Concrete slab calculator
Concrete Slab Calculator
Estimate concrete volume and bag quantities for rectangular slabs.
Results
- Base cubic feet
- 33.33 cu ft
- Base cubic yards
- 1.23 cu yd
- Cubic yards including waste
- 1.36 cu yd
- 40 lb bags
- 123 bags
- 60 lb bags
- 82 bags
- 80 lb bags
- 62 bags
10% waste allowance
Save this calculation to a project estimate.
What this calculator does
This calculator focuses on rectangular concrete slabs. It estimates cubic feet, cubic yards, and common bag quantities from slab length, width, thickness, quantity, and waste allowance.
Use it for early material planning on simple slab shapes. For non-rectangular pours, split the area into simpler sections and compare the combined estimate with supplier requirements.
Formula used
Base cubic feet equals length times width times thickness converted from inches to feet, multiplied by quantity. Cubic yards are calculated by dividing cubic feet by 27. Waste is applied to the volume before bag counts are calculated.
Waste and bag estimates
Waste is added before final volume and bag counts are calculated. Bag estimates use approximate 40 lb, 60 lb, and 80 lb yields and round up to whole bags.
Common estimating mistakes
Avoid mixing inches and feet in the same field, forgetting slab quantity, or using planned thickness when the formed excavation is deeper. A small thickness change can noticeably change concrete volume.
Frequently asked questions
- Is this different from the concrete calculator?
- It uses the same rectangular slab volume logic, but this page is framed specifically around slab estimating and bag planning.
- Should I enter slab thickness in inches?
- Yes. Length and width are entered in feet, while thickness is entered in inches and converted inside the calculation.
- Why are bag counts rounded up?
- Concrete bags are purchased as whole bags, so fractional bag estimates are rounded upward after waste is applied.