Calculators/Concrete volume

Concrete volume calculator

Slabs, continuous footings and round piers — in cubic feet and yards, with a waste allowance and bag counts for the pours small enough to mix.

The pour

5–10% is usual, more over a rough subgrade. Running short mid-pour leaves a cold joint, which is permanent; a little left over is not.

What to order

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Check the result against the job. This does the arithmetic from the values you enter. Building codes, span tables and material specifications vary by jurisdiction and by product, and the governing document is your approved drawings and your local code. Nothing you type here leaves your browser: the calculation runs on your device and we never receive it.

How this is checked. Every formula on this site runs from one shared calculation engine, and every one has a test fixture whose expected answer was worked out from the geometry or taken from a figure the trade already publishes — never from what the code returns. Some land exactly, which is the point: a 6/12 hip over a 144″ run is exactly 216″, a 45° crown in a square corner bevels at exactly 30°, and a 30° conduit bend has a multiplier of exactly 2. Where a number is a convention rather than arithmetic — a code limit, a waste allowance, bundles per square — it is an input with a stated default rather than a hidden constant. Found something wrong? Tell us at [email protected] and we will fix it and say so.

Everything divides by 27

Concrete is ordered by the cubic yard, and a cubic yard is 3 × 3 × 3 feet. So the whole job is: get the volume into cubic feet, then divide by 27.

  • Slab — length ft × width ft × (thickness in ÷ 12).
  • Continuous footing — length ft × (width in ÷ 12) × (depth in ÷ 12).
  • Round pier — π × radius ft² × height ft, times the number of piers.

A 20 × 24 slab at 4 inches is 160 cubic feet, which is 5.93 yards — call it 6.5 with waste. Six 12-inch piers 4 feet tall are 18.85 cubic feet, or about 0.7 of a yard, which is squarely bagged-mix territory.

Where the extra goes

The waste allowance is not padding. It covers overexcavation, an uneven subgrade, spillage, and the concrete that stays in the chute and the barrow. On a rough base a nominal 4-inch slab can easily average 4½.

The asymmetry is what matters: too much concrete costs you the overage, while too little stops the pour and leaves a cold joint through the slab. One is an expense, the other is a defect that does not come out.

Bags or ready-mix

Bag counts here use the yields printed on the bag — about 0.6 cubic feet from an 80 pound bag and 0.45 from a 60. A full yard is roughly 45 of the 80s, which is a day of mixing and a strong argument for ready-mix on anything approaching that size.

Common questions

How do you calculate concrete in cubic yards?

Work out the volume in cubic feet, then divide by 27, because a cubic yard is 3 by 3 by 3 feet. For a slab that is length in feet times width in feet times thickness in feet — so a 20 by 24 slab at 4 inches is 20 × 24 × 0.333, which is 160 cubic feet, or 5.93 cubic yards.

How much waste should you add to a concrete order?

Between 5 and 10 percent is usual, and the rougher the subgrade the more you want. Overexcavation, spillage and an uneven base all consume concrete that never appears in the drawing. Running short mid-pour is far more expensive than a little left over, because a cold joint is a permanent defect.

How many bags of concrete are in a cubic yard?

About 45 of the 80 pound bags, or 60 of the 60 pound bags, at the yields printed on the bag. That is a lot of mixing, which is why bagged mix makes sense for piers and small footings and ready-mix makes sense for anything approaching a yard.

How do you calculate concrete for a round pier?

Area of the circle times the height. Take the radius in feet — half the diameter in inches, divided by 12 — square it, multiply by pi, then multiply by the height in feet and the number of piers. Six 12 inch piers 4 feet tall come to about 18.85 cubic feet, or 0.7 of a yard.

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