Post Hole Concrete Calculator: Exact Bags, Gravel & Cost
Planning to set fence, deck, gate, or barn posts? This Post Hole Concrete Calculator gives you precise material quantities before you mix a single bag.
Enter your hole dimensions, post size, number of posts, gravel base, and waste buffer — and get instant results for:
- Number of 40–80 lb concrete bags
- Gravel volume
- Water needed
- Total weight and project cost
Works for round or square holes, full or partial fills, and includes stability ratings, frost line guidance, and a live cross-section diagram. Built for accuracy and speed so you buy the right amount every time. (478 characters)
SteelSolver · Construction Calculators
Post Hole Concrete Calculator
Work out exactly how many bags of concrete, how much gravel, and how much it'll cost to set fence, deck, gate, or pole-barn posts — before you mix a single bag.
Quick answer: Enter your hole size, post size, and post count below — your bag count updates instantly as you type.
Your Results
Updates instantly as you change any input above.
You need
380 lb bags total
For 1 hole · 10" diameter × 30" deep · net of 6×6 post
Concrete volume
— ft³
Gravel needed
— ft³
Water required
— gal
Total weight
— lb
Total cost
—
Cure / set time
—
Estimated hole stability
Cross-section updates live with your inputs above
Per-post breakdown
| Post role | Count | Concrete/hole | Bags/hole | Subtotal bags |
|---|---|---|---|---|
| Project total | — | — | — | — |
Formulas Used in This Calculation
Full transparency — here's exactly how your numbers above are calculated.
Hole volume (round hole)
\[ V_{hole} = \pi \times \left(\frac{D}{2}\right)^2 \times d_{hole} \]Where \(D\) = hole diameter and \(d_{hole}\) = hole depth.
Hole volume (square / rectangular hole)
\[ V_{hole} = w \times l \times d_{hole} \]Buried post volume (displacement)
\[ \begin{aligned} V_{post,\,round} &= \pi \times \left(\frac{D_{post}}{2}\right)^2 \times d_{embed} \\ V_{post,\,square} &= w_{post} \times t_{post} \times d_{embed} \end{aligned} \]Gravel volume & net concrete volume
\[ \begin{aligned} V_{gravel} &= A_{hole} \times d_{gravel} \\ V_{concrete} &= V_{hole} - V_{post} - V_{gravel} \end{aligned} \]Waste buffer & bags required
\[ \begin{aligned} V_{final} &= V_{concrete} \times (1 + waste\%) \\ bags_{hole} &= \left\lceil \frac{V_{final}}{yield_{bag}} \right\rceil \end{aligned} \]Bags always round up — concrete can't be bought by the fraction of a bag.
Project totals, cost & water
\[ \begin{aligned} bags_{total} &= \sum_{i=1}^{n} bags_{hole,\,i} \\ cost_{total} &= bags_{total} \times price_{bag} + V_{gravel,total} \times price_{gravel} + n \times price_{labor} \\ water_{total} &= bags_{total} \times ratio_{water/bag} \end{aligned} \]A note on accuracy: these formulas match the methods used by major ready-mix and bagged-concrete manufacturers. Real-world yield varies slightly by brand, water ratio, and how well concrete is packed into the hole — buy your rounded-up bag count plus a few extra if you're pouring more than 3–4 holes in one trip.
Reference Guides
Background, charts, and common mistakes — expand any section for details.
Planning a bigger project?
These calculators cover the rest of the job — different search intent, same project.
Estimates only — always confirm hole depth and footing requirements against your local building code and frost-line guidance before digging. SteelSolver.com · Construction & Engineering Calculators