How Much Concrete Do I Need?

Estimating concrete comes down to three measurements and one conversion. Measure the length, width and thickness of your pour, multiply them together to get cubic feet, then divide by 27 to get cubic yards — the unit concrete is ordered and delivered in. Get those two numbers right and add a small waste margin, and you'll order the correct amount the first time.

Step 1 — measure everything in feet

The single most common estimating mistake is mixing inches and feet. Convert thickness to feet before you multiply: a 4-inch slab is 0.333 ft thick, a 5-inch slab is 0.417 ft, and a 6-inch slab is 0.5 ft. Length and width are usually already in feet for a slab; for footings and walls, convert those too.

Step 2 — multiply to cubic feet, then divide by 27

Volume in cubic feet is simply length × width × thickness. There are 27 cubic feet in a cubic yard (a yard is a 3 × 3 × 3 ft cube), so divide by 27 for cubic yards. A 10 × 12 ft patio at 4 inches is 10 × 12 × 0.333 = 40 cubic feet, or about 1.48 cubic yards.

Step 3 — add a waste margin

Real pours are never perfect: the subgrade is uneven, some concrete sticks to the chute or wheelbarrow, and forms aren't exact. Add 5–10% to your figure to cover it, then round the order up to the nearest half yard. Running a fraction of a yard short mid-pour means a cold joint or a frantic bag run, so it's worth the small overage.

Step 4 — bags or a ready-mix truck?

Once you know the volume, the supply decision is mostly about size. Under about 1 cubic yard, bagged concrete usually wins — no delivery minimum, no truck. Above roughly 2 cubic yards, a ready-mix truck is cheaper and far less work than mixing dozens of bags by hand. Between the two it's close, and delivery and short-load fees tip the math. Compare your exact size on the bags-vs-ready-mix calculator.

To skip the arithmetic entirely, the slab calculator takes your dimensions and returns cubic yards, bag counts and truck loads at once, and the cost estimator adds gravel, rebar and pricing for a full material list. For odd shapes, see the full list of calculators — columns, footings, stairs, curbs and more each have their own.