Bags come from the hole volume minus the post
The concrete fills the gap around the post, so the amount is the hole volume less the volume the post occupies. Divide that by how much one bag yields, then round up.
- Round hole volume — π × radius² × depth. A 10 in hole, 2 ft deep is about 1.09 ft³.
- Bag yield — a 50-lb bag makes about 0.375 ft³, an 80-lb bag about 0.6 ft³, a 25-kg bag about 0.47 ft³.
- Add ~10% for waste and spillage, and always round each post up to a whole bag.
Bury a third of the post, and clear the frost line
Two rules set the depth. The first is structural: sink about one-third of the post length so it stays upright. The second overrides it in cold climates: the base must sit below the frost line, or freezing ground heaves the post up over the winters.
The one-third rule for a 6-foot post; deepen the buried part if the local frost line is lower than 2 feet.
Frost line runs near zero in the far south, 2 to 3 ft across the central US, and 3 to 4+ ft in the north. Put 4 to 6 inches of gravel under the post for drainage and set the concrete an inch below grade.
Common setups
| Post & hole | Depth | Concrete | 50-lb bags |
|---|---|---|---|
| 4x4 post, 8 in hole | 24 in | ~0.55 ft³ | 2 |
| 4x4 post, 10 in hole | 24 in | ~0.86 ft³ | 3 |
| 4x4 post, 12 in hole | 30 in | ~1.24 ft³ | 4 |
| 6x6 post, 12 in hole | 36 in | ~1.61 ft³ | 5 |
Concrete shown includes a 10% waste margin. A gate or corner post carries more load, so oversize the hole one step.
Common questions
How many bags of concrete do I need per fence post?
A common 4x4 post in a 10-inch hole about 2 to 2.5 feet deep takes two to three 50-lb bags. Bigger holes for gate or corner posts can run four to six. The exact count comes from the hole volume minus the post, so use the calculator for your own sizes.
How deep should a fence post hole be?
Bury about one-third of the post length, and always get below the local frost line. A 6-foot-tall post wants roughly 2 feet in the ground; in cold regions the frost line can force 3 to 4 feet regardless.
How wide should the hole be?
About three times the post width. A 4x4 post takes a hole around 10 to 12 inches across. A hole too close to the post size lets frost grab the post and can split it.
