BuildProjectCalc
Concrete & Masonry guide

How to Calculate Concrete for a Slab

Measure slab length, width, and thickness, then convert the resulting volume to cubic yards with a clearly stated waste allowance.

The short answer

Treat a rectangular slab as a shallow box: multiply its finished length, width, and thickness. Convert thickness to feet or meters first, then convert the total volume to cubic yards or cubic meters and add the selected waste allowance.

Use the Concrete Slab Calculator

Measure and calculate the slab

Use finished slab dimensions rather than mixing excavation measurements with form dimensions.

  1. Measure the formed length

    Measure the inside length that the concrete will occupy.

  2. Measure the formed width

    Take the width independently and split tapered or irregular slabs into simpler sections.

  3. Record the planned thickness

    Keep thickness as a separate measurement; in US customary units, convert inches to feet by dividing by 12.

  4. Multiply for cubic volume

    Multiply the three converted dimensions, then add the volumes of any additional slab sections.

  5. Convert and apply waste

    Divide cubic feet by 27 for cubic yards, then multiply by 1 plus the chosen waste rate.

Rectangular slab formula

Thickness in feet = thickness in inches ÷ 12
Cubic feet = length (ft) × width (ft) × thickness (ft)
Cubic yards = cubic feet ÷ 27

For metric dimensions, convert centimeters to meters by dividing by 100, then multiply length × width × thickness to obtain cubic meters.

Worked example: 16-by-12-foot slab

A slab is 16 feet long, 12 feet wide, and 5 inches thick. The planning allowance is 8%.

  1. Thickness: 5 in ÷ 12 = 0.4167 ft
  2. Base volume: 16 × 12 × 0.4167 = 80 ft³
  3. Base cubic yards: 80 ÷ 27 = 2.963 yd³
  4. With 8% allowance: 2.963 × 1.08 = 3.20 yd³

Result: The geometry produces 80 cubic feet, or about 2.96 cubic yards before waste and 3.20 cubic yards with the stated 8% allowance.

Slab-specific checks before using the result

Thickness changes
A thickened edge or separate grade beam is additional geometry and should not be hidden inside the nominal slab thickness.
Irregular footprints
Divide L-shaped or stepped slabs into non-overlapping rectangles and total their volumes.
Units on field notes
Label feet, inches, meters, and centimeters explicitly; an unlabeled thickness is a common source of large errors.
Waste is an assumption
The example percentage is not a universal requirement. Choose it for the measured project conditions.

What this calculation does not decide

Volume arithmetic does not establish slab thickness, reinforcement, base preparation, drainage, joint layout, or load capacity. Those decisions are outside a material-volume estimate.

If the slab includes curves, slopes, voids, or thickened sections, calculate those shapes separately or use project documents that provide a verified volume.