Concrete Slab Calculator: Cubic Yards and Bags for Any Slab
Quick answer: a 10 ft x 10 ft slab, 4 inches thick, needs 1.23 cubic yards (33.3 cu ft) of concrete. With 10% extra, that is about 1.36 cubic yards, or 62 bags of 80 lb. For anything over about 1 cubic yard, a ready-mix truck is usually cheaper and easier than bags.
Concrete slab calculator
Concrete needed for common slab sizes (4 inches thick)
How thick should a concrete slab be?
| Use | Typical thickness |
|---|---|
| Walkway, patio, shed floor | 4 in |
| Garage floor, car driveway | 4 to 6 in |
| Hot tub pad, RV or truck parking | 5 to 6 in or more |
These are common practice, not code requirements. Soil, base gravel, reinforcement and local rules all matter, so follow your local building department for anything structural.
How the calculation works
- Volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12.
- Cubic yards = cubic feet ÷ 27.
- Bags = cubic feet ÷ bag yield, rounded up.
Common questions
How many 80 lb bags of concrete in a yard?
About 45 bags (27 cu ft ÷ 0.60 cu ft per bag).
How much concrete for a 12x12 slab?
At 4 inches thick, 1.78 cubic yards (48 cu ft). Add 5 to 10% for waste.
Bags or ready-mix?
Bags make sense for small jobs, up to roughly 1 cubic yard. Above that, mixing 45 or more bags by hand is hard work, and a truck delivery is usually cheaper per yard. Many suppliers have a minimum order or a short-load fee, so ask.
Checked by: Engr. Abdul Latif, civil and structural engineer. Volumes are exact geometry; bag yields are typical manufacturer values. Estimates for planning only.