How to Calculate Concrete Volume for a Slab

Why Get This Right?

Ordering too little concrete means a return trip to the supplier (and delays). Ordering too much means wasted money and disposal headaches.

The good news: calculating slab volume is simple. It's literally just length × width × depth.

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The Formula

Volume (m³) = Length (m) × Width (m) × Depth (m)

That's it. Everything else is just unit conversion and rounding up.

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Worked Example

Let's say you're pouring a concrete floor slab for a garage:

• Length: 6 metres
• Width: 4 metres
• Depth: 100mm (which is 0.1m)

Calculation:
6m × 4m × 0.1m = 2.4 m³

So you'd order 2.5 m³ of concrete (rounding up slightly for waste and uneven ground).

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Common Mistakes

• Forgetting to convert millimetres to metres. If your depth is 150mm, that's 0.15m, not 150m.
• Not allowing for waste. Always round up by 10-15% for uneven ground, spillage, and over-thickness.
• Measuring on the flat. Sloped or uneven ground means you might need 15-20% extra.

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Quick Reference: Common Slab Depths

• 50mm (0.05m): Thin paving, paths
• 75mm (0.075m): Shed bases, light structures
• 100mm (0.1m): Garage floors, driveways
• 150mm (0.15m): Heavy-duty industrial floors

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Pro Tip

If you're dealing with an irregular shape (L-shaped garage, etc.), break it into rectangles, calculate each one separately, then add them together.

Always call the concrete supplier and double-check your numbers. They've seen the mistakes before and can spot if something looks wrong.