Concrete Calculator
Use your own numbers after reviewing the formula and worked example.
Quick answer
A slab calculation multiplies length, width and thickness after converting all dimensions to compatible units.
Formula or method
Keep units, percentages and time periods consistent. The formula is useful as a transparent check on the calculator result and helps explain why changing one input changes the answer.
Step-by-step
- Use 10 ft × 20 ft for area.
- Convert 4 inches to 4/12 ft.
- Multiply to get about 66.67 cubic feet.
- Divide by 27 to get about 2.47 cubic yards.
Worked example
A 10×20 slab at 4 inches thick needs about 2.47 cubic yards before waste. A 10% allowance gives about 2.72 cubic yards.
How to interpret the result
Actual ordering may need an allowance for uneven subgrade, spillage and thickness variation.
When the answer is used for a purchase, loan, payroll decision, construction order, school grade, health estimate or other real-world choice, verify the assumptions that matter in that context. Accurate arithmetic still depends on accurate inputs.
Common mistakes
- Using 4 feet instead of 4 inches.
- Forgetting the cubic-feet-to-yards conversion.
- Ordering exactly the theoretical amount with no practical allowance.
Frequently asked questions
How many cubic feet are in a cubic yard?
27.
Why add waste?
Real pours can use more than exact geometric volume.
Does this determine required slab thickness?
No; it only calculates volume from the thickness entered.