How to Calculate Concrete Yards — Formula & Examples
Ready-mix is sold by the cubic yard, bags are sold by the pound, and job sites think in feet and inches. This guide is the conversion chain, with worked numbers you can check your own takeoff against.
Quick answer: Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27. A 10×12 slab at 4 inches: 10 × 12 × 4 ÷ 12 ÷ 27 = 1.48 yd³ — order 1.6 with a 10% waste factor. 45 eighty-lb bags make one yard before waste.
The formula
Everything reduces to cubic feet first, then ÷ 27 for yards:
- Slab: L ft × W ft × (thickness in ÷ 12) = ft³ → ÷ 27 = yd³
- Strip footing: L ft × (W in ÷ 12) × (D in ÷ 12) = ft³ → ÷ 27 = yd³
- Round pier: π × (diameter in ÷ 24)² × (depth in ÷ 12) = ft³ → ÷ 27 = yd³
Worked example — patio slab
12 × 16 patio at 4 inches: 12 × 16 × 0.333 = 64 ft³ → 64 ÷ 27 = 2.37 yd³. Add 10% waste: 2.6 yd³. Most plants sell in quarter- or half-yard steps, so you'd order 2.5-2.75 yards and confirm the truck's minimum load.
Worked example — deck piers
Six piers, 12 inches diameter, 48 inches deep: one pier = π × (0.5)² × 4 = 3.14 ft³ → 0.116 yd³ each; six = 0.70 yd³. That's 35 eighty-pound bags with waste — bag territory, no truck. This is exactly the crossover decision the bag calculator is built for.
The bag conversion table
- 80-lb bag: 0.60 ft³ yield → 45 bags per cubic yard (50 with 10% waste)
- 60-lb bag: 0.45 ft³ yield → 60 bags per cubic yard (66 with waste)
- 40-lb bag: 0.30 ft³ yield → 90 bags per cubic yard (99 with waste)
- Crossover: past roughly 1 yd³, ready-mix beats bags on price and hours
Waste factor honesty
10% covers uneven subgrade, spillage, and stiffened leftovers on formed slab work; 15% for irregular or broken forms. Ordering exactly the calculated number is how pours end one wheelbarrow short — and short loads at the plant cost more than the extra half yard would have.