Construction & Home
Concrete Calculator
This calculator estimates how much ready-mix concrete you need for a slab, footing, or similar pour, based on its length, width, and thickness.
The formula
cubic feet = length × width × (depth in inches ÷ 12)
cubic yards = cubic feet ÷ 27
Worked example
A 10 ft × 10 ft slab poured 4 inches thick:
cubic feet = 10 × 10 × (4 ÷ 12) = 33.3 ft³
cubic yards = 33.3 ÷ 27 = 1.23 yd³
Frequently asked questions
How much concrete do I need for a slab?
Multiply the slab's length by its width by its depth (converted to the same unit) to get the volume, then convert that volume to cubic yards, since concrete is typically ordered by the cubic yard.
Why is depth measured in inches while length and width are in feet?
Slabs and footings are usually specified in inches of thickness (e.g. a 4-inch slab) but measured in feet across their length and width, so this calculator matches that common convention and converts depth to feet internally.
Should I order extra concrete beyond the calculated amount?
Many contractors add 5–10% extra to account for uneven subgrade, spillage, and form imperfections, since running short mid-pour is far more costly than having a little extra.