Concrete planning tool
Concrete Slab Calculator
Estimate the concrete volume for a rectangular slab. Enter the finished dimensions and an optional waste allowance to see the total update instantly.
Slab dimensions
Enter your measurements
Waste allowance is added to the slab volume and shown separately in the estimate.
Volume estimate
Concrete required
Total concrete required
2.96 yd³80 ft³ total- Base concrete volume
- 80 ft³2.96 yd³
- Waste allowance
- 0 ft³0 yd³
- Total volume
- 80 ft³2.96 yd³
Live diagram
Slab proportions
Diagram shows relative proportions, not scale.
How it works
From slab dimensions to concrete volume
The calculator uses the slab length, width, and thickness to determine concrete volume, converts the result to cubic yards, and applies your selected waste allowance.
Concrete slab formula
For a rectangular slab, the volume formula is:
Volume = Length × Width × Thickness
In US Customary mode, this calculator accepts length and width in feet and inches, and thickness in inches. It converts the thickness to feet before calculating volume in cubic feet. To convert that result to cubic yards, divide by 27 because 1 cubic yard equals 27 cubic feet. Metric inputs are normalized to the same physical dimensions before calculation.
Worked example: 20 ft × 12 ft × 4 in
This example uses a 20 ft by 12 ft slab with a thickness of 4 in.
- Convert thickness to feet4 in ÷ 12 ≈ 0.3333 ft
- Calculate cubic feet20 × 12 × 0.3333 ≈ 80 ft³
- Convert to cubic yards80 ÷ 27 ≈ 2.96 yd³
With a 10% waste allowance:
80 × 10% = 8 ft³ additional
80 + 8 = 88 ft³ total
88 ÷ 27 ≈ 3.26 yd³
Understanding waste allowance
The Waste field adds the percentage you enter to the calculated slab volume. An allowance can help account for dimensional variation, uneven excavation or base conditions, placement losses, and other job-specific uncertainty. The appropriate percentage depends on the project, so you control the amount used in the estimate.
Cubic feet vs. cubic yards
One cubic yard equals 27 cubic feet. The calculator shows both units so you can see the slab's volume in cubic feet and the cubic-yard quantity commonly used when estimating concrete volume.
Estimate limitations
This calculator estimates the mathematical volume of a rectangular slab using the dimensions you enter. It does not inspect site conditions or determine:
- structural design or required slab thickness
- reinforcement requirements
- local building-code requirements
- the amount a concrete supplier should deliver
Actual material requirements can vary with job and site conditions.
Frequently asked questions
- How do I calculate concrete for a slab?
- Multiply the slab's length by its width and thickness, using compatible units. This calculator normalizes either US Customary or Metric inputs, calculates the slab volume, and presents the result in the selected measurement system.
- How many cubic feet are in a cubic yard?
- There are 27 cubic feet in 1 cubic yard.
- Should I add waste to my concrete estimate?
- An allowance may help account for real-world variation and placement losses, but the appropriate percentage depends on your project. Enter the percentage you want the calculator to add.
- Does slab thickness change how much concrete I need?
- Yes. With length and width unchanged, a thicker slab has more volume and requires more concrete.
- Can I use this calculator for an irregular-shaped slab?
- No. This calculator is specifically for rectangular slabs and does not accurately model arbitrary shapes.
- Does this calculator tell me how many bags of concrete I need?
- No. This page calculates concrete volume, not bag quantity. Bag requirements depend on the yield of the specific product per bag.