How Many Bags of Concrete Per Post Hole?
Tells you the bags per hole and the bags for the whole run. It subtracts the volume the post itself takes up, which most calculators forget — on a long fence that is several bags you would otherwise buy for nothing.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
-
Size the hole to the post
Three times the post width is the standard. A 4×4 post takes a 10 to 12 in. hole; a 6×6 takes 16 to 18. Narrower saves concrete but gives the post far less to hold on to.
-
Go deep enough, and below the frost line
A third of the post's above-ground height is the usual minimum — so a 6 ft fence post wants about 2 ft in the ground. But frost depth overrides that: in cold regions the hole must go below the frost line or the ground will heave the post up over a few winters. Your building department publishes the local figure.
-
Put gravel in the bottom
Three to six inches of gravel under the post lets water drain away instead of standing against the end grain. The calculator takes that depth off the concrete, because it is not concrete.
-
Subtract the post
The post occupies part of the hole. A 4×4 is really 3.5 in. square, which over a 30 in. hole is about a fifth of a cubic foot — a third of an 80 lb bag, on every single post. Across a 30-post fence that is real money.
Bags per hole for a 4 × 4 post
Post volume already subtracted. No gravel base.
| Hole size | Concrete needed | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8 in. × 24 in. | 0.53 cu ft | 2 | 2 | 1 |
| 10 in. × 24 in. | 0.92 cu ft | 3 | 3 | 2 |
| 10 in. × 30 in. | 1.15 cu ft | 4 | 3 | 2 |
| 10 in. × 36 in. | 1.38 cu ft | 4 | 4 | 3 |
| 12 in. × 30 in. | 1.75 cu ft | 5 | 4 | 3 |
| 12 in. × 36 in. | 2.10 cu ft | 6 | 5 | 4 |
| 12 in. × 42 in. | 2.45 cu ft | 7 | 6 | 5 |
The common rule of thumb — two 80 lb bags per 4×4 post — matches a 10 in. × 30 in. hole almost exactly.
What each bag yields
| Bag | Yield | Holes per bag (10×30 in.) | Weight to carry |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 0.26 | Easy |
| 50 lb | 0.375 cu ft | 0.33 | Manageable |
| 60 lb | 0.45 cu ft | 0.39 | Two hands |
| 80 lb | 0.60 cu ft | 0.52 | Heavy — plan the lift |
80 lb bags are the cheapest per cubic foot but the hardest to move. For a long fence, the saving is usually worth it.
How many bags of concrete for a 4x4 fence post?
For the usual 10 in. wide, 30 in. deep hole, a 4×4 post needs about 1.15 cubic feet of concrete once the post itself is subtracted. That is two 80 lb bags, three 60 lb bags, or four 50 lb bags.
Put a 4 in. gravel base in the bottom — which you should — and the concrete drops to about 1.0 cubic foot. Still two 80 lb bags, because concrete is sold whole. That is the calculator's default, which is why it may show a slightly smaller volume than the table below.
Go to a 12 in. hole at the same depth and it jumps to 1.9 cu ft, over three 80 lb bags. Hole diameter costs far more concrete than depth does, because volume rises with the square of the radius.
How many bags for a whole fence?
Set the post count and the calculator gives you the full order. A 100 ft fence with posts at 8 ft centers needs 13 posts, which at two 80 lb bags each is 26 bags — about 1,040 lb, or half a pallet. Order it delivered rather than making four car trips.
Add two or three spare bags. A hole that hits a rock and has to be widened will eat them, and returning unopened bags is easy.
Do you need to mix it, or can you pour it dry?
Both work for fence posts, and opinion is genuinely divided.
- Mixed — stronger and more predictable, because you control the water. Necessary for anything structural or load-bearing.
- Dry-set — pour the dry mix in around the post, then add water on top and let it soak down. Fast-setting products are formulated for this and reach usable strength in 20 to 40 minutes. Fine for a fence, and it is what most fence crews actually do.
Dry-setting in very dry soil is the one case to avoid — the ground pulls the water out before the mix hydrates properly.
Should the concrete finish above or below ground?
Above, and sloped away. Bring the concrete an inch or two proud of grade and trowel a slope on it so rain runs off rather than pooling against the post. A collar finished flat, or below grade, holds water exactly where the post rots first.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
-
hole volume = π × (diameter ÷ 2)² × depthThe hole is a cylinder. Volume rises with the square of the radius, which is why width costs more than depth. -
depth = hole depth − gravel baseThe gravel takes up space at the bottom that concrete does not fill. -
− post volume (3.5 × 3.5 in. for a 4×4)Nominal sizes are not actual sizes. This is the step most calculators skip. -
÷ 1728Cubic inches to cubic feet. -
÷ bag yield, rounded up80 lb yields 0.60 cu ft, 60 lb yields 0.45, 50 lb 0.375, 40 lb 0.30. -
× number of postsRounding happens per hole, because you cannot split a bag between two holes.
Common questions
How many bags of concrete for a 4x4 post?
Two 80 lb bags for a standard 10 in. wide, 30 in. deep hole. That works out at about 1.15 cubic feet once the post is subtracted. A wider 12 in. hole needs three or four.
How deep should a fence post hole be?
At least a third of the post's height above ground — 2 ft for a 6 ft fence — and always below your local frost line, whichever is deeper. Frost heave will lift a shallow post out of the ground over a few winters.
How wide should the hole be?
Three times the post width. A 4×4 takes 10–12 in., a 6×6 takes 16–18 in. Going wider than that mostly buys concrete rather than strength.
Do I really need gravel at the bottom?
It is worth the few minutes. Three to six inches of gravel lets water drain away from the end grain instead of sitting against it, which is where posts rot first. Set the depth above and the calculator adjusts the concrete for you.
Can I pour the concrete in dry?
Yes for fence posts, using a fast-setting mix made for it — pour dry around the post, then add water on top. It is what most fence crews do. Use properly mixed concrete for anything structural, and avoid dry-setting in very dry soil.
How long before I can attach the rails?
Fast-setting mix is firm enough to work against in about 4 hours and reaches full strength in 28 days. Standard mix needs 24 to 48 hours before you put any load on the post. Leave the bracing in place the whole time.
Why subtract the post from the hole?
Because the post is occupying space the concrete would otherwise fill. On a 30 in. hole a 4×4 displaces about 0.21 cu ft — roughly a third of an 80 lb bag. On a thirty-post fence, ignoring it means buying ten bags you do not need.
How far apart should fence posts be?
Six to eight feet on center for most wood fences. Eight is the common maximum for 6 ft panels; go to six in windy places or with heavier boards. Divide your run by the spacing and add one for the end post.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Quikrete technical data — bag yields in cubic feet for 40, 50, 60 and 80 lb bags
- Sakrete product data sheets — fast-setting mix yields and dry-set installation method
- International Residential Code R403.1.4 — footings must extend below the frost line
- American Wood Protection Association — ground-contact treatment and drainage at the post base
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.