To estimate concrete for a slab, multiply length × width × thickness to get volume, then divide by 27 to convert cubic feet to the cubic yards suppliers sell — and add 5 to 10 percent for waste. A 10 ft × 12 ft slab at 4 inches thick needs about 1.5 cubic yards before that cushion. Guessing wrong is expensive both ways, so this guide walks through the calculation step by step so you order the right amount the first time.
Concrete is sold by volume, not area
The single biggest mistake is thinking in square feet. Concrete is a three-dimensional material sold by the cubic yard in the US or the cubic meter elsewhere, so you need length, width, and thickness. The formula is simply volume = length × width × thickness, with every measurement in the same unit before you convert.
Thickness is where people slip because it's usually given in inches while length and width are in feet. A standard patio or shed slab is 4 inches thick, a driveway that carries vehicles is often 5 to 6 inches, and a light footpath might be 3.5 inches. Always convert thickness to the same unit as your other dimensions before multiplying — mixing feet and inches is how estimates go badly wrong.
A worked example in cubic yards
Say you're pouring a 10 ft by 12 ft slab at 4 inches thick. First convert the thickness: 4 inches is 4 / 12 = 0.333 feet. Now multiply: 10 × 12 × 0.333 = 40 cubic feet. Since concrete is ordered in cubic yards and there are 27 cubic feet in a cubic yard, divide 40 / 27 = 1.48 cubic yards.
That 1.48 is your bare volume — you would never order exactly that. Ready-mix suppliers typically sell in quarter-yard increments and expect you to round up, and you also need to account for waste. In metric the same slab is about 3.05 m × 3.66 m × 0.1 m = 1.12 cubic meters, which you would round up for the same reasons.
Always add for waste and uneven ground
Real-world pours never use exactly the calculated volume. Sub-grade rarely sits perfectly level, forms bow slightly under the weight of wet concrete, and some material is lost in the wheelbarrow and on the tools. The standard allowance is 5 to 10 percent extra, so our 1.48-yard slab becomes about 1.63 yards at 10 percent — round up and order 1.75 yards.
That cushion is cheap insurance. Running short mid-pour is far more costly than a little leftover: a second small delivery carries a short-load fee, and if the first batch has started to set you get a cold joint — a visible, structurally weak seam where old and new concrete never fully bond. When in doubt, round up rather than down.
Beyond the slab: footings, thickened edges, and other materials
Many slabs aren't a simple rectangle of uniform depth. A thickened edge or perimeter footing adds volume you must calculate separately and add to the total — a slab with a footing can need 15 to 25 percent more concrete than the flat area alone suggests. Break any complex shape into rectangles, calculate each, and sum them.
Concrete is also rarely the only material in a project. A slab usually sits on a compacted gravel base, and walls built on it may need brick or block, while interior finishing brings in drywall. Estimating each material with the same careful volume-or-area math — and the same waste allowance — keeps your whole material order accurate and your budget intact.
Run the numbers yourself
Frequently Asked Questions
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft). This is the conversion that trips most people up: calculate your slab volume in cubic feet, then divide by 27 to get the cubic yards concrete is actually ordered in.
How much extra concrete should I order for waste?
Add 5 to 10 percent to your calculated volume to cover uneven sub-grade, form movement, and spillage, then round up to the supplier's nearest increment (usually a quarter yard). Running short mid-pour is far more expensive than a small leftover.
How thick should a concrete slab be?
A patio, shed, or walkway is typically 4 inches thick, a driveway carrying cars is usually 5 to 6 inches, and a light footpath can be 3.5 inches. Thicker slabs and any that bear vehicle or structural loads generally also need reinforcing mesh or rebar.