How Much Concrete Do I Need?
Updated June 2026 · Estimates only — verify before quoting
Concrete is sold by the cubic metre (m³), so the first job on any slab, footing or base is to work out your volume. Get it wrong and you either run short mid-pour or pay for waste.
// Do the maths instantly
Concrete Volume Calculator
Work out the m³ of concrete for any slab, base or footing.
Open the Concrete Volume CalculatorThe simple formula
Volume (m³) = length (m) × width (m) × depth (m). For example, a 4m × 3m slab at 100mm (0.1m) deep is 4 × 3 × 0.1 = 1.2 m³. Always add about 5–10% for sub-base unevenness and spillage.
- Slab/base: length × width × thickness
- Footing/trench: length × width × depth
- Round up and add 5–10% contingency
Bags vs. ready-mix
As a rough guide, a 25kg bag of pre-mixed concrete yields about 0.01 m³, so you need roughly 100 bags per m³ — fine for small jobs only. For anything above ~0.5 m³, ready-mix is usually cheaper and far less work. The Concrete Volume Calculator below gives you the m³ figure to order from.
Frequently asked questions
How do I calculate concrete in m³?
Multiply length × width × depth in metres. A 4×3m slab at 100mm deep = 1.2 m³. Add 5–10% for waste.
How many bags of concrete in a cubic metre?
Roughly 100 × 25kg bags per m³. Above about half a cubic metre, ready-mix is usually more economical.
From estimate to winning quote
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