CalcArcade.

Concrete Slab Calculator

Enter the length, width and thickness of your slab to see how many cubic yards of ready-mix to order or how many bags of concrete mix to buy.

Your numbers

Result

How we calculate it

Volume in cubic feet = length × width × thickness (in feet). Thickness is divided by 12 to convert inches to feet. Cubic yards = cubic feet ÷ 27. The waste allowance is added on top.

Bag counts use typical yields: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.30, always rounded up.

Assumptions and limits

  • A 4 inch slab is typical for patios and walkways; driveways and garage floors are often 5 to 6 inches.
  • Ready-mix suppliers often have a minimum order and charge extra for small loads, so compare bags against delivery for jobs under about 1 cubic yard.
  • Reinforcement, base gravel and forms are not included.

Frequently asked questions

How many bags of concrete do I need for a 10x10 slab?

A 10 ft by 10 ft slab 4 inches thick is about 33 cubic feet, or roughly 56 bags of 80 lb mix before any waste allowance.

How thick should a concrete slab be?

Four inches suits most residential patios and sidewalks. Driveways for vehicles are commonly 5 to 6 inches. Follow local code and your engineer or supplier for structural slabs.

Should I order extra concrete?

Yes. Ordering 5 to 10 percent extra covers spillage, uneven subgrade and slightly thicker edges. Running short mid-pour is far costlier than a little excess.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Length (feet) = 12; Width (feet) = 10; Thickness (inches) = 4; Extra for waste and uneven ground (%) = 10.

  • Concrete needed (with waste) 1.63 cubic yards
  • Cubic yards, no waste 1.48
  • Cubic feet (with waste) 44.0
  • Cubic meters (with waste) 1.25
  • 80 lb bags 74
  • 60 lb bags 98
  • 40 lb bags 147

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.