Post Hole Calculator
Work out the concrete for fence and deck post holes from hole diameter, depth and post size — bags per hole and for the whole run, or gravel fill instead.
The hole is a cylinder — minus the post
A post hole is a cylinder of known diameter and depth, and the fill only occupies what the post does not. That subtraction is the step most estimates skip: a 4 × 4 post in a 36-inch hole displaces about a quarter of a cubic foot, and over a long fence run it is the difference between ordering right and ordering two bags too many.
Fill per hole = π × (diameter ÷ 2)² × depth − post cross-section × depth
Example: a 12 in diameter, 36 in deep hole around a 4 × 4 post needs 2.10 cu ft of concrete exactly — with waste, 4 × 80-lb bags per hole, so a four-post job takes 16 bags.
How much concrete is in a bag?
| Bag | Yield |
|---|
| 80-lb concrete mix | 0.60 cu ft |
| 60-lb concrete mix | 0.45 cu ft |
| 50-lb fast-setting mix | 0.375 cu ft |
| 50-lb gravel bag | 0.5 cu ft |
Bag counts are rounded up per hole, deliberately: concrete for a hole is mixed and placed hole by hole, and half a bag saved on paper is a hole left short on site. The fast-setting 50-lb bags (the ones you pour dry and water in place) are the fence trade's standard for a reason — no mixing, and the posts can be worked the same day. Gravel fill, offered as the alternative, drains freely and is the right choice where frost heave or rot is the main enemy and the post carries no lateral load worth concreting against; it is figured at full hole volume around the post, in yards and 0.5 cu ft bags.
Depth is a local question: posts should reach below the frost line, which is 12 inches in mild regions and 48 or more in the upper Midwest and Canada. The calculator takes the depth you give it — check your local code or frost map before you dig.
Frequently asked questions
How wide should a post hole be?
The trade rule is three times the post width: a 4 × 4 post (3.5 in actual) wants a 10–12 in hole. That collar of concrete is what gives the post its lateral strength.
Dry-pour fast-setting mix or pre-mix in a wheelbarrow?
Both work. Dry-pour (fill, then water) is faster and standard for fences; pre-mixing gives a more uniform result for structural deck posts. Bag counts are identical — only the method differs.
Should the post sit on the concrete or be buried in it?
Best practice is a gravel pad at the bottom of the hole, the post standing on it, concrete around the sides — so the end grain is not sealed in a cup of water. The calculator counts fill to full depth; a 4–6 inch gravel pad comes out of the concrete a bag at a time on most jobs, which the waste allowance roughly covers.
How many bags for a 6 × 6 deck post?
At the same 12 in × 36 in hole, the bigger post displaces about 0.63 cu ft all by itself, and that volume comes straight off the concrete — pick the 6 × 6 in the form and the subtraction is done for you.
CalcCoverage