Fence post concrete calculator
Work out how many bags of concrete mix each fence post footer needs, accounting for the post itself, based on hole and post dimensions.
Fence calculator
Enter your specs. Every result updates live and shows its formula.
Overview
A footer is not a solid cylinder of concrete: the post sits inside the hole and displaces mix. This calculator builds the footer volume from the hole diameter, hole depth and post width, subtracts the post volume, and converts the remaining concrete to 50, 60 and 80 lb bag counts.
The result is per post. Multiply by the number of posts in your run, or use the full concrete-for-holes calculator when you buy bags for a set of identical footers at once.
Common mistakes
- Ordering bags from the hole volume without subtracting the post volume, which over-buys mix by roughly the space the post occupies.
- Assuming the same bag count for a 4 by 4 and a 6 by 6 post at the same hole size.
- Rounding bag totals down "for the noise", when one bag short means a mid-install hardware store run.
Related calculators
- Fence post calculator Calculate line posts, corner posts and end posts from run length and spacing, with concrete for each post included in the take-off.
- Fence post depth calculator Get the right embedment depth for a fence post from frost line and above-ground height, so corner posts hold and winter frost cannot heave them.
- Fence cost calculator Estimate total fence cost from run length, height and your local material and labor rates, with the budget math shown.
Related reading
In-depth guides
Frequently asked questions
How many bags of concrete does a 10 by 24 inch post hole need?
With the hole volume minus a 4 by 4 post, a 10 inch by 24 inch footer needs around 2 to 3 bags of 60 lb mix (about 3.5 cubic feet of concrete). The calculator reports exact-per-post counts for all three common bag sizes.
What is the formula for concrete per post?
Compute the hole volume from diameter and depth, subtract the post volume below grade (post width squared times embedded depth), and divide the remainder by the yield of your bag size (0.375 cu ft at 50 lb, 0.45 at 60 lb, 0.6 at 80 lb per published mix yields).
Why subtract the post volume?
The post occupies space inside the hole, so the concrete fill equals the hole volume minus the post volume. Skipping the subtraction means buying roughly a half-bag or more of unneeded mix per post.
Is a 60 lb bag or an 80 lb bag the better buy?
Compare cost per cubic foot, not cost per bag: 80 lb bags yield 0.6 cu ft versus 0.45 for 60 lb, so fewer 80 lb bags cover the same footer, but the price per bag is higher. The per-post count for each size is on screen to make that comparison visible.
How deep should the concrete footer go?
Below the frost line where it applies, and deep enough for at least a third of the post height by the common rule. The depth is an input here that feeds directly into the bag count, so changing it changes the purchase list.
Can I backfill with gravel instead of concrete?
Yes, in free-draining sandy or rocky ground gravel backfill works and drains well, but it offers less rigid hold. If you switch to gravel, the gravel calculator gives the volume and tonnage for that plan instead.
Do gate and corner posts need more concrete?
Often yes, because they carry tension and gate load. Many footers spec a wider or deeper hole at corners, which this calculator handles when you widen the hole diameter input for those posts and multiply by their count.
Make quickset or regular concrete for fence posts?
Quickset (fast-set) bags are common for fence work because a post can carry load within an hour or two, versus a day for standard mix. The bag math is identical; only the working time differs.