Concrete calculator: cubic yards and bags for a slab

Order too little and the pour stops half way; order too much and you pay for it. The volume is length times width times thickness. Here is how to get from there to cubic yards and bags.

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How do you calculate concrete for a slab?

Volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12. Divide by 27 to get cubic yards, the unit ready-mix is sold in.

Example. A 10 ft by 10 ft patio, 4 in thick: 10 × 10 × 4 ÷ 12 = 33.33 cu ft = 1.23 cu yd. With 10 percent for waste and an uneven base, order about 1.36 cu yd.

How many bags of concrete do I need?

A bag's yield is printed on it. Typically an 80 lb bag makes about 0.6 cu ft, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.3 cu ft. Divide the volume by the yield and round up.

Example. 33.33 cu ft ÷ 0.6 = 55.6, so 56 bags of 80 lb, before waste. With 60 lb bags it is 75.

Concrete in metres

In metric, volume in m³ = length × width × thickness, all in metres. A 3 m by 3 m slab 100 mm thick is 3 × 3 × 0.1 = 0.9 m³.

Concrete steps and footings

Split steps into simple blocks, work out each block's volume and add them. For a footing, use length × width × depth. Always check the depth the frost line and your local code ask for.

Ready-mix or bags?

For a few cubic feet, bags are easier. Past roughly one cubic yard, ready-mix is usually quicker and gives a more even pour, but delivery minimums differ by supplier, so ask.

Concrete in Clad

Clad's concrete tool gives cubic yards and the number of bags for the size you choose, with waste added, and the result goes into the job's shopping list with its cost.

See what Clad does. Clad is coming soon to the iPhone.

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How this page was made

Written with the help of an AI assistant (Claude, by Anthropic). Every worked example was computed again by a program and checked against its formula. Building codes differ by place: check yours before you build.