Fence Post Concrete Calculator
Find the concrete to set fence posts — hole volume minus the post, cubic feet per hole and total 50 lb bags for the run.
How it works
per hole = π × (dia ÷ 24)² × (depth ÷ 12) − post displacement · bags = ceil(volume ÷ bag yield)
Each hole is a cylinder, so its volume is π r² times depth with all measurements converted to feet; the buried part of the post is subtracted so you do not over-buy. Multiply by the number of posts and divide by the bag yield, rounding up per hole (you cannot pour a fraction of a bag neatly). A common rule of thumb is a hole three times the post width and one-third of the post’s above-ground height deep.
Worked example
A 10 in hole 30 in deep is π × (5 in)² × 30 in = 0.34 cu ft after subtracting a 3.5 in post. For 20 posts that is 6.8 cu ft; at 0.375 cu ft per 50 lb fast-set bag, 1 bag per hole and about 19 bags for the run.
Frequently asked questions
How deep should a fence post go?
Bury about one-third of the post’s total length, and always below the frost line — commonly 24–36 in for a 6 ft fence. Corner and gate posts carry more load, so set them deeper and wider.
Do I need to mix fast-set concrete?
No — fast-setting mixes are designed to be poured into the hole dry, then have water added on top; they set in 20–40 minutes. For standard mixes, mix to a stiff consistency before placing.
Should concrete dome above ground?
Yes. Slope the top of the collar away from the post so water sheds off instead of pooling against the wood or steel, which is what rots and rusts posts at the ground line.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.