Fence Post Concrete

Concrete for a fence post is the volume of the hole minus the volume of the post, minus the gravel at the bottom. That last subtraction is the one most estimates skip, and it is worth about a sixth of the total.

Fence posts are the one job where dry-pouring works. Fast-setting mix goes into the hole as powder, water goes in on top, and the bag is designed for exactly that. Do not do this for a structural pier — but for a fence, it is what the product is for.

The two rules that decide whether the fence stands

The hole should be about three times the width of the post. A 4×4 needs at least a 10½″ hole. Narrower than that and the collar of concrete is too thin to resist the leverage that wind puts on a fence panel.

Bury about a third of the post. A post standing 6 ft above grade wants roughly 2 ft in the ground. Depth does far more for stability than width does, which is the opposite of what most people assume when they widen the hole instead of digging deeper.

Gravel first, always, on wood posts

Four inches of ¾″ drainage rock in the bottom before any concrete. Concrete poured straight to the bottom of the hole makes a cup that holds water against the end grain, and the post rots from the bottom up while the visible part still looks perfect.

Frost line beats everything

A post set above the frost line will heave, and no amount of concrete stops it. Thirty-six inches clears most of the southern and central US; the northern tier runs 48–60″. Check your local figure before you dig, because the alternative is doing the whole fence again in three years.