How to use this calculator
Enter the length, width, and uniform thickness of a rectangular slab. For premixed bags, use the mixed-concrete yield printed on the bag. Bag weight alone is not enough to determine its yield.
The formula
A worked example
A 10 × 12 ft slab at 4 inches thick is 40 cubic feet. With 10% extra it needs 44 cubic feet, or 1.63 cubic yards. At 0.6 cubic feet per bag, buy 74 bags.
Assumptions & limitations
Estimates material volume only. It does not design foundations, assess loads, specify reinforcement, or determine an appropriate slab thickness. Footings and thickened edges require separate volumes.
Common questions
Why use yield instead of bag weight?
Different products and bag sizes produce different mixed volumes. The label yield is the relevant input.
Does this account for a sloping base?
No. It assumes uniform depth. Use separate sections or a measured average depth for a volume estimate.
Units reference
The calculation follows the method above. This reference supports the units and conversions.
NIST: units and conversion factors ↗Formula and worked example checked against automated test cases. About our methodology.