Concrete is easy to under-buy because the post takes up part of the hole. The trick is to work out the hole volume and then subtract the post. Here is the method and a worked example.
The formula
A round post hole is a cylinder. Its volume is π × radius² × depth, all in feet. Then subtract the volume of the post that sits in it:
concrete = (π × (hole diameter ÷ 2)² × depth) − (post width² × depth)
Divide that by the yield of your bag size and round up to get the bags per post.
Worked example: a 4×4 in a 12 in hole
A common setup is a 4 in post in a 12 in (1 ft) diameter hole, 2 ft deep:
- Hole: π × 0.5² × 2 = 1.57 cubic feet
- Post: 0.33² × 2 = 0.22 cubic feet
- Concrete: 1.57 − 0.22 ≈ 1.35 cubic feet
At 0.6 cubic feet per 80 lb bag, that is 1.35 ÷ 0.6 = 2.25, which rounds up to three 80 lb bags per post. For a 14-post fence, that is 42 bags. Mixing a little extra is wise, since concrete settles and the last bag is rarely full.
Sizing the hole
A good rule is a hole about three times the post width, a 12 in hole for a 4×4. Depth should be roughly a third of the post’s height above ground, and never above the frost line, or winter heave will push the post up. Frost depth varies a lot by region, so check your local figure before you dig.
Count it in seconds
The concrete per fence post calculator takes the post width, hole size, depth and bag size and returns the cubic feet and bags per post, then for the whole fence. Planning the layout too? The fence drawing tool folds concrete into the full materials list.