How Much Concrete Do I Need?
Concrete quantity is based on volume. For a rectangular slab, multiply length by width by thickness, then convert cubic feet to cubic yards by dividing by 27.
Last updated October 6, 2026 · Written for practical home-project planning.
1. Measure the slab
Measure length and width in feet. Confirm the actual formed dimensions rather than relying on a nominal drawing size.
2. Convert thickness to feet
A 4-inch slab is 4 ÷ 12 = 0.333 feet thick. Always convert inches to feet before multiplying.
3. Calculate cubic feet
Multiply length × width × thickness. A 20 × 20 ft slab at 4 inches is about 266.7 cubic feet.
4. Convert to cubic yards
Divide 266.7 cubic feet by 27 to get about 9.88 cubic yards. Add a project-appropriate allowance and confirm the order quantity with the ready-mix supplier.
Different shapes
For circular pads use π × radius² × thickness. For stairs or complex forms, break the pour into simpler shapes and add the volumes.
Ready-mix vs. bagged concrete
Ready-mix suppliers sell by cubic yard and may have minimum loads. Bagged concrete uses a manufacturer-specific yield. Always use the actual product yield when buying bags.
Structural work
A quantity calculator does not determine slab thickness, reinforcement, subgrade requirements, strength, joints, or engineering. Follow the project design and local requirements for structural work.