Weld Heat Input Calculator (kJ/mm & kJ/in)
SteelSolver
Welding calculator
Weld heat-input calculator
Calculate heat input in kJ/mm or kJ/in from welding current, arc voltage and travel speed. Switch between SI and US customary units; the calculation updates as you type.
Current stays in amperes and voltage in volts in both unit systems, as it does on a welding procedure sheet. Only travel speed and the heat-input output change.
Calculate heat input
Enter the arc current, arc voltage and average travel speed. All three values must be positive.
Use the set value for planning, or the measured value for a production check.
Arc voltage, not open-circuit voltage.
Average linear travel speed along the joint, not wire-feed speed. Measure over the steady part of the pass.
Process efficiency factor optional
Use 1.00 to report arc energy. Apply a factor greater than 0 and no more than 1.00 only when the governing procedure or code specifies it.
Heat input per unit length
kJ/mm
k = 1.00 · arc energy
Q = 1.00 × (60 × 25 × 200) ÷ (1000 × 300) = 1.00 kJ/mm
Heat input is energy per unit length for one pass at steady travel. It does not by itself establish procedure qualification or code compliance.
Use
What the result is used for
Heat input is the energy delivered to the weld per unit length — arc energy when k = 1.00. It is a derived quantity that a welding procedure limits, alongside current, voltage, travel speed, preheat and interpass temperature. Fabricators use it to compare a proposed parameter set with a procedure limit, to check measured shop values against the procedure, and to see how strongly a change in travel speed moves the result.
It applies to arc processes with a measurable steady travel speed — manual metal arc, MIG/MAG, TIG, flux-cored and submerged-arc welding. It is not a prediction of hardness, HAZ width, distortion or toughness; those depend on the steel, heat treatment, joint design and the whole thermal cycle.
Method
How this is calculated
Heat input per unit length is arc power divided by travel speed, corrected by k where the procedure requires it. The calculator uses the conventional arc-welding relationship:
- Q
- Heat input per unit length (kJ/mm with v in mm/min; kJ/in with v in in/min).
- k
- Process efficiency factor, dimensionless. Use 1.00 for arc energy; use another value only if the governing procedure requires it.
- U
- Arc voltage in volts (V).
- I
- Welding current in amperes (A).
- v
- Average travel speed in mm/min or in/min. The factor 60 converts minutes to seconds; 1000 converts joules to kilojoules.
Setting k = 1.00 gives arc energy, the electrical energy delivered per unit length. Some procedures and codes ask for heat input corrected by a process efficiency factor; others use arc energy and call it heat input. Use the definition and factor required by your governing specification. k is not a safety factor.
Worked example
200 A, 25 V, 300 mm/min, k = 1.00
SI calculation, followed by the US customary equivalent.
-
Arc power
P = U × I = 25 V × 200 A = 5,000 W = 5.00 kW -
Travel speed
v = 300 mm/min ÷ 60 s/min = 5.00 mm/s -
Arc energy
E = P ÷ v = 5.00 kJ/s ÷ 5.00 mm/s = 1.00 kJ/mm -
Heat input
Q = k × E = 1.00 × 1.00 = 1.00 kJ/mm -
US equivalent
Q = 1.00 kJ/mm × 25.4 = 25.4 kJ/in
Check the round trip: 300 mm/min ÷ 25.4 = 11.811 in/min. Using the same formula with v in in/min gives 60 × 25 × 200 ÷ (1000 × 11.811) = 25.4 kJ/in, which is 1.00 kJ/mm.
Reference
Units and conversions
| Quantity | SI | US customary |
|---|---|---|
| Current, I | A | A |
| Arc voltage, U | V | V |
| Travel speed, v | mm/min | in/min |
| Heat input, Q | kJ/mm | kJ/in |
| From | To |
|---|---|
| 1 in | 25.4 mm |
| 1 in/min | 25.4 mm/min |
| 1 kJ/mm | 25.4 kJ/in |
| 1 kJ/in | 0.0393701 kJ/mm (1/25.4) |
| 1 kJ/mm | 10 kJ/cm |
| 1 mm/s | 60 mm/min |
| 1 kW | 1 kJ/s |
Checks
Assumptions, mistakes and limits
The calculation is only as good as the current, voltage and travel-speed values entered. It assumes a steady arc and a constant average travel speed for the pass.
Common mistakes
- Using wire-feed speed instead of travel speed. Wire-feed speed controls deposition rate; travel speed is how fast the arc moves along the joint.
- Entering open-circuit voltage. Use the arc voltage at the welding point, not the open-circuit value on the machine or the no-load reading.
- Mixing mm/min and in/min. The formula gives kJ/mm with mm/min and kJ/in with in/min. Convert the speed before entering it.
- Treating k as a safety factor. It is a process efficiency correction, not a margin. Use the value, or none, required by the governing procedure.
Limits of the calculation
- For AC or pulsed welding, use the averaging convention required by the governing procedure. A single instantaneous reading may not represent average arc power.
- Use the k factor defined by the applicable code or welding procedure. This page does not supply process efficiency factors because they depend on process, consumable, shielding, polarity and equipment.
- A heat-input limit may apply per pass or across a procedure. This calculator reports one pass only, from the values entered.
- Heat input alone does not predict hardness, HAZ width, dilution, distortion or mechanical properties. Preheat, interpass temperature, joint geometry and pass sequence also matter.
- No specific standard clause is applied. Confirm the governing specification's definition of heat input, its units and whether it requires an efficiency correction.
- Results are displayed to three significant figures; the calculation itself uses the entered values at full precision. Do not round a result across a specified acceptance limit.
