Over-ordering concrete is one of the most common — and most expensive — mistakes on a small build. Order too little and you stall mid-pour; order too much and you pay for material that ends up wheelbarrowed into a corner of the site. The good news: working out the right volume is straightforward once you know the formulas.
The basic formulas
Concrete volume is measured in cubic metres (m³). Every shape on a typical job site reduces to one of three formulas:
- Rectangular slab — length × width × depth
- Square footing — side × side × depth
- Round column — π × radius² × height
Keep every measurement in metres. A 4m × 3m driveway slab at 100mm thick is 4 × 3 × 0.1 = 1.2m³. Simple.
The five percent rule
Always add around 5% on top of your calculated volume. That allowance covers three real-world losses you can't avoid:
- Uneven ground — formwork rarely sits perfectly level.
- Over-excavation — you nearly always dig slightly deeper than spec.
- Spillage — losses around the chute, pump line, and wheelbarrows add up quickly.
Volumetric vs. ready mix
If you're not 100% sure of the volume, volumetric mixing is forgiving — you only pay for what's used, and the mix is made on the truck. Ready mix is the right call when you've got a precise spec and a confirmed pour window. Both have their place; the calculator on this site gives you the volume figure either way.
Common mistakes to avoid
- Mixing units — measuring length in metres and depth in millimetres throws the answer out by a factor of 1000.
- Forgetting the waste allowance — a 0.5m³ shortfall is enough to halt the pour.
- Using internal diameter for columns — always use the full outside dimension of the formwork.
"Measure twice, pour once. The thirty seconds you spend double-checking your numbers is the cheapest insurance on site."
The shortcut
If maths isn't your favourite part of the day, use the volume calculator at /concrete-calculator. Plug in your dimensions, get the figure, click through to a quote. Three minutes, end to end.
