Pouring a slab and not sure how much concrete to buy? The math is simpler than it looks. Here's the exact method the pros use, a worked example, and a free tool to do it for you.

The formula in one line

Concrete volume is just length × width × thickness. The only trick is units: length and width are in feet, but thickness is almost always in inches. Convert the thickness to feet by dividing by 12, multiply everything together for cubic feet, then divide by 27 to get the cubic yards a ready-mix supplier quotes.

In short: cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27.

Worked example: a 10 × 20 ft slab, 4 inches thick

Say you're pouring a 10 by 20 foot patio at the standard 4-inch thickness:

Area: 10 × 20 = 200 square feet
Thickness in feet: 4 ÷ 12 = 0.333 ft
Volume: 200 × 0.333 = 66.7 cubic feet
Cubic yards: 66.7 ÷ 27 = 2.47 cubic yards

Add a 10% waste allowance and you'd order about 2.7 cubic yards of ready-mix.

Skip the math

Concrete Calculator

Enter your slab size and get the cubic yards and exact number of 60 lb and 80 lb bags instantly.

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How many bags is that?

For smaller jobs you'll buy bags of pre-mix instead of ordering a truck. Each bag lists its yield: an 80 lb bag makes about 0.60 cubic feet, and a 60 lb bag about 0.45 cubic feet. Divide your cubic feet by that number:

66.7 ÷ 0.60 = 112 bags of 80 lb
66.7 ÷ 0.45 = 149 bags of 60 lb

That's a lot of bags — which is exactly why anything over about one cubic yard is usually cheaper and far easier to order as ready-mix delivered by truck.

Why you should always add waste

Real ground is never perfectly level and forms are never perfectly tight. If you order the exact volume and come up short mid-pour, you get a cold joint — a weak seam where fresh concrete meets partly-set concrete. Adding 5–10% is cheap insurance; lean toward 10% on rough or over-excavated ground.

What thickness should the slab be?

Patios and walkways are typically 4 inches. Driveways and anything carrying vehicles are usually 5–6 inches. Always follow your local building code and any engineer's specification — this guide covers the concrete volume only, not the gravel base, rebar or forms you'll also need.