How many bags of concrete for a fence post?
A 4x4 post in a 10" hole, 30" deep, takes 3 bags of 60 lb mix. Bag counts by hole size and depth, with the post's own volume already taken out.
One 10" hole 30" deep takes 3 bags of 60 lb mix.
The hole is 1.36 ft³, but the post fills 0.21 ft³ of it — so you are placing 1.15 ft³ of concrete, not the full hole.
How that is worked out
- A round hole is π r² × depth: 10" across and 30" deep is 1.36 ft³.
- Take out the post. A 4x4 is really 3 1/2" × 3 1/2", so 30" of it displaces 0.21 ft³.
- 1.15 ft³ ÷ 0.45 ft³ per 60 lb bag = 2.6, so 3 bags. Always round up.
Bags per hole, with a 4x4 post in it
| Hole | 40 lb bags | 60 lb bags | 80 lb bags | Concrete placed |
|---|---|---|---|---|
| 8" × 24" deep | 2 | 2 | 1 | 0.53 ft³ |
| 8" × 30" deep | 3 | 2 | 2 | 0.66 ft³ |
| 10" × 24" deep | 4 | 3 | 2 | 0.92 ft³ |
| 10" × 30" deep | 4 | 3 | 2 | 1.15 ft³ |
| 10" × 36" deep | 5 | 4 | 3 | 1.38 ft³ |
| 12" × 36" deep | 8 | 5 | 4 | 2.10 ft³ |
| 12" × 42" deep | 9 | 6 | 5 | 2.45 ft³ |
Worth knowing
- Buy one more bag than the table says. Concrete you did not need is cheaper than a second trip in the middle of a pour.
- How deep the hole goes is a local question, not one this table answers. The usual guidance is a third of the post's length in the ground and the bottom of the hole below the frost line — and the frost line is set by where you are, so ring the building department. Some jurisdictions also have rules about fence footings specifically.
- The hole wants to be about three times the post's width. A 4x4 gets a 10" to 12" hole — wide enough to get concrete all round the post, and the width is what resists the post being pushed over.
- Gravel in the bottom of the hole lets water drain away from the end grain instead of sitting against it. It is the single cheapest thing you can do for how long the post lasts.
Work out your own numbers
- Concrete footings and slabs
Cubic yards for a run of post footings or a slab, and the bags of ready-mix that buys.
- Fence post spacing
Post count and the actual on-centre spacing for any run and maximum span.
Related questions
- How many bags of concrete per fence post?
- Three 60 lb bags for the common case — a 4x4 in a 10" hole, 30" deep. A shallower 8" hole drops it to two; a 12" hole or a deeper one takes four or five. The table above has the combinations, and every one of them already has the post's own volume taken out, which is a step most estimates skip.
- Do I need to subtract the post from the hole?
- Yes, and it is worth about a fifth of the concrete on a typical hole. A 4x4 sitting 30" into a 10" hole displaces a bit over 0.2 ft³ — just under half a 60 lb bag. Over a twenty-post fence that is several bags you would otherwise have bought and carried.
- Should I mix the concrete or pour it dry?
- Mix it. Dry-setting — pouring the bag in and adding water on top — is common and it does eventually set, but it cures unevenly and you have no idea what strength you ended up with. For a gate post or anything carrying real load, mix it properly in a tub or a barrow.