How to Calculate Concrete Quantities
Quick Answer
Volume of concrete = Length × Width × Depth (all measurements in the same unit)
- In metres: Volume in m³ = L(m) × W(m) × D(m)
- In feet: Volume in ft³ = L(ft) × W(ft) × D(ft). Convert to yd³: ft³ ÷ 27
Example: A driveway 6 m long, 3 m wide, 0.1 m (100 mm) thick. Volume = 6 × 3 × 0.1 = 1.8 m³. Always add 10% for wastage: 1.8 × 1.10 = 1.98 m³ (order 2 m³).
Calculating concrete quantities correctly saves money, prevents shortfalls and reduces waste. For volume conversions between cubic metres, cubic feet and cubic yards, use the free Volume Converter on CalConvs.
Concrete for a Rectangular Slab
- Example 1 (metric): House extension base slab 4 m × 5 m × 0.15 m thick. Volume = 4 × 5 × 0.15 = 3.0 m³. With 10% wastage: 3.3 m³.
- Example 2 (US measurements): Garden path 30 ft × 4 ft × 4 inches (0.333 ft) thick. Volume = 30 × 4 × 0.333 = 39.96 ft³ = 39.96 ÷ 27 = 1.48 yd³. With 10% wastage: order 1.7 yd³.
Concrete for Cylindrical Columns or Posts
Volume = π × radius² × height (π = 3.14159)
Example: Concrete fence post footing, diameter 300 mm = radius 0.15 m, depth 0.9 m. Volume = 3.14159 × (0.15)² × 0.9 = 0.0636 m³ per post. For 20 posts: 20 × 0.0636 = 1.27 m³ total.
Concrete Bag Coverage
| Bag Size | Approximate Coverage |
|---|---|
| 20 kg bag (Australia, UK) | Approximately 0.009 m³ per bag |
| 25 kg bag (UK and India) | Approximately 0.011 m³ per bag |
| 40 lb bag (US) | Approximately 0.011 ft³ per bag |
| 60 lb bag (US) | Approximately 0.017 ft³ per bag |
| 80 lb bag (US) | Approximately 0.022 ft³ per bag |
Example: 0.5 m³ of concrete using 25 kg bags. Bags = 0.5 ÷ 0.011 = 45.5 bags. Round up: order 50 bags. For large projects over 2 m³, ready-mix concrete is more economical and consistent than bagged mix.
Common Concrete Thickness Guidelines
| Application | Thickness |
|---|---|
| Footpath or garden path | 75 to 100 mm (3 to 4 inches) |
| Driveway (cars) | 100 to 125 mm (4 to 5 inches) |
| Driveway (heavy vehicles) | 150 to 200 mm (6 to 8 inches) |
| House floor slab | 100 to 150 mm (4 to 6 inches) |
| Foundation footing | 200 to 300 mm (8 to 12 inches) |
Concrete Standards by Country
- United Kingdom: British Standard BS 8500. Ready-mix ordered in m³. C25/C30 is standard for domestic slabs.
- United States: ASTM standards. Ready-mix ordered in cubic yards. 3,000 to 4,000 psi is standard residential concrete.
- Australia: AS 1379 standard. Ready-mix in m³. N25 is standard residential.
- Pakistan: PS:385 standard. Mix designs M15, M20, M25 standard.
Frequently Asked Questions
How many cubic metres of concrete do I need for a 6m x 4m slab?
Volume depends on thickness. At 100 mm thick: 6 × 4 × 0.1 = 2.4 m³. At 150 mm thick: 6 × 4 × 0.15 = 3.6 m³. Always add 10% for wastage.
How many bags of cement in 1 cubic metre of concrete?
For a standard M20 mix (1:1.5:3), approximately 8 bags of 50 kg cement are needed per cubic metre of concrete.
What is the difference between concrete and cement?
Cement is a binding powder (Portland cement). Concrete is the finished product made by mixing cement with sand, aggregate and water. Cement is approximately 10 to 15% of a concrete mix by weight.
Related Tools
- Volume Converter: m³, ft³, yd³, litres and more
- Length Converter: metres, feet, inches for project dimensions
- Area Converter: m² and ft² for surface area