FIELD CALCULATIONS / WORKED EXAMPLE
Concrete slab volume: feet, inches and cubic yards
By Eutopix Labs · 9 September 2026
For a rectangular slab, volume equals length × width × thickness. Put all three dimensions in the same unit before multiplying. Divide cubic feet by 27 to convert to cubic yards.
Example: 12 ft × 10 ft × 4 in
- Convert thickness: 4 in ÷ 12 = ⅓ ft.
- Calculate area: 12 × 10 = 120 square feet.
- Calculate volume: 120 × ⅓ = 40 cubic feet.
- Convert volume: 40 ÷ 27 = approximately 1.481 cubic yards.
Add an allowance separately
If you choose a 10% allowance for this example, multiply 40 by 1.10. That gives 44 cubic feet, or approximately 1.630 cubic yards. Ten percent is an illustration, not a universal ordering recommendation: uneven ground, formwork, delivery rules and your supplier affect the final quantity.
Common unit mistakes
Multiplying 12 × 10 × 4 without converting the thickness produces a value twelve times too large. Also keep square feet, cubic feet and cubic yards distinct: an area calculation cannot tell you the volume until you include thickness.
What the example excludes
This is a uniform rectangular slab. Footings, thickened edges, trenches, openings and varying depths need separate measurements. The example does not select a structural thickness, reinforcement or concrete specification.
Keep quantities and cost assumptions together
Use Onsite Math for construction calculations and material estimates, then keep the measurements, allowance and unit cost with the job. Confirm the final order with your supplier and the project specification.