Post Hole Concrete Calculator
Your estimate
- Bags per hole
- 2.7
- Concrete in total
- 10.13 ft³0.38 yd³ · 0.287 m³
- Concrete per hole
- 0.92 ft³0.03 yd³ · 0.026 m³
For fence posts, mailbox posts and deck posts — the post itself takes up space, so you need less than a full hole.
Updated 2026-10-20 · Based on: bag yields printed by concrete-mix makers (0.30–0.60 ft³ per bag) and a 1.54 dry-volume factor.
How to use the Post Hole Concrete Calculator
- Enter the hole diameter and depth.
- Enter the post width (3.5 in for a 4×4, 5.5 in for a 6×6).
- Type the number of posts and choose the bag.
- Buy the total number of bags shown.
Formula
Concrete per hole = π × (hole diameter ÷ 2)² × depth − post width² × depth.
Fast-setting 50 lb bags yield about 0.375 ft³; regular 80 lb bags about 0.60 ft³.
Worked example
With the values preset in the calculator above:
- Hole diameter
- 10 in
- Hole depth
- 2 ft
- Post width
- 3.5 in
- Number of posts
- 10
- Bag size
- 50 lb fast-setting
- Extra for waste
- 10 %
- Bags in total
- 28
- Bags per hole
- 2.7
- Concrete in total
- 10.13 ft³ (0.38 yd³)
- Concrete per hole
- 0.92 ft³ (0.03 yd³)
How big should a fence post hole be?
A common rule: hole diameter three times the post width and depth one third of the post height above ground (plus 6 in of gravel at the bottom). A 4×4 post for a 6 ft fence: a 10–12 in hole about 2–3 ft deep.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole 2 ft deep: about 2.7 bags of 50 lb fast-setting concrete per post (with 10 % extra).
Do I mix fast-setting concrete with water first?
No — pour water into the hole, then the dry mix. It sets in 20–40 minutes, so set and brace the post first.
Should the concrete come to the top of the hole?
Stop just below ground level and slope the top away from the post so water drains off.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.