Concrete Calculator for Metric Measurements

If you work in metric units — metres, centimetres, and kilograms — this guide shows you exactly how to calculate concrete volume for any project. While the underlying maths is identical regardless of unit system, working in metric simplifies many calculations because the conversions between metres, litres, and kilograms are based on clean multiples of ten. This guide covers metric formulas, unit conversions, worked examples, and the most common pitfalls for metric users.

Metric Volume Formulas

The core formulas are the same as imperial, just expressed in metres:

• Rectangular slab: Volume (m³) = Length (m) × Width (m) × Depth (m)
• Circular slab: Volume (m³) = π × Radius² (m) × Depth (m)
• Column: Volume (m³) = π × Radius² (m) × Height (m)

The critical point is that all dimensions must be in metres. Slab thickness is often measured in millimetres (e.g., 100 mm), so you must divide by 1,000 to convert to metres before multiplying. A 100 mm thick slab has a depth of 0.1 m.

Converting Millimetres and Centimetres to Metres

This is where most metric calculation errors happen. Here are the conversions you need:

• 1 metre = 100 centimetres = 1,000 millimetres
• To convert mm to m: divide by 1,000 (e.g., 150 mm = 0.15 m)
• To convert cm to m: divide by 100 (e.g., 75 cm = 0.75 m)

Always convert before multiplying. If your slab is 3 metres long, 4 metres wide, and 100 mm thick, the calculation is 3 × 4 × 0.1 = 1.2 m³. Writing it as 3 × 4 × 100 gives you 1,200, which is clearly wrong.

Worked Example: Garden Patio

Suppose you want to pour a rectangular patio that is 5 metres long, 3.5 metres wide, and 100 mm (0.1 m) thick.

Volume = 5 × 3.5 × 0.1 = 1.75 m³

Adding a 10% waste factor: 1.75 × 1.10 = 1.925 m³. Round up to 2.0 m³ for ordering.

At a concrete density of 2,400 kg per cubic metre, this weighs approximately 2.0 × 2,400 = 4,800 kg, or 4.8 tonnes. That is a significant weight, so ensure your base preparation is adequate.

For bags: A 25 kg bag covers approximately 0.0104 m³. You would need 1.925 ÷ 0.0104 = 186 bags. For this volume, ready-mix delivery is clearly the better option.

Worked Example: Circular Column

A cylindrical column with a diameter of 300 mm (radius 0.15 m) and a height of 2.5 metres:

Volume = π × 0.15² × 2.5 = 3.14159 × 0.0225 × 2.5 = 0.177 m³ per column

For four identical columns: 0.177 × 4 = 0.707 m³. With 10% waste: 0.778 m³.

Metric Bag Sizes and Coverage

In metric countries, concrete is typically sold in 20 kg, 25 kg, or 40 kg bags. The approximate coverage per bag (assuming standard concrete density) is:

• 20 kg bag: ~0.0083 m³
• 25 kg bag: ~0.0104 m³
• 40 kg bag: ~0.0167 m³

To find the number of bags, divide your total volume (including waste) by the coverage per bag and round up. Our calculator does this automatically when set to metric mode — just toggle the metric switch and select your bag size.

Metric to Imperial Conversion

If you need to communicate with suppliers who use imperial units, or if you are reading plans in mixed units, here are the key conversions:

• 1 metre = 3.281 feet
• 1 cubic metre = 1.308 cubic yards = 35.315 cubic feet
• 1 kilogram = 2.205 pounds

Common Mistakes for Metric Users

• Not converting mm to m: The single most common error. A 150 mm depth entered as 150 in a formula gives a volume 1,000 times too large.
• Confusing cm and m: Similar to above. Ensure your tape measure reading matches the unit you enter into the formula.
• Ordering in the wrong unit: Confirm with your supplier whether they price per cubic metre or per cubic yard. In the UK, Australia, and most of Europe, ordering is per m³.
• Ignoring weight: Concrete is heavy. At 2.4 tonnes per cubic metre, even a modest pour produces significant loads. Plan your delivery access and base preparation accordingly.

For imperial formulas and more detail on each shape type, see our concrete formula guide. To see a worked scenario using metric measurements, check the garden shed scenario.

Frequently Asked Questions

How do I calculate concrete in cubic metres?

Multiply length × width × depth, all in metres. For example, a 3 m × 4 m slab at 100 mm (0.1 m) thick equals 3 × 4 × 0.1 = 1.2 cubic metres. Always convert millimetres to metres by dividing by 1,000 before calculating.

How much does 1 cubic metre of concrete weigh?

Standard concrete weighs approximately 2,400 kg (2.4 tonnes) per cubic metre. Lightweight aggregate concrete can weigh less, while heavy mixes used for radiation shielding weigh more. For most residential projects, 2,400 kg/m³ is the correct figure.