How to Calculate Concrete: Formula and Worked Examples
Concrete is volume: length times width times thickness, all in the same unit, then divided by 27 ft³ to get cubic yards. A slab 10 ft by 10 ft and 4 in thick is 33.33 ft³, which is 1.23 yd³.
Estimates for planning, not professional advice. Read the full notice.
Work it out for your own sizes
Check your own slab in the concrete calculatorExample to try: Length 10 ft, Width 10 ft, Thickness 4 in
Step by step
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Step 1: Measure length, width and thickness
Measure the length and the width of the area to be poured, and decide the thickness. Thickness is usually in inches while length and width are in feet, so write all three in one unit before multiplying. The example is 10 ft by 10 ft and 4 in thick, and the thickness is 0.333 ft in feet.
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Step 2: Multiply for the volume
Length times width times thickness gives the volume in cubic feet. For the example the volume is 33.33 ft³.
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Step 3: Divide to get cubic yards
Concrete is ordered by the cubic yard, and one cubic yard is 27 ft³. Divide the cubic feet by that number: the example is 1.23 yd³.
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Step 4: Add the waste allowance
The ready-mix industry body says the quantity ordered should be 4%[1] to 10%[1] more than an estimate worked from the plan dimensions, to cover contingencies. Taking the upper figure, the example comes to 1.36 yd³.
Quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies.
The same four steps on three jobs
Volume in cubic feet, in cubic yards, and in cubic yards with a waste allowance added, for three slabs
| Slab (length × width × thickness) | Volume (cubic feet) | Volume (cubic yards) | With the waste allowance (cubic yards) |
|---|---|---|---|
| 10 ft × 10 ft × 4 in | 33.33 ft³ | 1.23 yd³ | 1.36 yd³ |
| 12 ft × 24 ft × 5 in | 120 ft³ | 4.44 yd³ | 4.89 yd³ |
| 24 ft × 24 ft × 6 in | 288 ft³ | 10.67 yd³ | 11.73 yd³ |
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
Each figure is worked from the sizes in the first column; the waste allowance is added once, on the total.
Checking the answer against a calculator
The yards calculator follows the same working and shows it. For the same job counted in bags, see how many bags make a cubic yard. One cubic yard is 1 yd³ of concrete.
Questions
What is the formula for concrete?
Volume is length times width times thickness in one unit. Divide cubic feet by the number of cubic feet in a cubic yard to get yards, then add the waste allowance.
How do I calculate concrete for a slab?
Follow the four steps on this page: measure, multiply, divide to cubic yards, add waste. The table repeats them for three sizes.
How many cubic feet are in a cubic yard?
The number is given in the third step of the guide, worked out from the units themselves rather than typed.
Calculate it for your project
Enter your own sizes to get the amount to buy or the layout figures, with the working shown and a page to print.
Compare your hand working with the calculatorSources (1)
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CIP 31, Ordering Ready Mixed Concrete (opens in new tab), National Ready Mixed Concrete Association, 2000, 2016, 2023.
Standards or industry body.
“Quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies.”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.