Concrete Calculator for a Slab
Estimate how much concrete a rectangular slab, patio, walkway, pad, or similar project needs from length, width, and thickness.
Enter your slab dimensions
Use the ATHENA Concrete Calculator with length and width in feet and slab thickness in inches. The calculator returns cubic yards and approximate bag quantities for planning.
How to calculate concrete for a slab
Convert the slab thickness from inches to feet, multiply length × width × thickness to get cubic feet, then divide by 27 to convert cubic feet to cubic yards.
Cubic yards = length × width × thickness in feet ÷ 27
Common slab thickness conversions
4 inches is 1/3 foot, 6 inches is 1/2 foot, and 8 inches is 2/3 foot. Converting the thickness before calculating helps keep the units consistent.
Example: 20 × 20 ft slab at 4 inches
A 20 × 20 ft slab has 400 square feet. At 4 inches (1/3 foot) thick, it is about 133.3 cubic feet, or about 4.9 cubic yards before the planning allowance. With ATHENA's 10% planning allowance, the recommended planning quantity is about 5.4 cubic yards.
Should I add extra concrete?
ATHENA adds a 10% planning allowance to the recommended order. Treat this as a planning buffer rather than a universal requirement, and confirm the final quantity with your supplier or project professional.
What if my slab is not a perfect rectangle?
For an irregular slab, divide the project into smaller rectangular sections, calculate each section separately, and add the cubic-yard results. This can give you a more practical estimate than forcing an irregular shape into one set of dimensions.
Bagged concrete or ready-mix?
Smaller projects may use bagged mix, while larger pours are often ordered as ready-mix from a supplier. Compare your volume with the product yield or supplier requirements before ordering.
Ready to calculate?
Run your own measurements here: Free Concrete Calculator.
Read: How Much Concrete Do I Need? · Read: How Many Bags of Concrete Do I Need? · Read: Concrete Bag Calculator
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