How to Calculate Concrete Volume for a Slab

The direct answer

Concrete volume for a rectangular slab is length × width × depth, with all three measurements converted to the same unit before you multiply. The result comes out in cubic feet (if you worked in feet) or cubic metres (if you worked in metres) — a raw volume, not yet a cubic-yard order quantity.

Why depth is where estimates go wrong

Length and width are usually measured in feet or metres without much confusion. Depth is almost always given in inches or centimetres — a 4-inch patio slab, a 6-inch driveway — and it's the one number that has to be converted before it goes into the formula. Multiplying 20 ft × 10 ft × 4 (as if it were 4 ft) overstates the volume by a factor of 12.

Convert depth to feet by dividing inches by 12 (4 in ÷ 12 = 0.333 ft), or work entirely in metres and convert centimetres by dividing by 100.

From cubic feet to cubic yards

Ready-mix concrete is ordered by the cubic yard in the US, and 1 cubic yard = 27 cubic feet. Once you have a volume in cubic feet, divide by 27 to get cubic yards. In metric regions, concrete is ordered directly in cubic metres, so no second conversion is needed.

Irregular slabs

An L-shaped or stepped slab isn't a single length × width × depth calculation. Split it into rectangular sections, calculate each section's volume separately, and add them together. A slab with a thickened edge (a common footing-and-slab combination) is two volumes: the main slab plus the extra depth of the thickened strip.