Concrete Calculator for Fence Posts
Fence post concrete = (hole volume − post volume) × number of posts. A 10 in hole 24 in deep holds about 1.09 ft³; a 3.5 in (4×4) post takes 0.17 ft³, leaving 0.92 ft³. Six posts need 5.5 ft³ (0.20 yd³), or 0.23 yd³ with 10% waste. For posts, rails and pickets, use the Fence Calculator.
How it's calculated: Concrete = (π × (hole diameter ÷ 2)² − post width²) × hole depth × number of posts.
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?
- Bag yield varies by product, so use the yield printed on your bag. For example, Quikrete Concrete Mix 1101 lists 0.45 ft³ per 60 lb bag. In the worked example, 5.524 ft³ is about 12.3 bags exact (buy 13), or about 13.5 with the 10% allowance (buy 14). Other products' yields differ, so check the yield printed on the product you buy.
- 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.
- How much concrete do I need per fence post?
- About 0.92 ft³ for a 10 in hole 24 in deep around a 4×4 post: the hole holds 1.09 ft³ and the post takes up 0.17 ft³. Wider holes need much more, because volume grows with the square of the diameter.
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, so this calculator gives the volume rather than a bag count. Divide by the yield printed on the bag you buy — for example, Quikrete's Concrete Mix 1101 data sheet lists 0.45 ft³ per 60 lb bag.
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. This calculator gives the volume, not a bag count — divide the total ft³ by the yield printed on your bag (for example, 0.45 ft³ per 60 lb bag of Quikrete Concrete Mix 1101).
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.
- Fence Calculator — Posts, rails, pickets and concrete for a fence run of a given length.
Part of the Concrete calculators.