Concrete Volume Calculator
Calculate concrete in cubic yards and meters for slabs, footings, columns, and steps.
Returns bag count or ready-mix loads with 10% waste allowance.
Concrete volume calculation determines how much ready-mix or bagged concrete is needed for a pour. Ordering too little causes cold joints (structural weakness); ordering too much wastes money. Always include a 5–10% overage buffer.
Volume formulas by shape:
Rectangular slab: Volume (ft³) = Length × Width × Thickness Convert to cubic yards: Volume (yd³) = Volume (ft³) / 27
Circular slab (patio, column footing): Volume = π × Radius² × Thickness
Cylindrical post or tube form: Volume = π × (Diameter/2)² × Height
Trapezoidal footing (wider at base): Volume = Thickness × (Top Width + Bottom Width) / 2 × Length
Unit conversions:
- 1 cubic yard = 27 cubic feet = 0.7646 cubic meters
- 1 cubic meter = 1.308 cubic yards
Bagged concrete yield:
| Bag Size | Yield |
|---|---|
| 40 lb bag | 0.30 ft³ |
| 60 lb bag | 0.45 ft³ |
| 80 lb bag | 0.60 ft³ |
Ready-mix ordering: Sold by the cubic yard. Minimum orders typically 1 yard. Delivery trucks carry 8–10 yards.
Worked example: A 12 ft × 20 ft concrete patio, 4 inches (0.333 ft) thick: Volume = 12 × 20 × 0.333 = 79.92 ft³ In cubic yards = 79.92 / 27 = 2.96 yd³ With 8% overage = 2.96 × 1.08 = 3.20 yd³ → order 3.5 yards
Using 80 lb bags: 79.92 / 0.60 = 133 bags (without overage) With 8% buffer: 144 bags — and at $8/bag, that’s $1,152 vs. ready-mix at ~$200/yd³ × 3.5 = $700.
Ready-mix is cheaper at scale. Bags make sense for volumes under 1 cubic yard or remote locations without truck access.