Fence post concrete calculator

Diameter costs more than depth

Volume grows with the square of the diameter but only linearly with depth. Widening a hole from 8 to 12 inches more than doubles the concrete; going 6 inches deeper adds a modest fraction.

That is worth knowing in both directions. Do not shave the diameter to save bags — the collar thickness is what spreads the load. But if the budget is tight and the frost rule lets you, depth is the cheaper dimension to be generous with.

Worked example

A 10-inch hole 36 inches deep is about 1.6 cubic feet. A 4×4 in that hole displaces about a quarter of a cubic foot, leaving about 1.4 to fill. At a typical 80-pound bag yield that is three bags per hole, rounded up — and on a 14-post run, forty-two bags. Which is a pallet, a delivery, and a good reason to work it out before Saturday morning.

Ten inches is the narrow end for a 4×4. The three-times-the-post guide in how deep should fence posts be puts the hole nearer 10½ to 12 inches. At 12 inches the same hole holds about 2.1 cubic feet of concrete: four 80-pound bags a post, fifty-six on the run.

Buy a couple over. Running short mid-run means a half-set hole and a trip, and a post reset the next day never quite matches its neighbours.

Mixing, and the gravel underneath

Fast-setting products designed to be poured dry and wetted in the hole are a different method from mixing in a barrow. Follow the bag: too much water is the commonest way to weaken a footing, and the yield figure assumes the correct mix.

Whatever you use, the few inches of gravel under the post come first, before any concrete — that is what lets water leave the end grain instead of standing in a concrete cup. And crown the top of the collar above grade and slope it away, so surface water runs off rather than sitting against the timber. Both details are in post depth.

Sources

Nothing here is asserted as a current figure. The tool is arithmetic on what you type, and every default is a round placeholder for your yard's quote.