Concrete for Fence Posts and Footings: How Much Per Hole
For a cylindrical hole, calculate the volume filled by concrete and subtract the volume of the post within that concrete. Add the volumes for all holes, then convert to bags or yards. Use the actual geometry for tapered, belled or irregular holes.
The formula
Hole volume (cu in) = π × (Hole diameter ÷ 2)² × Concrete depth
Post volume (cu in) = Post width × Post thickness × Concrete depth (square post)
= π × (Post diameter ÷ 2)² × Concrete depth (round post)
Concrete per hole = Hole volume − Post volume
Cubic feet = cubic inches ÷ 1,728
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag, rounded up
Concrete depth is the depth the concrete fills. If you put gravel in the bottom of the hole, subtract it.
Use actual post size, not nominal. A 4×4 is actually about 3.5 in × 3.5 in.
Hole size and depth come from the post size, fence height, wind exposure, soil, frost depth and local code. The 10 in hole used below is an example input, not a sizing recommendation. Follow the post manufacturer's and your local requirements. This page covers volume only.
Worked example: 20 wood posts
Inputs: 10 in diameter hole, 36 in deep, with 4 in of gravel in the bottom, so the concrete is 32 in deep. 4×4 post (3.5 × 3.5 in actual). 20 posts.
Carry unrounded values through the calculation; displayed values below are rounded.
Hole volume = π × (10 ÷ 2)² × 32 ≈ 2,513.2741 cu in
Post volume = 3.5 × 3.5 × 32 = 392 cu in
Per hole ≈ 2,121.2741 cu in ≈ 1.2275892 cu ft
20 holes ≈ 24.5517838 cu ft
With 10% ≈ 27.0069622 cu ft
Cubic yards ≈ 1.0002579 cu yd; round the order up to the supplier's increment
80 lb bags ≈ 45.0116037 → 46 bags, assuming 0.60 cu ft per bag
Per hole, that's about 1.23 ÷ 0.60 = 2.05 bags (80 lb) before waste. Buy for the whole job and round up once, so you're not rounding every hole up to a full extra bag.
The 10% waste is a common practice, not a rule. Holes dug by hand or in rocky ground are rarely perfect cylinders, and bell-shaped or oversized holes take more.
Bag yields
Check the yield listed on the bag. Commonly listed yields for standard concrete mix are about 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag. Fast-setting post mixes list their own yield, so use that number when you're using that product.
FAQ
Do I subtract the post from the hole volume?
Yes. The post takes up space the concrete doesn't fill. On a 10 in hole with a 4×4, the post is about 16% of the hole volume (392 ÷ 2,513).
Should I use nominal or actual post size?
Actual. A 4×4 is about 3.5 × 3.5 in and a 6×6 is about 5.5 × 5.5 in.
Do gate and corner posts need more concrete?
They're often set in bigger or deeper holes. Run the math separately for those holes with their own size and depth.
How many yards is that for a whole fence?
Multiply concrete per hole by the number of holes, add your waste allowance, then divide cubic feet by 27.
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