How much concrete do I need?
Enter the area and depth and we'll work out the volume of concrete in cubic metres, plus the cost — with the calculation shown. Concrete is priced by volume, so it's all about area × depth.
1 m³ per m³.
| Ready-mix concrete (C25) | 4 m³ | £520 |
- 1Volume: 3 m³
- 2Usage rate: 1 m³ per m³
- 3Allowing for waste: 3 × 1 × 1.05 waste = 3.15 m³
- 4Quantity: 4 m³ @ £130 each
- Volume = area × depth. A 20 m² slab at 150mm = 3 m³.
- Indicative trade-counter price; confirm with a current supplier quote.
How to work out concrete volume
- 1Measure length × width
For a slab or base, measure the area in metres. A 4 m × 3 m base is 12 m².
- 2Multiply by the depth
Depth in metres. A 100 mm slab is 0.1 m, so 12 m² × 0.1 m = 1.2 m³ of concrete.
- 3Add ~5–10% for waste
Ground rarely sits perfectly level, so allow a margin — under-ordering means a cold joint or a second, small (expensive) load.
- 4Ready-mix vs bags
Above ~0.5 m³, ready-mix delivered is usually cheaper and easier than mixing bags; below that, bagged concrete is convenient.
Concrete is one of the few materials where getting the quantity slightly wrong is genuinely costly — order too little and you're paying a short-load charge for a second delivery, or risking a cold joint in a structural pour. Working from area × depth, with a sensible waste margin, is the way to order once and order right.
Concrete quantities by job (indicative)
| Job | Typical depth | Volume guide |
|---|---|---|
| Shed / garden base | 100 mm | 0.1 m³ per m² |
| Driveway / patio base | 100–150 mm | 0.1–0.15 m³ per m² |
| Extension foundation (strip) | varies | engineer-specified |
| Bagged concrete (25 kg) | — | ~90 bags per m³ |
1 m³ of concrete weighs roughly 2.4 tonnes. Foundations are set by the structural design. Confirm with quotes.
Frequently asked questions
How much concrete do I need for a slab?+
Multiply the area by the depth in metres. A 12 m² base at 100 mm deep needs 12 × 0.1 = 1.2 m³ of concrete, plus about 5–10% for waste and uneven ground.
How many bags of concrete are in a cubic metre?+
Roughly 90 × 25 kg bags of pre-mixed concrete make one cubic metre. Above about half a cubic metre, ready-mix delivered is usually cheaper and far less work.
How much does 1m³ of concrete cost?+
Ready-mixed concrete is typically priced per cubic metre delivered, with part-load and short-load charges for small quantities. The calculator estimates the cost from your volume.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.