Concrete calculator: cubic yards and bags

Work out how much concrete to order for slabs, footings, round columns and steps. Add as many sections as the job has and the calculator totals them in cubic yards for the ready-mix truck and in bags for small pours, with waste included.

The formula

Concrete is ordered by volume. Multiply length by width by thickness with everything in feet to get cubic feet, then divide by 27 for cubic yards. Thickness is the number people get wrong, because it is measured in inches: 4 inches is 0.333 feet, not 4.

A 20 × 12 ft slab at 4 in: 20 × 12 × 0.333 = 80 cubic feet, which is 2.96 cubic yards. With 10 percent overage, order 3.25 yards.

Round columns: 3.1416 × radius² × height, with the radius in feet. A 12 in tube 4 ft deep holds 3.14 cubic feet.

Steps: each step is a slab as wide as the stair and as deep as the run, stacked one riser higher than the one below. The calculator adds them up as solid concrete, which is how most small stairs are poured.

Slab thickness by use

Bags or a truck?

Past about one cubic yard, bags stop making sense: that is 45 bags at 80 lb, nearly two tons to move and mix. Ready-mix plants usually charge a short-load fee under 3 to 4 yards, so price both ways for pours between one and three yards.

Why add 10 percent. Subgrade is never perfectly flat, forms bow, and some concrete stays in the chute and the wheelbarrow. Running short in the middle of a pour leaves a cold joint. Having a wheelbarrow left over costs a few dollars.