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.

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.

Common questions

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. Subtract it from the depth you enter, or you buy concrete for space the gravel fills.

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
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.

Unit conversions are exact by definition and need no source.

How we choose and cite these figures

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