How to Calculate Concrete Yards (Step by Step)
Updated 2026-07-01
Ready-mix concrete is ordered by the cubic yard, so every concrete job starts by turning your slab's dimensions into yards. It's one formula. Here's how it works, why the number 27 shows up, and a couple of worked examples.
Open the concrete calculator →How to calculate cubic yards of concrete
- Measure in feet: Measure length and width in feet, and thickness in inches.
- Convert thickness to feet: Divide the thickness in inches by 12 to get feet.
- Multiply for cubic feet: Multiply length × width × thickness (all in feet) to get cubic feet.
- Divide by 27: Divide cubic feet by 27 to get cubic yards, because a cubic yard is 3×3×3 = 27 cubic feet.
- Add waste: Add about 10% and round up — you can't pause a pour to get more.
The formula
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27.
The thickness is usually given in inches, so convert it first: inches ÷ 12 = feet. A 4-inch slab is 4 ÷ 12 = 0.333 ft thick.
Why divide by 27?
A cubic yard is a cube 1 yard (3 feet) on each side: 3 × 3 × 3 = 27 cubic feet. Your measurements give cubic feet, so dividing by 27 converts them to the cubic yards suppliers price by.
Worked example: a slab
A 12 ft × 12 ft slab, 4 inches thick.
Thickness: 4 ÷ 12 = 0.333 ft. Volume: 12 × 12 × 0.333 = 48 cubic feet. Yards: 48 ÷ 27 = 1.78 cubic yards, or about 1.96 with 10% waste. Order 2 yards.
Standard thicknesses
Get thickness right — it drives the whole number:
- Patios, walkways, shed floors: 4 inches.
- Driveways and load-bearing slabs: 5–6 inches.
- Footings: follow your local code or an engineer's spec.
Always round up
Concrete is delivered in one continuous pour — you can't stop halfway to order more. Add ~10% for spillage and uneven subgrade, then round up to the next quarter-yard. The calculator includes the waste factor for you.
Frequently asked questions
How many cubic feet in a cubic yard of concrete?
27 cubic feet, because a cubic yard is 3 feet × 3 feet × 3 feet.
How much concrete for a 10x10 slab at 4 inches?
About 1.23 cubic yards before waste, or roughly 1.36 cubic yards with 10% added. Round up to 1.5 yards when ordering.
Why do I add 10% waste?
Real subgrades aren't perfectly flat, some concrete spills, and you can't top up a continuous pour. A 10% cushion keeps you from coming up short mid-pour.