How to Calculate Concrete Volume for a Slab
Concrete volume for a rectangular slab is calculated as length × width × thickness, all converted to the same unit before multiplying — since thickness is usually specified in inches while length and width are usually in feet, converting thickness to feet first (dividing inches by 12) keeps the units consistent, and the resulting cubic feet is then divided by 27 (the number of cubic feet in a cubic yard) to get the volume in cubic yards, the unit ready-mix concrete is typically ordered in.
This same basic formula scales up directly from a small patio slab to a full driveway or foundation footing.
Working through a concrete example
A 10-foot by 10-foot slab poured at a 4-inch thickness: convert 4 inches to 4/12, or about 0.33 feet, then multiply 10 × 10 × 0.33, giving roughly 33 cubic feet — dividing by 27 gives about 1.24 cubic yards of concrete needed for that slab.
Why thickness is the detail most often miscalculated
Length and width are usually measured directly and accurately, but thickness is often estimated or assumed rather than precisely verified — since thickness is a direct multiplier in the volume formula, even a small error there (an extra half-inch across a large slab) meaningfully changes the total volume needed.