Concrete
Concrete Slab Calculator
Enter the length, width and thickness of a rectangular slab to get the volume in cubic feet and cubic yards. Ready-mix suppliers sell by the cubic yard, so the calculator adds a waste factor and rounds up to the nearest quarter yard.
Formula
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12. Cubic yards = ft³ ÷ 27, because one cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³. Order quantity = yd³ × (1 + waste% ÷ 100), rounded up to the next 0.25 yd³.
Worked example
A 20 ft by 12 ft patio poured 4 in thick: 20 × 12 × 4 ÷ 12 = 80 ft³. 80 ÷ 27 = 2.96 yd³. Adding 10% waste gives 3.26 yd³, so order 3.5 yd³.
Assumptions and limits
- Slab is rectangular with a uniform thickness.
- Subgrade is flat; low spots, thickened edges and over-excavation need extra concrete.
- A 5% to 10% waste allowance is typical for spillage, form deflection and uneven subgrade.
Frequently asked questions
How thick should a concrete slab be?
Residential patios and sidewalks are commonly 4 in thick. Driveways that carry cars are often 4 to 5 in, and 6 in where heavier trucks are expected. The IRC (Section R506) sets a 3.5 in minimum for concrete floor slabs on ground.
Why round up to a quarter yard?
Most ready-mix plants batch in quarter or half yard increments, and running short mid-pour creates a cold joint. Ordering slightly over is cheaper than a second truck.
How much does a yard of concrete weigh?
Normal-weight concrete is about 145 to 150 lb/ft³, so one cubic yard weighs roughly 3,900 to 4,050 lb (about 2 tons).