Concrete Volume Calculator

How to estimate concrete for a slab

Concrete is sold and priced by the cubic yard, so the whole point of a concrete calculator is turning your slab dimensions into a yardage number you can hand to a supplier. The math is simple, but the details around thickness and waste are where most first time pours go wrong.

The formula

Volume equals length times width times thickness. The trick is that thickness is usually specified in inches, while length and width are in feet. Convert thickness to feet by dividing by 12. So a 12 by 10 patio at 4 inches thick is 12 x 10 x (4/12) = 40 cubic feet. Divide by 27 to convert to cubic yards, since one cubic yard is 27 cubic feet. That patio comes out to about 1.5 cubic yards.

Why you order 10 percent extra

Never order exactly what you calculate. The subgrade is never perfectly flat, some concrete sticks in the chute, forms bulge slightly, and you round up bag counts. Ordering 10 percent extra is standard practice, and coming up short on a pour is far more painful than having a little left over. This calculator shows a recommended volume that already includes the 10 percent buffer.

Bags versus ready-mix

Bagged concrete mix is convenient for small jobs like setting a mailbox post, patching a step, or pouring a single fence footing. Once you approach one cubic yard, the number of bags gets unwieldy fast: roughly 45 eighty-pound bags per cubic yard. At that point a ready-mix truck saves time, effort, and usually money per yard.

Picking the right thickness

Thickness depends on load. Walkways and patios are typically 4 inches. Driveways for cars are usually 4 to 6 inches over a compacted base. Garages, workshops, and anything holding heavier vehicles or equipment are commonly 6 inches or more. Reinforcement (rebar or mesh) is a separate decision, but thicker slabs are stronger and last longer against freeze thaw cycles.

Handling non-rectangular pours

Real slabs often have odd shapes. The trick is to break the pour into rectangles, calculate each one, and add them together. For a round pad, area equals pi times the radius squared, then multiply by thickness in feet. When in doubt, round up rather than down.

These are planning estimates. Your concrete supplier and contractor will confirm the final order based on site conditions.