Concrete calculator
Estimate concrete volume for a rectangular slab and include an allowance for waste. Enter all dimensions in metres.
Reference calculation or estimate for example inputs; assumptions and limitations apply.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| Length (m) | Enter length in m; use the same basis as the other measurements. |
| Width (m) | Enter width in m; use the same basis as the other measurements. |
| Depth (m) | Enter depth in m; use the same basis as the other measurements. |
| Waste allowance (%) | Enter percentage units: 5 means 5%, not a fraction of 0.05. |
Understanding your result
The answer estimates quantity, coverage or cost from the dimensions and allowances you entered. It does not size a structural member, assess capacity or determine installation adequacy.
Common mistakes
- Mixing centimetres with metres, or outside dimensions with the actual filled volume.
- Treating allowance, compaction, supplier yield and density as interchangeable adjustments.
Check your calculation
- Measure independently, subdivide irregular shapes, and compare supplier pack/yield information with the units used here.
Calculation checks, sources and review limits
How much concrete do I need for a rectangular slab?
Enter the slab’s dimensions in metres to find its concrete volume and an order quantity including the waste allowance. The calculation estimates material quantity.
Common uses
- Plan a rectangular concrete slab or pad.
- Add a waste allowance before comparing order quantities.
How it works
Multiply length, width and depth to find the slab volume. The order quantity adds your chosen waste allowance. Use the average depth for a level slab; uneven excavation can require more material. This estimates volume only, not structural strength or reinforcement.
Worked example
A slab 5 m long, 4 m wide and 0.1 m deep needs 2 m³. With a 10% allowance, order 2.2 m³.