Concrete Calculator
Enter the length, width, and thickness of your slab or footing to find the volume of concrete you need — in cubic feet and cubic yards — plus how many pre-mixed bags that works out to.
How the Concrete formula works
Concrete volume for a rectangular slab or footing:
Volume (ft³) = Length(ft) × Width(ft) × Thickness(in) / 12 Volume (yd³) = Volume (ft³) / 27
Bag counts assume a standard yield of roughly 0.45 ft³ per 60 lb bag and 0.6 ft³ per 80 lb bag of pre-mixed concrete.
Step-by-step calculation
- Convert the slab thickness from inches to feet by dividing by 12.
- Multiply length × width × thickness (in feet) to get cubic feet.
- Divide cubic feet by 27 to convert to cubic yards, the unit ready-mix trucks price by.
- Divide cubic feet by each bag's yield to estimate the number of 60 lb or 80 lb bags needed.
Worked example
A 10 ft × 12 ft patio slab poured 4 inches thick: Volume = 10 × 12 × (4/12) = 40 ft³. In cubic yards, that's 40 / 27 ≈ 1.48 yd³. At 0.6 ft³ per 80 lb bag, that's about 67 bags — most people would instead order 1.5 yd³ of ready-mix for a pour this size.
Frequently asked questions
Should I order extra concrete beyond the calculated amount?
Yes — most contractors add 5–10% extra to account for uneven subgrade, spillage, and minor form deviations, since running short mid-pour is far costlier than a small surplus.
When should I use bags versus ready-mix delivery?
Bagged concrete is practical for small pours under roughly 1 cubic yard, such as fence posts or small pads. Larger pours are almost always cheaper and faster with a ready-mix truck delivery.
Does thickness really make that much difference?
Yes — thickness scales the volume linearly, so doubling a slab from 4 to 8 inches doubles the concrete needed. Structural slabs (driveways, garage floors) are typically thicker than decorative slabs.