A 4×4 fence post in a 10 inch wide, 36 inch deep hole with 6 inches of gravel in the bottom takes about 1.15 cubic feet of concrete: 2 bags of 80 lb mix or 4 bags of 50 lb mix. The exact number depends on the hole diameter, the depth and how much of the hole the post itself fills.
| Post | Hole diameter | Hole depth | Concrete (cu ft) | 50 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 4×4 | 8 in | 24 in | 0.40 | 2 | 1 |
| 4×4 | 10 in | 30 in | 0.92 | 3 | 2 |
| 4×4 | 10 in | 36 in | 1.15 | 4 | 2 |
| 4×4 | 12 in | 42 in | 2.10 | 6 | 4 |
| 6×6 | 12 in | 36 in | 1.44 | 4 | 3 |
| 6×6 | 12 in | 42 in | 1.73 | 5 | 3 |
| 6×6 | 14 in | 48 in | 3.01 | 9 | 6 |
For your own hole sizes, the concrete calculator has a post hole mode that does this math for any number of posts.
How the number is worked out
A post hole is a cylinder with a square post standing in it. The concrete fills the space between them, from the top of the gravel to the top of the hole:
- Hole area: π × (diameter ÷ 2)². A 10 in hole is 3.1416 × 5 × 5 = 78.5 sq in.
- Post area: width × width. A 4×4 is 3.5 × 3.5 = 12.25 sq in.
- Concrete area: 78.5 − 12.25 = 66.3 sq in.
- Concrete depth: hole depth − gravel. 36 − 6 = 30 in.
- Volume: 66.3 × 30 = 1,989 cubic inches.
- Cubic feet: 1,989 ÷ 1,728 = 1.15 cu ft.
- Bags: 1.15 ÷ 0.60 = 1.9, so 2 bags of 80 lb mix.
The post only takes up about 15% of a 10 inch hole, so leaving it out of the math will not change the bag count much. The gravel base matters more: 6 inches of gravel saves about a sixth of the concrete in a 36 inch hole.
Worked example: a 10-post fence
A 60 ft run of privacy fence with posts every 8 ft needs 9 posts. Add a gate post and you have 10 holes. Each is 10 in wide and 36 in deep with a 6 in gravel base, holding a 4×4:
- Concrete for 10 holes: 10 × 1.15 = 11.5 cu ft
- With 10% extra for uneven holes and spills: 12.7 cu ft
- In bags: 22 bags of 80 lb or 34 bags of 50 lb
That is just under half a cubic yard. Bags are the practical choice at this size, and they let you set posts one at a time over a weekend.
How wide and deep should the hole be?
Two rules of thumb are common among fence builders. They are starting points, not code:
- Width: about three times the post width. That gives a 10 to 12 inch hole for a 4×4 and a 16 inch hole for a 6×6, though 12 to 14 inches is common for 6×6 posts in firm soil.
- Depth: about one third of the post’s height above ground, and below your local frost line. A 6 ft fence on an 8 ft post usually goes about 2 to 3 ft deep.
Frost depth varies from a few inches in the southern US to 4 ft or more in the northern states. Your local building department can tell you the required depth, and many fence permits list it.
Gravel in the bottom: worth it?
A few inches of gravel under the post lets water drain away from the end grain instead of pooling in a concrete cup around it. That slows rot in wood posts. It also gives the post a firm base to sit on while the concrete sets. Use clean crushed stone, tamp it, then set the post on it.
Fast-setting mix or standard concrete?
Fast-setting post mix is poured dry into the hole and then soaked with water. It sets in about 20 to 40 minutes, so you can move to the next post quickly. Check the yield on the bag, though: some fast-setting mixes make less concrete per bag than standard mix of the same weight. If the yield is lower, your bag count goes up.
Standard mix is mixed in a wheelbarrow first. It takes longer to set but often costs less per cubic foot and gives you time to plumb the post carefully.
Mistakes to avoid
- Doming the top wrong. Slope the concrete away from the post at the surface so rain runs off instead of collecting at the wood.
- Digging a bell-shaped hole by accident. A hole that is wider at the top than the bottom holds less and resists frost heave poorly. Keep the sides straight.
- Not bracing the post. Check plumb on two adjacent faces and brace the post before the concrete stiffens.
- Buying exactly the calculated amount. Holes are never perfect cylinders. Keep a spare bag or two.