Post-Hole Concrete Calculator
Setting fence or deck posts in concrete? Enter the hole diameter and depth, the number of posts, and the yield of your bag to get the bags per hole and the total bags to buy.
Calculator
Each 12 in × 24 in hole holds about 1.57 cu ft of concrete → 4 bags per hole, or 56 bags for 14 posts (before post displacement). Bag yield is a labeled typical — confirm it on the bag.
Concrete for posts is a volume problem: each hole is a cylinder, and you buy enough bags to fill it. The trick is that concrete is sold by bag yield in cubic feet, so you find the hole volume, divide by what one bag makes, and round up — then multiply by the number of holes.
The result is deliberately a little conservative: it ignores the volume the post itself takes up in the hole, so you are unlikely to run short on the last post. Pair it with the fence material calculator, which gives you the post count to drop straight into this tool.
Formula
Each hole is a cylinder; bags per hole round up, then scale by the post count:
hole_volume = π × (hole_diameter_in ÷ 24)² × hole_depth_in ÷ 12 bags_per_hole = ceil(hole_volume ÷ bag_yield_cuft) total_bags = bags_per_hole × num_posts
Dividing the diameter by 24 gives the radius in feet (÷2 for radius, ÷12 for inches-to-feet), and dividing the depth by 12 puts it in feet too, so the volume lands in cubic feet to match the bag yield. Because you buy whole bags, each hole rounds up.
Worked example
A 12 in diameter hole 24 in deep, 14 posts, using 60-lb bags that yield 0.45 cu ft each:
- Radius: 12 ÷ 24 = 0.5 ft
- Hole volume: π × 0.5² × (24 ÷ 12) = π × 0.25 × 2 = 1.571 cu ft
- Bags per hole: 1.571 ÷ 0.45 = 3.49 → 4 bags
- Total: 4 × 14 = 56 bags
So plan on 4 bags per hole and 56 bags for the run (before subtracting the volume the post takes up). Buy a couple extra so a slightly wide hole does not send you back to the store.
Setting posts in concrete
Hole size rule of thumb. Make the hole about three times the post width and dig it a third of the post's above-ground height deep — a 6 ft fence post typically goes about 2 ft down. Both are labeled typicals you can override; the deciding factor is often the local frost line.
Dig below the frost line. In cold climates the footing must sit below the depth the ground freezes to, or frost heave will lift the posts over the winters. Check your local building department for the required depth — it can be well past 24 in in northern regions.
Bag yield. A 60-lb bag of concrete mix makes about 0.45 cu ft and an 80-lb bag about 0.60 cu ft; fast-setting post mixes list their own yield. Enter the figure printed on your bag — it is a labeled typical, and brands differ.
Post displacement. This count fills the whole hole and ignores the space the post occupies, so it runs slightly high — a safety margin, not an error. For a big job you can subtract the post volume, but the extra bag or two of buffer is cheap insurance. Once posts are set, finish the lumber take-off with the fence material calculator and price the job with the fence or deck cost calculator.
Reference table
Bags of 60-lb mix (~0.45 cu ft each) per hole, before post displacement:
| Diameter | Depth | Hole volume | 60-lb bags |
|---|---|---|---|
| 8 in | 18 in | ≈ 0.44 cu ft | 1 bag |
| 10 in | 24 in | ≈ 0.91 cu ft | 3 bags |
| 12 in | 24 in | ≈ 1.57 cu ft | 4 bags |
| 12 in | 30 in | ≈ 1.96 cu ft | 5 bags |
Bag yield is a labeled typical — confirm it on your bag; an 80-lb bag yields ~0.60 cu ft.