How many bags of concrete for a fence post?
A 4x4 post set 24 in deep needs 2 bags of 80 lb concrete.
A 6x6 post set 36 in deep needs 7 bags. That is for the standard 10 in and 16 in holes — three times the post width, which is the rule every yard uses.
Every common post and depth
| Post | Depth | Hole | Concrete | 80 lb | 60 lb | 40 lb |
|---|---|---|---|---|---|---|
| 4x4 | 24 in | 10 in | 0.92 cu ft | 2 | 3 | 4 |
| 4x4 | 30 in | 10 in | 1.15 cu ft | 3 | 3 | 5 |
| 4x4 | 36 in | 10 in | 1.38 cu ft | 3 | 4 | 6 |
| 6x6 | 30 in | 16 in | 2.97 cu ft | 6 | 8 | 11 |
| 6x6 | 36 in | 16 in | 3.56 cu ft | 7 | 9 | 14 |
| 6x6 | 42 in | 16 in | 4.15 cu ft | 8 | 11 | 16 |
| 8x8 | 42 in | 22 in | 7.96 cu ft | 15 | 20 | 30 |
Counts include 10% over, which is not padding — a hole is never the neat cylinder the arithmetic assumes. Soil caves in slightly, the bottom is rarely flat, and running half a bag short on the last post means the whole batch sets while you drive to the store.
Where the number comes from
You are filling a cylinder with a square post already standing in it, so the concrete is the difference between the two.
- Hole volume = π × radius² × depth. A 10 in hole 24 in deep is 1.09 cu ft.
- Subtract the post. A 4x4 is actually 3.5 in square, which occupies 0.17 cu ft over that depth.
- That leaves 0.92 cu ft of concrete.
- An 80 lb bag yields about 0.6 cu ft, so 0.92 ÷ 0.6 = 1.5, rounded up with waste to 2.
The post displaces less than people expect — about 16% of the hole here. It is the hole diameter that drives the bag count, and diameter is squared, so going from a 10 in hole to a 12 in one costs roughly 44% more concrete.
Mixing in the hole, and when not to
Fence posts are the one job where dry-pour is genuinely accepted: pour the bag in dry around a plumb post, add water on top, let ground moisture finish it. It is faster, and for a fence panel carrying nothing but wind it holds up.
Do not dry-pour anything structural. A deck post, a pergola leg or a gate post takes real load, and a dry-pour cures unevenly with dry pockets you cannot see. Mix those in a tub or a barrow. The bag count is the same either way.
Common questions
- Is one bag per post ever enough?
- Only for a light post in a narrow hole — a 4x4 at 18 in in an 8 in hole is about one and a half bags. At the standard 10 in by 24 in you need 2, and buying one is the most common way people end up short.
- Should I put gravel in the bottom?
- Yes, 4 to 6 in of it. It drains water away from the end grain, which is what actually rots a post. Remember it shortens the concrete: 6 in of gravel in a 24 in hole leaves 18 in of concrete and drops a 4x4 from 2 bags to 2.
- What about a gate post?
- Size it up. A gate hangs its whole weight off one post and swings, so go one post size larger, deeper, and into a wider hole — a 6x6 at 36 in is the usual answer, 7 bags.
- Can I use fast-setting concrete?
- Yes, and for fences it is the normal choice. Yields are close enough that this table still applies. It goes off in 20 to 40 minutes, so plumb the post before you pour, not after.
- How much does the post size really matter?
- Less than the hole. Going from a 4x4 to a 6x6 at the same depth removes more concrete from the middle, but the wider hole a 6x6 needs adds far more than the post displaces — 0.92 cu ft becomes 2.37 at the same 24 in.