How to Calculate Concrete for a Slab
The cubic-yard formula for a concrete slab, standard thickness by use case, and how to convert your total into bag count or a ready-mix order.
Concrete is priced and delivered by volume, so the core task is converting a flat slab area into cubic yards — the unit ready-mix trucks and most suppliers actually quote.
The formula
- Area (sq ft) = length × width
- Volume (cu ft) = area × thickness (in feet — divide inches by 12)
- Cubic yards = volume in cu ft ÷ 27
Standard slab thickness by use
| Use | Typical thickness |
|---|---|
| Walkway / sidewalk | 4 in |
| Patio | 4 in |
| Standard driveway | 5-6 in |
| RV pad / heavy-duty | 6 in or more |
Worked example
A 10×10 ft patio at 4 inches thick: area = 100 sq ft, thickness = 4 ÷ 12 = 0.333 ft, volume = 100 × 0.333 = 33.3 cu ft, cubic yards = 33.3 ÷ 27 ≈ 1.23 cu yd. For bagged concrete, a 60 lb bag yields about 0.45 cu ft and an 80 lb bag about 0.60 cu ft — so 33.3 cu ft needs roughly 56 bags at 60 lb or 42 bags at 80 lb, before any waste margin.
The Concrete, Mulch & Topsoil Calculator runs this conversion automatically and shows both the bag count and the cubic yard total to give a ready-mix supplier.