How to calculate concrete for fence posts
All measurements are in inches; dividing by 1,728 converts cubic inches to cubic feet. The concrete depth is the hole depth minus any gravel you put in the bottom. Then divide by the yield of your bag: an 80 lb bag makes about 0.6 ft³, a 60 lb bag 0.45 ft³ and a 50 lb bag 0.375 ft³.
Worked example
Ten 4×4 posts in 10 in holes, 36 in deep, with 6 in of gravel at the bottom. The hole is 78.5 sq in, minus 12.25 sq in for the post, times 30 in of concrete = 1,989 cubic inches, or 1.15 ft³ per post. With 10% extra that's 1.27 ft³, about 2.1 bags of 80 lb per post, or 22 bags for all ten.
Bags per post at a glance
| Post | Hole | Concrete depth | 80 lb bags per post |
|---|---|---|---|
| 4×4 | 8 in | 24 in | about 1 |
| 4×4 | 10 in | 24 in | about 1.5 |
| 4×4 | 10 in | 30 in | about 2 |
| 6×6 | 12 in | 36 in | about 3 |
Figures are before any extra for waste.
How deep and wide should post holes be?
A common rule is to bury one-third to one-half of the post's height above ground: a 6 ft fence needs holes about 2–3 ft deep. In cold climates, go below the frost line so the post doesn't heave in winter. Make the hole about three times the width of the post, so a 4×4 (3.5 in actual) needs a 10–12 in hole.
Put 4–6 in of gravel in the bottom so water drains away from the end of the post instead of rotting it. Slope the top of the concrete away from the post for the same reason.
Fast-setting or regular concrete?
Fast-setting post mix is poured dry into the hole and then soaked with water, with no mixing needed. It sets in 20–40 minutes, so you can keep building the same day. Regular concrete mix is cheaper per bag and stronger, but needs mixing and a day or two before you load the post. Both have about the same yield per pound. Plan the rest of the fence with the fence calculator.
Call 811 before you dig to have buried utility lines marked. It's free in the US.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole with about 2 ft of concrete, around 1.5 bags of 80 lb. Deeper or wider holes need 2 or more.
How much does an 80 lb bag of concrete fill?
About 0.6 cubic feet. A 60 lb bag makes 0.45 ft³ and a 50 lb bag 0.375 ft³.
Should I put gravel under a fence post?
Yes. 4–6 in of gravel at the bottom of the hole lets water drain away from the end of the post, which helps stop rot.
How long before I can attach rails to a post set in concrete?
Fast-setting mix is firm in 20–40 minutes and ready for rails in about 4 hours. Regular concrete needs 24–48 hours.
How deep should a fence post be?
About one-third to one-half of its height above ground, and below the frost line in cold areas. For a 6 ft fence, 2–3 ft is typical.
Run a landscaping, garden or contractor website? Add this calculator to your site for free.
Last reviewed 2026-11-14. Material weights are typical industry averages and vary by supplier, moisture and compaction.