Concrete volume formula for a rectangular slab
For a simple rectangular slab, footing or pad, multiply length by width by thickness. The main source of error is mixing units—for example, multiplying feet by feet by inches without first converting the thickness.
If a slab is 12 ft long, 10 ft wide and 4 in thick, convert 4 inches to 4/12 = 0.3333 ft. The volume is 12 × 10 × 0.3333 ≈ 40 cubic feet.
Convert cubic feet to cubic yards
Concrete in the United States is commonly ordered by cubic yard. One cubic yard contains 27 cubic feet.
The 40 ft³ example is about 1.48 yd³. In metric work, keep dimensions in meters to get cubic meters directly.
Metric example
A slab 4 m long, 3 m wide and 0.12 m thick has a volume of 4 × 3 × 0.12 = 1.44 m³. If the supplier sells in cubic meters, that is already in the required unit before any allowance for waste or site conditions.
Should you add a waste or overage allowance?
Real pours can require more material than the perfect geometric volume because excavation is uneven, forms move, the base is not perfectly level, dimensions vary, or a small amount remains in delivery and handling. Contractors often include an allowance, but the appropriate percentage depends on the job and supplier practices.
Instead of automatically using a fixed percentage for every project, calculate the geometric requirement first, then apply an overage that reflects the uncertainty of the specific pour. For a highly controlled form, the allowance may be smaller than for irregular excavation.
Irregular shapes
Break an irregular area into simple sections, calculate each section separately, then add the volumes. For example, an L-shaped slab can be divided into two rectangles. For cylinders, use the cylinder volume formula; for sloped or varying-depth areas, use a method that reflects the actual geometry rather than treating the entire pour as a uniform rectangle.
Why thickness matters so much
Concrete volume changes directly with thickness. Increasing a slab from 4 inches to 5 inches is a 25% increase in thickness, so the geometric concrete volume also rises by 25% when length and width stay the same.
Measure thickness in several places when estimating an existing excavation. A small thickness error across a large floor area can produce a large volume difference.
Before ordering concrete
- Confirm final form dimensions and slab thickness.
- Use one unit system consistently.
- Separate different pour areas if dimensions differ.
- Check supplier minimums, delivery increments and return rules.
- Allow for site-specific waste or variation.
- For structural work, follow the project drawings and engineering requirements.
Common estimating mistakes
- Using inches as if they were decimal feet.
- Forgetting that volume is cubic units, not square units.
- Rounding too early before converting to cubic yards or meters.
- Measuring only plan area and forgetting thickness.
- Applying one waste percentage without considering site conditions.
Frequently asked questions
How do I convert slab thickness from inches to feet?
Divide inches by 12. Four inches is 0.3333 ft; six inches is 0.5 ft.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard.
Can I use square footage alone to order concrete?
No. Square footage measures area. Concrete quantity requires volume, so thickness or depth is also required.
Enter your own values, review the result, then use the guide above to understand the formula and assumptions.