Concrete Calculator
Work out the volume of concrete and number of bags for a slab, path or base.
About this calculator
Ordering concrete is a one-shot decision: too little and you get a cold joint half-way through the pour, too much and you've paid to dispose of the excess. This calculator turns slab dimensions into cubic metres, adds a waste allowance for uneven ground and spillage, and converts the result into 25kg bags so you can compare bagged mix against a ready-mix delivery.
The formula
Worked example
A shed base 3m × 2m at 10cm deep with 10% waste:
- Volume = 3 × 2 × 0.10 = 0.60 m³
- With waste = 0.66 m³ → about 72 25kg bags (~1.6 tonnes)
Common mistakes
- Measuring depth in inches but entering centimetres — double-check units.
- Forgetting that soft or uneven ground swallows more concrete than the tape measure suggests.
- Mixing bagged concrete for volumes over ~1 m³, where ready-mix is cheaper and far less work.
Frequently asked questions
Assumptions & limitations
Bag yield of 0.0092 m³ per 25kg bag and wet density of ~2.4 t/m³. Always confirm structural requirements for load-bearing work.
Sources & verification
- The Concrete Centre — specification guidance — Industry body for concrete specification (BS 8500 / BS EN 206)
- Manufacturer bag-yield data — Yield per 20 kg / 25 kg bag is printed on each product’s data sheet and varies by brand — check your bag before ordering
All sources last accessed 24 July 2026.
Spotted an error or an out-of-date figure? Report it — corrections are prioritised over all other work (see our corrections policy).