Concrete slab calculator

A slab is the simplest concrete job to measure and the easiest to under-order, because a quarter inch of extra depth over a big area adds up fast.

How the slab calculator works

Volume is length x width x thickness. A 20 x 20 ft slab at 4 in is 133.3 cu ft. With 10% extra it is 146.7 cu ft, or 5.43 cubic yards.

If you enter a rebar spacing, the calculator lays out a grid both ways, stopping a few inches short of each edge so the steel stays covered. For the same slab at 18 in on center that is 14 bars each way, about 546 ft of bar before laps, or 28 bars 20 ft long. Weights use ASTM A615 nominal values; a #4 bar is 0.668 lb per foot.

Thickness

Slabs for patios, walks and shed floors are commonly 4 in. Garage floors and driveways that carry cars are often thicker. If you are pouring under a building, your permit drawings set the thickness, base and reinforcement, and they win over any rule of thumb.

Before you order

• Check the depth at several points after you set the forms. A low corner can eat your margin.
• Ask the plant what strength mix it recommends for outdoor slabs where you live, especially if you get freeze and thaw cycles.
• Have the next steps lined up: finishing tools, curing compound or plastic, and a plan to keep the slab damp for the first week (see how long concrete takes to cure).

Sources

• Quikrete Concrete Mix (No. 1101) data sheet: bag yields 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb
• ASTM A615 bar weights as published by Carroll Supply (#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb per ft)
• US Navy construction training manual: conventional concrete weighs about 150 lb per cubic foot

Sources last checked 1 November 2026.