How to use this calculator
- Enter the slab dimensions. Input the length, width, and thickness of your pour. Most residential slabs are 4 inches thick.
- Pick your units. Toggle between feet, inches, yards, meters, or centimeters — the calculator converts everything internally.
- Add a waste allowance. Most pros order 5–10% extra to account for spills, uneven sub-grade, and leftover in the chute.
- Read your result. You get cubic yards (what ready-mix trucks deliver), cubic feet, cubic meters, and the equivalent number of 60 lb and 80 lb bags.
Formula
Volume (yd³) = Length × Width × Thickness ÷ 27
Worked example
A 10 ft × 12 ft × 4 in slab equals 0.4 cubic yards (about 11 cubic feet). For one truck delivery you would order 0.5 yd³ minimum; if bagging you would need roughly 25 bags of 60 lb pre-mix or 19 bags of 80 lb pre-mix.
Tips for accurate results
- Order 5–10% extra. A short load is far more expensive than a few wasted dollars of concrete.
- Below-grade and structural slabs typically use a higher PSI mix (3,500+) than a sidewalk (3,000 PSI).
- For very small jobs, bagged pre-mix is cheaper than a short-load fee — break-even is usually around 0.5 yd³.
- Don't pour against frozen ground; cure above 50°F for at least 24 hours.