Setting fence posts
The calculator is set for 10 posts 3½ in square in holes 10½ in across, with 28½ in of concrete in each, counted in 50 lb bags of Quikrete Fast-Setting Concrete Mix. Change the number of holes, then measure your own post, hole and depth.
Measure the post itself: posts are sold by a nominal size, and the wood is smaller. Pick Round for a round post or a metal pipe. The calculator takes the post’s own cross-section away from the hole, because the concrete only fills the ring around it.
Sizing the holes
Quikrete’s rule of thumb on the Fast-Setting data sheet is a hole about 3 times as wide as the post and a third of the post’s length deep. The mix stops 3 to 4 in below the top of the hole, and the sheet fills that last part with soil. If some posts get bigger holes, like gate posts you dig deeper, add them as a second card with their own sizes and count.
Fast-setting or regular mix
Quikrete’s Fast-Setting mix goes into the hole dry, around the post, and is then soaked with about 1 gallon of water for each 50 lb bag; for holes deeper than 30 in, the sheet fills and soaks it in two goes. It reaches its final set in 20 to 40 minutes. Regular concrete mix is mixed in a barrow or mixer first, then shoveled in. Pick either bag in the calculator: the volume stays the same, only the bag count changes.
Holes dug by hand are rarely the exact size, so the 10% extra is a reasonable start, and an unopened bag left over is better than a post half set.
Concrete per post hole
Holes sized by Quikrete's rule of thumb: 3 times the post's width across, and a third of the post's length deep, with the concrete stopping 3½ in below the top. Bags for one hole, and fast-setting bags for ten holes, with no extra added.
| Post | Hole | Concrete | 50 lb Fast-Setting | 80 lb Concrete Mix | 10 holes, 50 lb |
|---|---|---|---|---|---|
| 3 in post, 6 ft long | 9 × 24 in | 0.65 ft³ | 2 | 2 | 18 |
| 3 in post, 8 ft long | 9 × 32 in | 0.9 ft³ | 3 | 2 | 25 |
| 3 in post, 10 ft long | 9 × 40 in | 1.15 ft³ | 4 | 2 | 31 |
| 3.5 in post, 6 ft long | 10.5 × 24 in | 0.88 ft³ | 3 | 2 | 24 |
| 3.5 in post, 8 ft long | 10.5 × 32 in | 1.23 ft³ | 4 | 3 | 33 |
| 3.5 in post, 10 ft long | 10.5 × 40 in | 1.57 ft³ | 5 | 3 | 42 |
| 5.5 in post, 6 ft long | 16.5 × 24 in | 2.18 ft³ | 6 | 4 | 59 |
| 5.5 in post, 8 ft long | 16.5 × 32 in | 3.03 ft³ | 9 | 6 | 81 |
| 5.5 in post, 10 ft long | 16.5 × 40 in | 3.88 ft³ | 11 | 7 | 104 |
Quikrete Fast-Setting Concrete Mix data sheet, checked October 2026: "Two 50 lb (22.6 kg) bags … will set a 3 in (75 mm) square post in a 9 in (225 mm) diameter hole 30 in (0.75 m) deep." By volume that hole holds 0.95 ft³ around the post filled to the top, and 0.84 ft³ filled to 3½ in below it, against 0.75 ft³ in two bags. Data sheet