Concrete Slab Calculator
Size a slab such as a patio or shed floor. The defaults are a 4-inch thickness and 10% waste for forms and an uneven grade.
Use it for a patio or floor slab when you need yards, not a guess in bags.
Compare scenarios
How we calculated this
Volume is length times width times thickness. Thickness is converted from inches to feet. Waste is added before converting to cubic yards (divide cubic feet by 27). Bag counts round up.
What does this mean?
A slab of this size needs about 5.43 cubic yards. Four inches is a common patio thickness; driveways are often thicker. Check the local requirement.
Your assumptions
Edit assumptions- Length
- 20.0 ft
- Width
- 20.0 ft
- Thickness
- 4.00 in
- Waste
- 10%
This calculator provides an estimate for planning. Material yield, waste, prices, and local requirements may vary.
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Paste this iframe to show the calculator on another page. Taller results scroll inside the frame.
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How the estimate is calculated
The math matches the concrete calculator: area times thickness, plus waste, in cubic yards and bags.
The higher default waste is the practical difference for a formed slab.
Slab volume
Cubic yards equal length × width × thickness in feet × (1 + waste) / 27.
Example
- Measure the slab, not the lawn around it.
- Keep 4 inches unless the job calls for more.
- Use 10% waste unless the base is very even.
With the numbers filled in on this page, the estimate is 5.43 cubic yards.
What changes the result
Use of the slab
A driveway or a shop floor often needs more than 4 inches. Confirm the thickness before you order.
Base
This slab tool does not add gravel. The pad calculator does.
Reinforcement
Wire and rebar are not included in the yardage.
Questions
Why is waste 10%?
Slabs lose volume to an uneven base and to concrete left in the chute. You can lower it.
Does 4 inches include the gravel?
No. Thickness is concrete only.
Can I calculate a round slab?
Not directly. Use a square that covers the circle and expect some extra, or calculate the circle’s area elsewhere and convert depth with the cubic yard calculator’s idea of area times depth.