How to Calculate Concrete in Cubic Yards
Use feet and inches to calculate concrete volume for a slab, footing or column, then convert cubic feet to cubic yards and add a sensible allowance.
Concrete in the United States is normally ordered by the cubic yard. The reliable way to estimate it is to calculate the actual volume in cubic feet first, then divide by 27. That separation matters: square feet describe area, while cubic yards describe volume.
The basic slab formula
For a rectangular slab:
cubic feet = length (ft) × width (ft) × thickness (ft)
cubic yards = cubic feet ÷ 27
Convert an inch thickness to feet by dividing by 12. A 4-inch slab is 0.333 feet thick; a 6-inch slab is 0.5 feet thick.
Worked example: 20 ft × 12 ft × 4 in
- Convert the thickness: 4 ÷ 12 = 0.333 ft.
- Find the volume: 20 × 12 × 0.333 = about 80 cu ft.
- Convert to cubic yards: 80 ÷ 27 = about 2.96 yd³.
- Add the project allowance separately. At 10%, the planning quantity is about 3.26 yd³.
Use the US concrete calculator to run this calculation without decimalizing the dimensions by hand.
Footings, columns and steps
A rectangular footing uses the same length × width × depth formula. For a round column, calculate the circular area first: π × radius², then multiply by the height. Measure the radius as half the diameter and keep every dimension in the same unit before multiplying.
Steps are easiest to understand as separate rectangular blocks. Calculate each block, add the volumes, and apply the allowance once to the combined volume. Do not add waste to every intermediate result and then add it again to the final order.
Why the measured volume and order quantity differ
The geometric result is not automatically the quantity to order. Excavation may be uneven, formwork may bow, subgrade elevations can vary, and some material stays in the chute, pump or tools. The allowance should reflect the actual job instead of being hidden inside the formula.
A simple, well-formed slab may need a smaller allowance than an irregular excavation. Keep these figures visible:
- measured volume;
- allowance percentage;
- volume including allowance;
- supplier order increment or minimum load.
That makes the estimate easier to check when site conditions change.
Before placing the order
Confirm the dimensions on site and ask the supplier how it accepts partial-yard quantities. Concrete volume does not select the required mix, strength, slump, air entrainment, reinforcement or curing method. Those decisions come from the project documents, local requirements and the appropriate licensed professional.
The National Institute of Standards and Technology lists 27 cubic feet in one cubic yard in its general tables of US customary units. Use that exact relationship for the volume conversion, then round only at the end of the estimate.