What this calculator does
Use this concrete calculator to estimate volume for a rectangular slab, base or footing. It gives cubic metres and a rough bag count.
Formula used
Volume equals length multiplied by width multiplied by depth. The calculator converts depth from millimetres to metres and adds the waste allowance.
How to read the result
Concrete ordering should allow for uneven ground, compaction, formwork and site waste. For structural work, check the specification with a qualified professional.
Assumptions
- Assumes a rectangular pour.
- Uses depth in millimetres.
- Bag estimate assumes 20 kg bags yield about 0.01 cubic metres each.
Sources and checks
This calculator uses a standard public formula. Where rules or thresholds can change, source links are listed on the relevant page.
Frequently asked questions
Why add waste?
Ground levels, spillage and formwork variation can increase the amount needed. Running short during a pour is usually worse than a small surplus.