Concrete slab formula
A rectangular concrete slab is a three-dimensional shape. Multiply length by width by thickness after converting all measurements to compatible units.
Convert slab thickness to feet
Slab thickness is commonly specified in inches while length and width are measured in feet. Divide inches by 12 before calculating cubic feet.
Worked example: 20 × 20 foot slab
For a slab measuring 20 feet by 20 feet and 4 inches thick, convert 4 inches to approximately 0.333 feet. Multiplying 20 × 20 × 0.333 gives about 133.3 cubic feet.
How thickness changes concrete quantity
Increasing thickness increases volume directly. A 6-inch slab requires roughly 50 percent more concrete than a 4-inch slab with the same length and width.
- 4 inches = 0.333 feet
- 5 inches = 0.417 feet
- 6 inches = 0.500 feet
- 8 inches = 0.667 feet
Should you add extra concrete?
Actual forms and subgrade conditions are rarely perfectly uniform. A reasonable planning allowance can help cover uneven excavation, minor overpour and placement loss.
The calculator estimates concrete quantity only. Slab thickness, reinforcement, subbase and concrete strength should be determined from the requirements appropriate to the project.