How the concrete calculator works
When someone asks how much concrete a slab, footing, pad, or walkway needs, the first step is finding its volume. This calculator converts your length, width, and depth into cubic feet and then into cubic yards, the unit commonly used when ordering ready-mix concrete.
Concrete calculator formula
Volume
cubic feet = length × width × depth in feet
cubic yards = cubic feet ÷ 27
order amount = cubic yards × (1 + waste ÷ 100)
Example
A 20 ft × 10 ft slab that is 4 inches deep is 3.33 cubic yards before waste.
With a 10% allowance, the estimate is about 3.67 cubic yards.
Why add a waste allowance?
Real jobs are rarely perfectly uniform. The ground may vary, forms may not be exact, and a little concrete can be left in the truck or lost during placement. The allowance is an estimate, not a guarantee, so use your supplier's advice when placing an order.
Frequently asked questions
How many cubic yards of concrete do I need?
Enter the slab or form dimensions and the calculator will convert the volume to cubic yards. Add a sensible waste allowance before ordering.
Why is concrete measured in cubic yards?
Ready-mix suppliers commonly sell concrete by the cubic yard, so the calculator reports the final estimate in that unit.
How much waste should I add?
A common planning allowance is around 5% to 10%, but the right amount depends on the site and shape of the pour.
Can I use this for a footing or pad?
Yes. Use the actual length, width, and average depth of the section you are estimating.
Is this a cubic yards of concrete calculator, or does it work in bags?
If you typed concrete calculator yards into a search engine, this page is built for exactly that: it reports in cubic yards, since that is the unit ready mix suppliers usually sell by. If you are looking for a cubic yards of concrete calculator to double check an order, this page does exactly that. For a bag count instead, use the Quikrete Bags Calculator linked above.