The basic concrete volume formula
For a rectangular slab, pad, footing or similar pour, start by multiplying length, width and thickness. All three measurements must use compatible units before they are multiplied.
Convert thickness from inches to feet
Concrete thickness is often measured in inches while length and width are measured in feet. Divide the thickness in inches by 12 before multiplying.
Convert cubic feet to cubic yards
Ready-mix concrete is commonly ordered by the cubic yard. One cubic yard contains 27 cubic feet.
Worked example
Suppose a slab is 20 feet long, 10 feet wide and 4 inches thick. First convert 4 inches to 0.333 feet. Multiplying 20 × 10 × 0.333 gives about 66.7 cubic feet. Dividing by 27 gives about 2.47 cubic yards before waste.
How much waste should you add?
Real projects rarely match perfect mathematical dimensions. Uneven excavation, form variation, spillage and finishing losses can increase material use.
- Small, well-controlled pours may need only a modest allowance.
- Irregular excavation can require more extra material.
- Ordering too little concrete can be more disruptive than having a small surplus.
- Always consider supplier minimums and delivery practices.
A calculator estimates volume. Structural thickness, reinforcement and concrete specification should come from the appropriate design requirements for the project.
Concrete bags vs ready-mix concrete
Premixed bags are convenient for smaller jobs. Larger pours are generally easier to estimate in cubic yards. Bag yield varies by product, so the printed manufacturer yield should be used when purchasing.