Household Figures

Concrete Calculator

Enter the size and thickness of your slab to see how much concrete to order.

4 inches is common for patios and walks.
5% to 10% is typical.
About 0.6 for an 80 lb bag. Check the bag.
Order: — cubic yards
Volume needed
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Volume needed, in cubic yards
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Volume with waste
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Bags needed
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Where to find your numbers

Not sure what to enter? Here is where to find each value for your own home. If you cannot find one, leave the example value and treat the result as a rough estimate.

Length and width

The size of the area you will pour.

  1. Measure the outside of the form or the marked-out area, in feet, with a tape measure.
  2. For an L shape or curves, split it into rectangles and add the cubic yards, or use the biggest rectangle that fits for a rough estimate.
  3. If you are pouring footings or posts, those are separate calculations.

Thickness (inches)

How deep the concrete will be.

  1. A thickness of 4 inches is common for patios and walkways.
  2. Driveways and garage floors are often thicker, and heavy loads or weak soil can call for more. Ask a contractor or check your local building rules for anything structural.
  3. If the ground is already dug, measure the depth of the hole or form at a few points. Uneven ground means you may need more than the calculation says, so use the waste field for that.

Extra for waste (%)

Extra to cover uneven ground, spills and a little left over.

  1. Add about 5% to 10%. Running short during a pour is a bigger problem than having a little left over.
  2. If you are ordering a truck, ask the supplier how much extra they suggest.

Bag yield (cubic feet per bag)

How much finished concrete one bag makes.

  1. The yield is printed on the bag, for example "yields approximately 0.6 cu ft".
  2. An 80 pound bag commonly yields about 0.6 cubic feet and a 60 pound bag about 0.45. Check the brand you buy, because it varies.
  3. If you plan to have it delivered by truck, ignore the bag count and use the cubic yards.

How it works

Volume in cubic feet is length x width x thickness in inches / 12. Cubic yards is cubic feet divided by 27. Waste is added as a percentage. Bags are the volume with waste divided by the yield of one bag, rounded up. For garden beds and gravel, use themulch, gravel and topsoil calculator.

Frequently asked questions

How much concrete do I need for a slab?

Multiply length by width by thickness (in feet) to get cubic feet, and divide by 27 for cubic yards. A 10 by 10 foot slab that is 4 inches thick is about 33 cubic feet, or 1.23 cubic yards. Add 5% to 10% extra for uneven ground and spills.

How thick should a concrete slab be?

Patios, walkways and similar light-duty slabs are commonly 4 inches thick. Driveways and garage floors are often thicker, and heavy loads or poor soil can call for more. Local building codes and a contractor or engineer should set the thickness for anything structural.

Bags or a truck?

Bags are practical for small jobs. Mixing many 80 pound bags by hand is hard work, so larger jobs, often a cubic yard or more, are commonly ordered as ready-mix delivered by truck. Small truck loads can carry extra fees, so ask the supplier.

How much does one bag make?

It is printed on the bag. An 80 pound bag of concrete mix commonly yields about 0.6 cubic feet, and a 60 pound bag about 0.45. The calculator uses the yield you enter.

Related tools

How the numbers are produced: see our methodology.