A post hole is a cylinder with a post standing in the middle of it, and both parts belong in the arithmetic. Most tools give you the volume of the hole and stop, which over a fence run overstates the order every time. This post hole concrete calculator subtracts the post.
The post takes up real room
Enter what the post measures rather than what it is called. A nominal 4x4 actually measures 3.5 inches, and a 6x6 measures 5.5.
Across ten 10 inch holes at 3 feet deep, that displacement is worth counting:
- Filling the holes solid, with no post: 30 bags.
- With a 4x4 in each: 26 bags, so the posts absorb about an eighth.
- With a 6x6 in each: 19 bags, closer to a third.
Leave the post width at zero for a hole you genuinely are filling solid, such as a sleeve or a footing pad.
Diameter costs more than depth
This is the one worth internalizing before you pick an auger. The radius is squared in the volume and the depth is not, so widening a hole is far more expensive than deepening it.
- 8 inch holes, ten of them at 3 feet: 15 bags.
- 10 inch holes: 26 bags.
- 12 inch holes: 39 bags.
Going from 10 inches to 12 is 50% more concrete, while going from 3 feet deep to 4 takes the same holes from 26 bags to 34, about 30%. Depth buys you more stability per bag than width does.
Two rules set the hole, and the stricter one wins. The usual guide is a diameter three times the post width and a depth of one third of the post height above ground. But in cold regions the hole has to reach below the frost line whatever that rule says, or the post lifts every winter. Your local building department publishes the depth, and it is not optional.
Bags, not yards
Post holes are bag territory. Ten holes is 0.56 cubic yards, far below any sensible ready-mix delivery, and fast-setting mix poured dry into the hole and watered is standard practice for fence posts. It is the manufacturer’s own method: Quikrete’s fast-setting mix data sheet says to pour the dry mix in to within 3 to 4 inches of the top, then add water until the powder is saturated.
Gravel in the bottom few inches of the hole gives the post a drainage path and is worth doing on anything wooden. It also reduces the concrete slightly, so subtract that depth from the hole depth above if you use it.
Counting the holes
A fence run has one more post than it has sections, because both ends are closed, and it is an easy one to miscount by one. The fence calculator works out the post count for a run and the rest of the parts list with it, and the concrete calculator covers anything poured flat.
Post hole concrete calculator FAQ
About 2 to 3 for a typical 10 inch hole 3 feet deep with a 4x4 post, which works out at 26 bags across ten holes. Wider holes rise steeply: 12 inch holes take 39 bags for the same ten posts.
One third of the post height above ground as a starting point, so 2 feet for a 6 foot fence. In cold regions the hole must also reach below the local frost line, which overrides that rule, and your building department publishes the depth.
About three times the width of the post, so roughly 10 to 12 inches for a 4x4. Width is expensive: going from a 10 inch hole to a 12 inch one raises the concrete by 50% across the same run.
Yes, and most calculators do not. A 4x4 measures 3.5 inches and absorbs roughly an eighth of a 10 inch hole. Over ten holes that is the difference between 30 bags and 26.