Post Hole Concrete Calculator — Bags per Post
Setting posts? Enter your hole size, post size, and number of posts to get the bags of concrete needed — per hole and in total. The post itself displaces concrete, so it is subtracted.
How to use this calculator
- Enter the number of posts and the hole diameter and depth.
- A good hole is about 3× the post width across and one-third to one-half the above-ground post height deep.
- Pick the post size so its volume is subtracted; choose "Round/metal" to ignore.
- Fast-setting concrete can be poured dry into the hole and soaked — handy for fence posts.
The formula
Each hole is a cylinder; the post fills part of it, and concrete fills the rest.
Hole volume (cubic feet) = π × (diameter ÷ 2 ÷ 12)² × depth. Post volume = (post width ÷ 12)² × depth.
Concrete per hole = hole volume − post volume. Bags = that ÷ bag yield, rounded up, then multiplied by the number of posts.
Worked example
Ten 4×4 posts, 10 in holes, 2 ft deep, 60 lb bags.
Hole = π × (10 ÷ 24)² × 2 = 1.09 cu ftPost = (3.5 ÷ 12)² × 2 = 0.17 cu ftPer hole = 1.09 − 0.17 = 0.92 cu ft → 0.92 ÷ 0.45 = 3 bagsTotal = 3 × 10 = 30 bags
Ten 4×4 posts in 10 in × 2 ft holes need about 3 bags of 60 lb concrete each — 30 bags total.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in wide, 2 ft deep hole, plan on about 3 bags of 60 lb (or 2 bags of 80 lb) concrete per post after subtracting the post itself.
How deep should a fence post hole be?
Bury about one-third of the post and at least below the frost line. For a 6 ft fence that usually means a 2–2.5 ft deep hole, deeper in cold climates.
Can I use fast-setting concrete dry?
Yes. For fence posts you can pour fast-setting mix into the hole dry, then add water per the bag — no mixing required. It sets in 20–40 minutes.