Post Hole Concrete Calculator
Work out the concrete (and bags) to set fence or deck posts — it accounts for the space the post takes up.
Last reviewed 2026-06-06
Setting fence or deck posts in concrete is the one job where people most often under-buy, because the maths is fiddly: a round hole minus the square post, times the depth, times the number of posts. This calculator does exactly that.
Enter the hole diameter and depth, the post width, and how many holes — we subtract the space the post occupies and tell you how many bags of concrete to buy.
How to use the Post Hole Concrete Calculator
- Decide your hole size — a common rule is a hole about 3× the post width, and one-third of the post’s above-ground height deep.
- Enter the hole diameter, depth, post width and number of holes.
- Read the number of bags. Fast-setting post mix is easiest for fences.
- Add gravel at the bottom of each hole for drainage.
Frequently asked questions
How much concrete do I need per fence post?
It depends on the hole. A typical 10-inch-wide, 24-inch-deep hole around a 4×4 post needs roughly 1 cubic foot of concrete — about two 80 lb bags. Enter your sizes above for an exact figure.
How deep and wide should a post hole be?
A good rule: the hole diameter is about three times the post width, and the depth is about one-third of the post’s above-ground height (so a 6 ft fence post is often set ~2 ft deep). Frost line depth may require deeper.
Do I need gravel at the bottom of the hole?
Yes — a few inches of gravel lets water drain away from the post base, which greatly slows rot (wood) and rust (metal). Set the post on the gravel, then pour the concrete around it.
Can I pour the concrete dry?
With fast-setting post concrete, yes: set the post, pour the dry mix into the hole, then add water per the bag. For standard concrete, mix it first. Either way, brace the post until it sets.