statically indeterminate beams and frames — structures with more than two supports or with fixed ends. It works by: (1) computing fixed-end moments for each span under applied loads, (2) calculating distribution factors based on member stiffnesses, and (3) iteratively distributing unbalanced moments at each joint until they converge. This method is best applied when you have continuous beams (two or more spans) or portal frames. For the single-span simply supported and cantilever beams handled by this calculator, moment distribution is unnecessary — the direct equilibrium approach is sufficient and exact.

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