How to estimate concrete for a slab or footing

Concrete is sold by the cubic yard, but you measure your project in feet and inches. The whole job of estimating is turning length, width, and thickness into yards — then deciding whether to mix bags by hand or call in a truck.

Reno Project Calculators · Last updated June 3, 2026

Start with volume, not area

Flooring and paint are flat, so you measure area. Concrete is a solid, so you measure volume — length times width times thickness. The catch is that all three numbers have to be in the same unit, and thickness is almost always given in inches while length and width are in feet.

Convert the thickness to feet first by dividing by 12. A 4 in slab is 4 ÷ 12 = 0.333 ft thick; a 6 in slab is 6 ÷ 12 = 0.5 ft. Then multiply through. A 10 ft by 12 ft patio at 4 in thick is 10 × 12 × 0.333 = 40 cubic feet.

Convert cubic feet to cubic yards

Concrete is ordered in cubic yards, and a cubic yard is 3 ft on every side — that's 3 × 3 × 3 = 27 cubic feet. So once you have your volume in cubic feet, divide by 27 to get cubic yards.

That 40 cubic ft patio works out to 40 ÷ 27 = 1.48 cubic yards. This single number is what the supplier wants when you call to order, so it's worth getting right.

Footings and round shapes

A footing is just a long, narrow slab — measure its length, width, and depth the same way and multiply. A continuous footing 18 in wide (1.5 ft), 8 in deep (0.667 ft), and 30 ft long is 1.5 × 0.667 × 30 = 30 cubic feet, or about 1.1 cubic yards.

Round columns and post holes

Round forms and post holes use the cylinder volume: pi times the radius squared times the height, all in feet. The radius is half the diameter. A 12 in (1 ft) diameter footing 3 ft deep has a radius of 0.5 ft, so its volume is 3.14 × 0.5 × 0.5 × 3 = 2.36 cubic feet. For several identical post holes, find one and multiply by the count before converting to yards.

Bags or ready-mix?

The deciding factor is volume. Bagged concrete is convenient for small jobs, but it gets impractical fast. An 80 lb bag yields about 0.60 cubic feet of mixed concrete, which means a full cubic yard takes roughly 27 ÷ 0.60 ≈ 45 bags. Mixing 45 bags by hand is a brutal afternoon.

That's why about one cubic yard is the practical threshold. Below it, bags are fine — a few sacks and a wheelbarrow or a rented mixer. At or above a cubic yard, order ready-mix from a truck. It arrives mixed, fills large forms in one continuous pour, and usually works out cheaper per yard than buying bags by the pallet (though most suppliers have a minimum load and a short-load fee under a few yards).