Concrete Slab Calculator

Enter your slab size and thickness to see how many cubic yards to order or how many bags to mix.

How the calculation works

cubic feet = length × width × (thickness ÷ 12) × (1 + waste %)

Divide by 27 for cubic yards, which is how ready-mix trucks sell concrete. For bagged mix, divide by the bag's yield: 0.30 cu ft for 40 lb, 0.45 for 60 lb and 0.60 for 80 lb.

Worked example

A 10 × 10 foot patio, 4" thick, is 33.3 cubic feet. With 10% extra, that's 36.7 cubic feet, or 1.36 cubic yards. In bags, that's 62 bags of 80 lb mix.

Bags or a truck?

Mixing more than about 1 cubic yard by hand (45 bags of 80 lb) is a long, heavy day, and the first batches start setting before the last ones are poured. For larger slabs, order ready-mix. Many suppliers have a minimum order or charge a "short load" fee under 3 to 4 yards, so ask when you call.

Frequently asked questions

How many bags of concrete for a 10 × 10 slab?

At 4" thick, about 56 bags of 80 lb mix without waste, or 62 with 10% extra. At 6" thick, about 84 bags without waste.

How thick should a concrete slab be?

Four inches works for patios, walkways and shed floors. Use 5" to 6" for driveways and anywhere that carries vehicles, and set it on 4" or more of compacted gravel.