Calculate concrete for your slab

The volume formula here is straightforward — length times width times thickness — but the units need care. Slab length and width are measured in feet, while thickness is almost always specified in inches, so this calculator converts thickness to feet (dividing by 12) before multiplying. Skipping that conversion is the most common way people end up with a volume estimate that's off by a factor of 12.

Once you have cubic feet, dividing by 27 converts to cubic yards, which is how ready-mix concrete is actually ordered and priced once you get past small bagged-mix jobs. Ready-mix trucks and most supplier price sheets quote by the cubic yard, so this is the number you'd give a concrete supplier over the phone.

The 0.45 and 0.6 cubic-feet-per-bag yield figures for 60lb and 80lb bags are the standard published yields printed on typical bagged concrete mix. Notice that an 80lb bag doesn't yield proportionally more than a 60lb bag per pound of dry mix — that's because bag yield accounts for the water and other components added during mixing, not just the dry weight of the bag itself.

Bagged concrete makes sense for small pours — fence post footings, a small patio patch, or anything roughly a cubic yard or less. Beyond that, mixing by hand or with a small mixer becomes labor-prohibitive, and ordering ready-mix from a truck ends up cheaper per unit of concrete and far less physically demanding for anything larger.

This calculator assumes a uniform rectangular slab poured at a constant thickness. It doesn't add extra for sub-base prep, formwork, spillage, or an over-pour buffer — many contractors add 5-10% on top of the calculated volume specifically to avoid running short mid-pour, since a concrete truck won't wait around for a second batch once it starts setting.