Concrete Calculator
How much concrete does your slab need? Enter length, width, and thickness to get cubic feet and cubic yards, plus an optional waste allowance. The bag table below converts yards into 40, 60, or 80 lb bags.
Quick answer
Volume is based on slab length x width x thickness.
What this tells you
- •Volume is based on slab length x width x thickness.
- •Thickness is converted from inches to feet for volume calculations.
- •Waste percentage increases recommended order quantity.
How to Use
- 1Enter slab length in feet.
- 2Enter slab width in feet.
- 3Enter slab thickness in inches.
- 4Add optional waste percentage.
- 5Calculate to get cubic feet and cubic yards required.
How It Works
Formula
Cubic Feet = Length(ft) x Width(ft) x Thickness(ft)
Cubic Yards = Cubic Feet / 27
Total with Waste = Cubic Yards x (1 + Waste%)Concrete is typically ordered in cubic yards, so the calculator converts slab volume from cubic feet to cubic yards.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Patio slab estimate
Concrete Bags Per Cubic Yard
How many bags of ready-mix concrete equal one cubic yard (27 cubic feet), based on standard bag yields.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags |
| 50 lb | 0.375 cu ft | 72 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 80 lb | 0.60 cu ft | 45 bags |
Yields are typical for standard ready-mix. Check the bag label, as high-strength and fast-setting mixes can differ slightly.
Common mistakes
- Entering thickness in feet instead of inches
- Ignoring waste allowance for overage and spillage
- Forgetting to convert cubic feet to cubic yards when ordering
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