How to use this calculator
- Measure. Get the length, width, and thickness of your slab.
- Enter values. Type the numbers and pick the unit (most users use feet + inches).
- Add waste. Set a 5–10% overage for any irregular sub-grade.
- Read result. Cubic yards is what you order from a ready-mix supplier. Bags are for hand-mixing.
Formula
Volume (yd³) = Length(ft) × Width(ft) × Thickness(ft) ÷ 27
Worked example
A 12 ft × 20 ft garage floor at 4 inches thick needs 2.96 cubic yards. With 10% overage you would order 3.25 yd³ — easily one truck.
Tips for accurate results
- Garage and shed slabs are usually 4 inches thick with #3 rebar at 18" o.c.
- A 4-inch slab equals exactly 0.0123 yd³ per square foot — useful for quick mental math.
- Use a 6-mil vapor barrier under heated floors and basement slabs.