Concrete Slab Calculator
Use our free Slab Calculator to accurately estimate concrete volume for your project. Simply enter your slab dimensions below, and the calculator will instantly tell you how many cubic yards of concrete you need, how many 80lb bags to buy, and an estimated material cost. Our Slab Calculator results include a standard 10% waste factor.
You might also like: Concrete Cost Calculator • Concrete Bag Calculator
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For estimation purposes only. Verify structural calculations with a licensed engineer.
How This Calculator Works
This concrete pad calculator uses the standard volume formula used by concrete contractors and structural engineers. There are no tricks or overcomplications — it's the same math you'd do by hand, just faster.
The calculator takes three measurements: length, width, and thickness (depth). It converts everything to feet, multiplies them together to get cubic feet, then divides by 27 to get cubic yards. From there it figures out bag counts based on your chosen bag size.
One thing worth mentioning: the 10% waste factor built into this tool is a real-world recommendation, not a padding trick to make you buy more concrete. Concrete delivery trucks have tolerances, forms can flex slightly under pressure, and you always want a small buffer to finish the job without a short pour.
The Formula Explained
Volume (cubic yards) = (Length × Width × Thickness in feet) ÷ 27
With waste factor:
Total Volume = Volume × 1.10
Bags needed (80lb):
Total bags = Total Volume (cubic feet) ÷ 0.60
Note: 1 cubic yard = 27 cubic feet. Each 80lb bag of Quikrete or Sakrete yields approximately 0.60 cubic feet of mixed concrete.
Step-by-Step Manual Calculation
Let's say you're pouring a 12×20 foot patio slab at 4 inches (0.33 feet) thick.
- Convert thickness to feet: 4 inches ÷ 12 = 0.33 feet
- Calculate volume in cubic feet: 12 × 20 × 0.33 = 79.2 cubic feet
- Convert to cubic yards: 79.2 ÷ 27 = 2.93 cubic yards
- Add 10% waste: 2.93 × 1.10 = 3.23 cubic yards
- Count 80lb bags if bagging it: 3.23 × 27 ÷ 0.60 = 145 bags
Common Slab Sizes (Quick Reference)
| Slab Size | Thickness | Cubic Yards | 80lb Bags |
|---|---|---|---|
| 10 × 10 ft | 4 inches | 1.35 yd³ | ~61 bags |
| 12 × 20 ft | 4 inches | 3.23 yd³ | ~145 bags |
| 20 × 20 ft | 4 inches | 5.41 yd³ | ~243 bags |
| 10 × 10 ft | 6 inches | 2.04 yd³ | ~92 bags |
| 20 × 40 ft | 4 inches | 10.81 yd³ | ~486 bags |
| 6 × 6 ft | 3.5 inches | 0.64 yd³ | ~29 bags |
All values include 10% waste factor. Bags rounded up to the nearest whole number.