Post hole concrete calculator

Subtracts the post from the hole, then gives bags per hole and bags for the run.

Units

Auger or spade width.

Actual, not nominal. A 4×4 measures 3.5 in.

Optional. 4–6 in is the usual allowance, and it goes below the hole depth above.

Why 10%?

Post holes waste less than a slab because you mix small batches and pour straight into a confined hole. Auger holes that belled out at the bottom, or ground that crumbled while you worked, are the usual reasons to add a little.

Add a price

Fast-setting post mix usually costs more per bag than standard mix.

Enter your hole and post sizes to get a bag count.

    Working out concrete for fence posts

    A post hole is a cylinder with a square timber standing in it. Concrete fills only what is left, so the honest figure is hole minus post, times the number of posts. Skipping that subtraction over-orders; forgetting the multiplier under-orders badly.

    Hole volume = π × (diameter ÷ 2 ÷ 12)² × depth in feet Post volume = (width ÷ 12) × (width ÷ 12) × depth in feet Net per hole = hole volume − post volume Total = net per hole × number of posts
    • diameter — the width of the auger or spade cut, in inches
    • depth — how far down the hole goes, in inches
    • width — the post's actual dimension: a nominal 4×4 measures 3.5 in, a 6×6 measures 5.5 in
    • 12 — inches per foot, converting both to the same units
    • π — the hole is round even though the post is square

    Worked example: six posts, 10 in holes, 24 in deep

    Hole = π × (10 ÷ 2 ÷ 12)² × 2 ft = 1.0908 cubic feet Post = (3.5 ÷ 12) × (3.5 ÷ 12) × 2 ft = 0.1701 cubic feet Net = 1.0908 − 0.1701 = 0.9207 per hole Total = 0.9207 × 6 = 5.524 cubic feet Bags = 5.524 ÷ 0.60 = 10 bags of 80 lb

    One hole alone needs 2 sacks, since 0.9207 divided by 0.60 rounds up. Six holes do not therefore need 12 — they need 10, because the leftover half of each sack goes into the next hole when you mix in batches. Buying by the hole rather than by the job is a quiet way to overspend by a fifth.

    Why the allowance starts at zero here

    A post hole loses almost nothing: small batch, tipped straight in, hole holds its shape. Watch the hole instead. Augers bell out in soft ground, and a spade-dug hole is rarely the neat cylinder the maths assumes.

    Common mistakes

    • Using the nominal post size. Entering 4 inches for a 4×4 overstates the post volume by a third and leaves you short.
    • Rounding the bag count per hole, then multiplying. Two sacks a hole across six holes reads as 12 sacks, but the job takes 10.
    • Measuring the hole at the top. An auger that wandered or a hole that crumbled is wider down the middle than it looks from above.
    • Filling the hole to the surface. Most fence details stop the concrete short of grade so soil or gravel can cap it and shed water away from the timber.

    When these numbers do not apply

    • Belled or footed holes. A hole deliberately widened at the base to resist frost heave holds considerably more than a straight cylinder.
    • Dry-set post mix. Some products are poured in dry and watered in place. Coverage is stated per hole on the sack and does not follow the yield used here.
    • Gravel bases. Several inches of drainage stone under the post displaces concrete you would otherwise have bought.

    Concrete per hole, around a 4×4 post (3.5 in actual)

    HoleConcrete (ft³)80 lb bags60 lb bags50 lb bags
    8 in × 24 in0.53122
    8 in × 30 in0.66222
    10 in × 24 in0.92233
    10 in × 30 in1.15234
    10 in × 36 in1.38344
    12 in × 30 in1.75345
    12 in × 36 in2.10456
    12 in × 42 in2.45567

    One hole each, with the post's own volume already subtracted. Bag counts round up per hole; a run of holes usually needs fewer in total, because part-bags combine when you mix in batches.

    Section through a 10 inch post hole 24 inches deep with a 3.5 inch post centred in it.
    Drawn to scale.

    Common questions

    I am in the UK, Canada or Australia. Does this work in metric?

