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.
Worked slab example
Consider a slab that is 20 feet long, 12 feet wide and 4 inches thick. First convert 4 inches to feet by dividing by 12, giving about 0.333 foot. Multiplying 20 × 12 × 0.333 gives roughly 80 cubic feet. Dividing by 27 converts the result to about 2.96 cubic yards before any waste or ordering allowance is added.
Why slab thickness has a large effect
Concrete volume changes directly with slab thickness. A small increase in thickness across a large area can add a significant amount of concrete. For this reason, use the intended finished thickness rather than guessing from an edge, excavation depth or form height. Structural thickness should be determined from the requirements of the project rather than from the volume calculator.
Allow for site and placement conditions
A calculated geometric volume assumes the excavation and forms match the entered dimensions exactly. Real projects may contain uneven subgrade, small over-excavations, variations in formwork and material left in equipment. A reasonable ordering allowance can help account for these differences, but the appropriate amount depends on the project and supplier arrangements.
Common slab-estimating mistakes
- Multiplying feet by inches without first converting the thickness.
- Using excavation depth instead of the intended concrete thickness.
- Forgetting thickened edges, beams, footings or separate sections.
- Rounding the cubic-yard result too early.
- Assuming the calculated volume automatically includes reinforcement, base material or other slab components.