Setting fence posts or deck posts in concrete? Enter the hole size, depth and post size to see how much concrete fills the space around each post, and how many bags to buy for the whole job.
Formula
Concrete = (π × (Hole diameter ÷ 2)² − Post area) × Depth × Number of posts
The post takes up part of the hole, so its cross-section is subtracted. A common rule is a hole three times the post width and about a third to a half of the post’s above-ground height deep.
Worked example
A 10-inch hole, 30 inches deep, with a 4×4 post (3.5 in square): (78.54 − 12.25) × 30 = 1,989 in³ ≈ 1.15 ft³ per post. Ten posts need 11.51 ft³ — 20 bags of 80 lb mix or 31 bags of 50 lb mix.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10-inch hole, 30 inches deep, about 2 bags of 80 lb mix or 3 bags of 50 lb mix per post.
How deep should a fence post hole be?
Typically one-third of the post’s total length, and at least 24 inches — deeper (below the frost line) in cold climates.
Last updated · Calculations checked by automated tests· Editorial policy