Concrete Calculator for Fence Posts
Concrete volume to set a run of fence posts, allowing for the posts displacing concrete in each hole.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Concrete per post = (π × (Hole diameter ÷ 2)² − Post width²) × Hole depth
Total concrete = Concrete per post × Number of posts
Run the calculation to see each step with your own numbers.
Six posts, 10 in holes, 24 in deep, 3.5 in (nominal 4×4) posts, default 10% waste.
Hole volume = π × (10 in ÷ 2)² × 24 in ≈ 1,885 in³ ≈ 1.09 ft³
Post volume in hole = 3.5² × 24 = 294 in³ ≈ 0.17 ft³
Concrete per post ≈ 0.92 ft³ → × 6 = 5.5 ft³ = 0.20 yd³ exact
0.20 yd³ × 1.10 = 0.23 yd³ recommended
- Square post section (enter the actual, not nominal, width).
- The hole is a straight cylinder and concrete fills it around the post to the full hole depth.
- No gravel base is included — if you set the post on gravel, reduce the hole depth accordingly.
- Bag counts are not calculated (see Sources).
- How many bags of concrete mix is that?
- BuildFactor does not answer this yet. Yield per bag differs between products and mix consistencies, and no validated figure is in the project documents. Use the volume figure and the yield printed on your chosen product's bag.
- How deep should the hole be?
- A common rule of thumb is about one-third of the above-ground post height, but frost depth and local code govern. This calculator uses the depth you enter — it does not recommend one.
Geometry
Cylinder volume minus a prism (the post): (π r² − s²) × depth. No empirical constant.
Unit conversion
NIST SP 811, Appendix B.
Bag counts are not estimated
Converting concrete volume to a number of 40/50/60/80 lb pre-mix bags depends on the yielded ft³ per bag, which varies by product. Rather than guess, this calculator gives you the volume — read the bag count off the yield figure printed on the bag you buy.
About the Concrete Calculator for Fence Posts
The Concrete Calculator for Fence Posts works out the total concrete to set a run of posts, subtracting the volume each post displaces in its hole so you don't over-order. Enter the post count, the hole diameter and depth, and the post size. For a single large footing or a deck pier, use the Sonotube or Concrete Pier calculators instead.
What you enter
- Hole diameter — Roughly 3× the post width is common.
- Hole depth — Often about ⅓ of the above-ground post height, plus a gravel base you set aside.
- Post width (square post) — A nominal 4×4 post is 3.5 in actual.
- Number of posts
- Waste allowance — Default 10%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Concrete volume — For small runs this is usually mixed from bags. BuildFactor does not convert to a bag count here — see the Sources section for why.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Concrete Calculator — How much concrete a rectangular slab or footing needs, in cubic yards or cubic metres.
- Sonotube Concrete Calculator — Concrete volume for round column forms (Sonotube-style tubes).
- Concrete Cost Calculator — Estimated material cost of a concrete pour from its volume and a unit price.
- Cubic Yard Calculator — Convert a length, width and depth into cubic yards, cubic feet or cubic metres.
Part of the Concrete calculators.