How big should a post hole be?
A common rule: make the hole three times the width of the post, and bury a third of the post's total length. So a post standing 6 ft above ground needs a hole about 3 ft deep. In cold areas the bottom of the hole must also sit below the frost line, which your local building department can tell you.
| Fence | Hole | cu ft per hole | 80 lb bags | 50 lb bags |
|---|---|---|---|---|
| 4 ft fence, 4×4 | 11 in × 24 in | 1.15 | 2 | 4 |
| 6 ft fence, 4×4 | 11 in × 36 in | 1.72 | 3 | 5 |
| 6 ft fence, 6×6 | 17 in × 36 in | 4.10 | 7 | 11 |
| 8 ft fence, 6×6 | 17 in × 48 in | 5.46 | 10 | 15 |
How the post hole calculation works
Most calculators skip the post and tell you to buy more than you need. Over a whole fence line, that adds up to several wasted bags.
Fast-setting or regular concrete?
Fast-setting mix is poured dry into the hole and then soaked with water, and the post can usually take weight within hours. Regular mix needs mixing first but gives you more time to line posts up. Both come in 50 and 80 lb bags. Check the bag for yield, since fast-setting mixes often make a little less per bag.
Common questions
How many bags of concrete per fence post?
For a 6 ft fence with 4×4 posts in 11 in × 36 in holes, about 3 bags of 80 lb per post. Smaller holes need less, and the calculator above works out your exact number.
How deep should a fence post hole be?
About a third of the post's total length, and below your area's frost line. For a 6 ft fence that's usually 2 to 3 ft deep.
Should I put gravel in the bottom of a post hole?
Many builders add a few inches of gravel under the post for drainage. If you do, measure the hole depth from the top of the gravel.
How many bags of concrete for a 4×4 post?
Usually 2 to 3 bags of 80 lb, or 3 to 4 bags of 50 lb, depending on hole size.