Fence Concrete Calculator
Concrete calculated the right way: hole volume − post displacement ÷ bag yield — not a flat "bags per post" guess. Enter your post count, hole size and bag format to get exact bag counts and cost.
Estimate
| Hole volume (per post) | 1.27 ft³ |
|---|---|
| Post displacement (−) | 0.26 ft³ |
| Net concrete volume (per post) | 1.01 ft³ |
| Bags per post (50 lb) | 3 |
| Total bags for 28 posts | 84 |
| Concrete cost | $546.00 |
How the concrete math works
Each post hole is a cylinder: π × (hole diameter ÷ 2)² × depth. The post itself takes up room in the hole, so its displacement is subtracted. The net volume divided by the bag's yield gives bags per post — always rounded up, because you cannot pour half a bag.
Worked example
- Hole: 10 in diameter × 28 in deep → 1.27 ft³
- 4×4 post displacement: 0.26 ft³ → net 1.01 ft³
- 50 lb bag yield 0.375 ft³ → ceil(1.01 ÷ 0.375) = 3 bags per post
- 28 posts → 84 bags
Depth: the quick rule vs your frost line
The calculator defaults to the quick rule — one-third of the above-ground height, 2 ft minimum — but you can enter an exact depth. Where you live, the frost line and local code override any rule of thumb. This is a planning estimate only.
Related tools
Count your posts first with the Fence Post Calculator, or get the whole project — posts, panels, pickets, concrete and cost — in the Fence Calculator. The price basis is published on our open price dataset.
FAQ
How many bags of concrete per fence post?
3–4 bags of 50 lb per 4×4 post in a 10 in × 24–28 in hole — the calculator gives the exact figure for your hole size.
How deep should fence post holes be?
⅓ of above-ground height, 2 ft minimum — frost line and local code take precedence.