How to use the post hole calculator
Enter how many posts you're setting, the post size and the hole you'll dig. The calculator works out the concrete between the gravel base and the top of the hole, subtracts the part of the post that sits in it, and turns that into bags. The headline number is the total if you can split a bag between holes; "bags per post" is the count if you mix each hole separately.
The formula
concrete depth = hole depth − gravel depth
per hole = π × (hole diameter ÷ 2)² × concrete depth − post area × concrete depth
bags = total ÷ bag yield, rounded up
Worked example: ten 4 × 4 fence posts
- Holes 10 in across and 30 in deep, with 6 in of gravel: 24 in (2 ft) of concrete.
- Hole volume: π × (5 ÷ 12)² × 2 = 1.091 ft³.
- Post: (3.5 ÷ 12)² × 2 = 0.170 ft³, so concrete per hole is 0.921 ft³.
- With 10% extra, ten holes need 10.13 ft³: 17 bags of 80 lb concrete (10.13 ÷ 0.60), or 28 bags of 50 lb fast-setting mix.
- Mixed hole by hole that's 2 bags of 80 lb per post, 20 in all.
Hole size guide
| Post | Hole diameter | Typical depth |
|---|---|---|
| 4 × 4 fence post, 6 ft fence | 10-12 in | 24-36 in, below frost line |
| 6 × 6 post, gate or deck | 14-18 in | 36-48 in, or per deck plans |
| Chain-link line post | 8-10 in | 24-30 in |
| Chain-link terminal post | 10-12 in | 30-36 in |
These are common guidelines. Deck posts that carry a structure are a different matter: most codes want them on a proper footing below the frost line, and many inspectors won't accept posts buried in concrete. See the sonotube calculator for formed piers.
Setting posts that last
- Gravel first. Tamp 4-6 in of gravel in the bottom so water drains away from the post's end grain.
- Crown the top. Slope the concrete away from the post at the surface so water sheds instead of pooling against the wood.
- Brace before it sets. Plumb each post on two faces and brace it. Fast-setting mix gives you only a few minutes.
- Use ground-contact lumber. Posts set in concrete need pressure-treated wood rated for ground contact.
Common mistakes
- Bell-shaped holes dug by hand hold far more concrete than a straight auger hole. The extra percentage covers some of it.
- Shallow holes in frost country let the post heave out over a few winters.
- Not subtracting the post. Overestimating is safe but on a 50-post fence it's a lot of spare bags.
Questions people ask
How many bags of concrete per fence post?
For a 4 × 4 post in a 10 inch hole with 24 inches of concrete, about 0.92 cubic feet: two 80 lb bags, or three 50 lb bags of fast-setting mix. Wider holes and 6 × 6 posts take more; enter your sizes above.
How big should a post hole be?
A common guideline is a hole about three times the width of the post: 10 to 12 inches for a 4 × 4, 14 to 18 inches for a 6 × 6. Depth is usually a third to a half of the post's height above ground, and below the frost line in cold climates.
Should I put gravel at the bottom of a post hole?
Yes, 4 to 6 inches of gravel under the post lets water drain away from the end grain, which is where wood posts rot first. Tamp it before setting the post.
Is fast-setting concrete better for fence posts?
It's quicker: QUIKRETE Fast-Setting mix is poured dry into the hole and water is added on top, and it sets in 20 to 40 minutes. Regular mix is cheaper per cubic foot and gives a longer working time. Both work for fence posts.
Do you subtract the post when calculating concrete?
You should, because a 4 × 4 takes up about 16% of a 10 inch hole and a 6 × 6 about 27% of a 12 inch hole. This calculator subtracts the post for you.
Sources and assumptions
- QUIKRETE Concrete Mix (No. 1101) data sheet: 80 lb bag 0.60 ft³, 60 lb 0.45 ft³
- QUIKRETE Fast-Setting Concrete Mix (No. 1004) data sheet: 50 lb bag 0.375 ft³
- Hole about 3× the post width and depth about ⅓-½ of the height above ground are common installation guidelines, not code. Local frost depth and your fence or deck plans govern.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.