Concrete Calculator
Enter the length, width and thickness of your slab. We tell you how much concrete to order, with a margin for waste.
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The formula
Volume (cubic feet) = length x width x thickness (in feet). Cubic yards = cubic feet / 27.
How to calculate it
Multiply length by width by thickness, with the thickness in feet (divide inches by 12). A 10 by 10 ft patio that is 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With 10 percent extra for waste, that is 1.36 cubic yards.
Bags or a truck?
An 80 lb bag makes about 0.6 cubic feet and a 60 lb bag about 0.45. With the waste allowance, that patio needs about 62 bags of 80 lb. Beyond roughly 1 cubic yard, ordering ready-mix delivered by truck is usually cheaper and easier.
Typical thicknesses
- Footpaths and patios: 4 inches
- Driveways: 5 to 6 inches
- Garage floors: 4 to 6 inches
- Footings: depends on local code, often 8 inches or more
Always add 5 to 10 percent for uneven ground and spillage, and check local building rules for structural work.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Length (ft): 10; Width (ft): 10; Thickness (inches): 4; Extra for waste (%): 10 | Concrete needed: 1.36 cubic yards |
| Length (ft): 20; Width (ft): 12; Thickness (inches): 6; Extra for waste (%): 10 | Concrete needed: 4.89 cubic yards |
| Length (ft): 6; Width (ft): 3; Thickness (inches): 4; Extra for waste (%): 5 | Concrete needed: 0.23 cubic yards |
Concrete Calculator: frequently asked questions
How many bags make a cubic yard?
About 45 bags of 80 lb (27 divided by 0.6), or 60 bags of 60 lb.
How accurate is this estimate?
It is as accurate as the numbers you give it. Real-world amounts such as paint, concrete or fuel vary, so allow a margin.
Is what I enter in this tool saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this tool free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026