    Yes, and the shopping list changes with it rather than just the arithmetic. Switch to metric and the answer comes in cubic metres and the bags become 20 kg and 30 kg — the sizes those merchants actually stock. It used to convert the volume and leave the packaging American, which gave you the right amount of concrete in a bag nobody near you sells. Yields are the published ones: Blue Circle put a 20 kg bag at about 0.01 m3, and Sakrete Canada put a 30 kg bag at about 0.014 m3. Prices can be entered in GBP, EUR, CAD or AUD.

    How much concrete does one fence post take?

    A 4×4 in a 10 inch hole 24 inches deep takes about 0.92 cubic feet — 2 sacks of 80 lb. A 12 inch hole climbs to roughly 1.4 cubic feet.

    How deep should the hole be?

    Depth is governed by frost line and post height, both local questions. A common starting point is a third of the post's above-ground height, but your building department's frost depth is the figure that matters.

    Why is the total less than bags per hole times posts?

    Because part-sacks carry over. Each hole rounds up on its own, but when you mix batches the remainder goes into the next hole rather than being thrown away.

    Should I put gravel in the bottom first?

    Many do, to drain water off the end grain — four to six inches is the usual allowance. Enter it in the gravel base field and the calculator takes it out of the hole for you: a six inch base in a twenty-four inch hole gives you concrete for eighteen inches, plus the crushed stone to buy. You no longer have to subtract it from the depth yourself.

    Does the gravel come out of the hole depth or go below it?

    On this page, out of it. You are telling the calculator how deep a hole you are digging, so the gravel fills the bottom of that hole and the concrete fills what is left. The fence post depth calculator frames it the other way round, because it asks for the structural embedment rather than the hole — there the gravel is dug additionally, below the depth it reports.

    Can I use fast-setting mix instead?

    Yes, and it is the usual choice for fencing. Check the yield printed on the sack — post mixes are often sold in 50 lb sacks with their own coverage figure rather than the standard yields used here.

    Sources

    Concrete mix density
    140 lb/ft3
    80 lb bag yield
    0.6 ft³
    60 lb bag yield
    0.45 ft³
    50 lb bag yield
    0.375 ft³
    40 lb bag yield
    0.3 ft³
    90 lb bag yield
    0.675 ft³
    30 kg bag yield
    0.49440533379999996 ft³
    20 kg bag yield
    0.35314666699999997 ft³
    Default waste allowance
    10%

    Bag yield is approximate and varies with aggregate and water content. Density 140 lb/ft3 is the ASTM C138 unit weight of mixed bagged concrete, about 3,780 lb/yd3, and is what every calculation here uses. Ready-mix delivered by truck is denser, about 150 lb/ft3 or 4,050 lb/yd3; that figure is stored separately as readyMixDensity and appears only where a page discusses ordering ready-mix. The two are not interchangeable. The 90 lb bag is regionally stocked rather than nationally, so it is flagged and listed last. Bag sizes are per market: the 40 to 90 lb bags are what North American merchants stock, and the 20 and 30 kg bags are what the UK, Canadian and Australian ones do. Switching the calculator to metric switches the shopping list with it, because a metric answer priced in pounds of product is not an answer anyone can buy. The 150 lb/ft3 ready-mix figure was carried for a long time as a point value with nothing behind it. NRMCA CIP 36 publishes it as the TOP of a range — normalweight concrete is 140 to 150 lb/ft3, and structural lightweight is 90 to 115 — so densityRanges now holds both and the weight calculator reports a span rather than a single number it cannot justify. Where one figure has to be picked, the top of the range is the safe end: over-stating a weight risks specifying too much truck, and under-stating it risks overloading one.

    Unit conversions are exact by definition and need no source.

    How we choose and cite these figures

    What else goes in the hole

    Most posts sit on four to six inches of compacted gravel so water drains away from the end grain rather than standing against it — size that with the gravel calculator. If you are setting posts for a deck rather than a fence, the deck board calculator works out the decking those posts will end up carrying.

    By Team MaterialMath. Published . Last checked . How these figures are sourced and checked.

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