How Much Concrete for a Fence Post?
The short answer: one 80 lb bag per hole. But the maths behind it is worth knowing.
One 80 lb bag of concrete per post for a standard 8-inch hole 24 inches deep, with a 4×4 post in it. The hole holds 0.70 cubic feet, but the post displaces 0.17 of that, leaving 0.53 cubic feet — and one bag yields 0.60.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete per post by hole size
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| 6" hole x 24" deep, 4x4 post | 0.2 cu ft | 0 cu yd | 1 × 80 lb | Bags |
| 8" hole x 24" deep, 4x4 post | 0.6 cu ft | 0 cu yd | 1 × 80 lb | Bags |
| 8" hole x 36" deep, 4x4 post | 0.9 cu ft | 0 cu yd | 2 × 80 lb | Bags |
| 10" hole x 36" deep, 6x6 post | 1.1 cu ft | 0 cu yd | 2 × 80 lb | Bags |
| 12" hole x 36" deep, 6x6 post | 1.9 cu ft | 0.1 cu yd | 4 × 80 lb | Bags |
| 8" hole x 24" deep, 20 posts | 11.6 cu ft | 0.4 cu yd | 20 × 80 lb | Bags |
One 4x4 post in each hole, subtracting 3.5 inches of post width. A 6x6 post is 5.5 inches across.
Why the post matters to the calculation
The concrete does not fill the whole hole — the post is standing in it. If you calculate from the hole volume alone, you overbuy.
Worked example, 8-inch hole 24 inches deep with a 4×4 post:
- Hole volume: radius of 4 inches is 0.333 ft. π × 0.333² × 2 ft = 0.70 cubic feet.
- Post displacement: a nominal 4×4 is actually 3.5 inches square, so 0.292 ft × 0.292 ft × 2 ft = 0.17 cubic feet.
- Concrete needed: 0.70 − 0.17 = 0.53 cubic feet, plus waste.
- Bags: 0.58 cubic feet with 10 % waste, divided by 0.60 per bag = 1 bag.
Note the nominal sizes. A "4×4" post is 3.5 inches square and a "6×6" is 5.5 inches. Using the nominal figure overstates the displacement and leaves you short of concrete.
How big and deep should the hole be
The concrete is doing two jobs: holding the post upright, and resisting frost heave. Both depend on the hole, not the concrete.
- Diameter: three times the post width is the old rule. For a 4×4 that means about 10 inches. In practice, 8 inches is common for a 4×4 and 10 to 12 for a 6×6.
- Depth: the general rule is one third of the post's above-ground height, with a minimum of 24 inches. A 6-foot fence therefore wants a 2-foot hole, and a 6-foot post in a windy spot is better with 30 to 36 inches.
- Below the frost line. This is the requirement that overrides the rules of thumb. If you are in a region where the ground freezes, the base of the hole must be below the local frost depth, which can be 36 to 48 inches in northern states. Ask your building department for the figure — it is not a detail you can estimate.
- Not deeper than the post is tall below ground. The post must reach the bottom of the hole. Concrete under the post is wasted.
Two mistakes that ruin fence posts
Mistake one: concrete as a column, not a collar. Pouring concrete and letting it set hard around the post is fine. But some people try to set the post in a concrete plug at the bottom of the hole and backfill with soil — that gives you no lateral support and the post will move.
Mistake two: no drainage. Water sitting around a wooden post rots it. Put 4 to 6 inches of gravel in the bottom of the hole before the post goes in. That lets water drain away from the end grain, which is where rot starts.
One more thing worth knowing: concrete is not what stops the post rotting. It is the drainage and the post treatment. Pressure-treated timber set in concrete with gravel underneath will outlast untreated timber by a decade.
Frequently asked questions
How many bags of concrete per fence post?
One 80 lb bag for a standard 8-inch hole 24 inches deep with a 4x4 post. Deeper holes, wider holes or bigger posts will need two.
How deep should a fence post hole be?
One third of the post height above ground, with a minimum of 24 inches — and always below the local frost line, which may be 36 to 48 inches in colder regions.
How wide should a fence post hole be?
About three times the post width. For a 4x4 that is roughly 10 inches, though 8 inches is commonly used.
Should I put gravel in the bottom of a fence post hole?
Yes. Four to six inches of gravel under the post lets water drain away from the end grain, which is where rot starts. The concrete is not what prevents rot.