BuildCalc

Post Hole Concrete Calculator

Find out how many bags of concrete a fence, deck or mailbox post needs. Unlike most post hole calculators, this one subtracts the volume the post itself takes up in the hole — on a 6×6 in a narrow hole that is nearly a fifth of the concrete, and ignoring it means buying bags you will not use.

Concrete per project
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    How it works

    Hole volume = π × radius² × depth, where radius = (hole diameter ÷ 12) ÷ 2 in feet. The post occupies its cross-sectional area × the depth, so concrete = hole volume − post volume. That net figure is multiplied by the number of posts, has your waste allowance added, then is divided by the bag yield: 0.60 cu ft for an 80 lb bag and 0.375 cu ft for a 50 lb bag of fast-setting mix.

    Frequently asked questions

    How many bags of concrete per fence post?

    A 4×4 post in a 10 in hole 2.5 ft deep needs about 1.1 cubic feet of concrete once the post is accounted for — roughly 2 bags of 80 lb, or 4 bags of 50 lb fast-setting mix. Wider or deeper holes climb quickly.

    How deep should a fence post hole be?

    The rule of thumb is one third of the post height above ground, or below the frost line — whichever is deeper. For a 6 ft fence that is usually 2 to 3 feet. Frost depth is set locally and can reach 48 in in cold climates.

    How wide should the hole be?

    About three times the post width: a 4×4 post wants a 10–12 in hole, a 6×6 wants 12–18 in. Wider holes use a lot more concrete, since volume grows with the square of the diameter.

    Does the post really change the amount of concrete?

    Yes, more than people expect. A 6×6 post in a 12 in hole takes up about 27% of the hole. Calculators that treat the hole as if it were empty will have you buying roughly a quarter more concrete than the job needs.

    Should I use fast-setting concrete for posts?

    Fast-setting mix can be poured dry into the hole and soaked, which suits fence posts and avoids mixing. A 50 lb bag yields about 0.375 cubic feet — less than an 80 lb bag of standard mix, so you need more bags. Both counts are shown above.

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    Estimates are for planning only. Material yields, densities and code requirements vary — confirm quantities with your supplier and local building code before ordering.