Free Concrete Calculator
Work out how much concrete you need for a slab, footing or column: volume in m³, weight, and a hand-mix bag estimate.
Concrete Calculator results
Fill in the details and select Calculate to see your figures.
Enter the length, width and depth of your pour to get concrete volume in cubic metres, weight in tonnes, and a rough cement/ballast bag count for hand-mixing. Adds a waste allowance and flags when ready-mix is the better option. UK metric units throughout.
How to use the Concrete Calculator
- Measure the plan length and width of the pour in metres.
- Enter depth in millimetres (100mm is common for light slabs).
- Pick the pour type that matches your job. The maths is the same L×W×D.
- Leave waste at 10% unless the ground is very uneven (use more) or the pour is tightly shuttered (use less).
- Read volume and weight for a ready-mix order, or cement bags + ballast for a hand mix.
How the Concrete Calculator works
Concrete volume is length × width × depth, with depth converted from millimetres to metres, then a waste percentage on top.
- 4m × 3m slab at 100mm: 4 × 3 × 0.10 = 1.2 m³.
- With 10% waste: 1.2 × 1.10 = 1.32 → round up to 1.4 m³.
- Weight ≈ 1.4 × 2.4 = 3.4 tonnes.
- Hand-mix cement ≈ 1.4 × 12 = 17 bags of 25kg; ballast ≈ 1.4 × 1.9 ≈ 2.7 tonnes.
Densities and C20 1:2:4 mix ratios are guidance only. Confirm mix grade (C20 general, C25 foundations) with your supplier or structural spec.
Typical UK benchmarks
- 100mm slab per m²
- 0.10 m³ concrete per m² at 100mm depth
- Concrete density
- ≈2400 kg/m³ typical reinforced concrete
- Shed base depth
- Typical garden shed/base slab 100mm
See it in your business.
Frequently asked questions
How much concrete do I need for a shed base?
A 3m × 2m shed base at 100mm needs about 0.6 m³ before waste, or ~0.7 m³ with 10% waste. That is roughly 1.7 tonnes. Most DIY shed bases are bagged concrete or a small ready-mix order.
What depth of concrete for a patio or path?
Light garden paths and patios are often 75–100mm on a compacted sub-base. Driveways and vehicle areas are typically 150mm or more, and foundations follow the structural design: never guess load-bearing depths.
When should I order ready-mix instead of bags?
Under about 1 m³, bagged or hand-mixed is usually practical. Over 1 m³, ready-mix by the cubic metre is faster, more consistent and often cheaper once you count labour and mixer hire.
How many 25kg bags of cement per m³?
For a standard C20 1:2:4 mix you need about 12 bags of 25kg cement per cubic metre (roughly 300 kg of cement), plus about 1.9 tonnes of ballast. Published mix tables put a 1:2:4 C20 mix at 285–320 kg of cement per m³, and BS 8500 sets a floor of 220 kg/m³ even for the weakest general-purpose grade, so treat anything near half that as a mis-order. Always check the bag/mix instructions for the grade you need.
Should I add a waste allowance?
Yes. 5–10% is normal for a well-shuttered slab; use up to 15–20% on uneven ground, trenches or first-time pours. Running short mid-pour is worse than a little left over.
