Post Hole Concrete Calculator
Calculate the concrete to set fence or deck posts — total cubic feet plus 60-lb and 80-lb bag counts — from the post and hole sizes.
How it works
per hole (cu in) = (π ÷ 4) × (hole diameter² − post diameter²) × depth · cu ft = ÷ 1728 · total = per hole × posts
Concrete fills the ring of space around the post inside the hole, so the volume is the hole’s cylinder minus the post’s cross-section, times the depth. Using (π ÷ 4) × (hole² − post²) works out that annular area in square inches; multiplying by the depth gives cubic inches, and dividing by 1,728 converts to cubic feet. A 60-lb bag of concrete mix yields about 0.45 cu ft and an 80-lb bag about 0.60 cu ft, so those set the bag counts.
Worked example
Six 4 in posts in 10 in holes 30 in deep: per hole = (π ÷ 4) × (10² − 4²) × 30 = 0.7854 × 84 × 30 ≈ 1,979 cu in ÷ 1,728 = 1.15 cu ft. Across 6 posts that is 6.9 cu ft — about 16 sixty-pound or 12 eighty-pound bags.
Frequently asked questions
How deep should a post hole be?
A common rule is one-third of the post’s above-ground height, or at least 2 ft, and always below the local frost line in cold climates. A 6 ft fence usually means an 8 ft post set about 2–2.5 ft deep. Corner and gate posts take more stress, so go deeper and wider.
How wide should the hole be?
About three times the post’s width — a 4 in post wants a 10–12 in hole — so there is a solid concrete collar all around. Too tight a hole leaves too little concrete to resist the leverage of wind and use, and the post works loose.
Do I need gravel at the bottom?
Yes — a few inches of compacted gravel under the post lets water drain away from the end grain instead of pooling and rotting it, and gives the post a firm bearing. Set the post on the gravel, plumb it, then pour the concrete collar around it and slope the top to shed water.
Related calculators
Estimate for planning. Add a waste margin and confirm against your supplier specs and local building codes.