Concrete Volume Calculator
Work out how much concrete a slab, path or footing needs — in cubic metres or yards, with a waste allowance, the number of bags and the weight.
How many cubic metres and bags a slab takes.
How the concrete volume calculator works
Concrete volume is length × width × thickness, with thickness converted from millimetres (or inches) to the same unit as the other two. Add a waste allowance — 10% is usual for uneven ground and spillage — then divide by the yield of a bag: a 20 kg bag of premix makes about 10 litres (0.01 m³), an 80 lb bag about 0.6 cubic feet.
Above roughly half a cubic metre it is usually cheaper and far less work to order ready-mix; the weight row (2,400 kg per m³) shows why.
Formula: volume = length × width × thickness · bags = volume × (1 + waste) ÷ bag yield
Worked examples
| Inputs | Concrete volume | Note |
|---|---|---|
| 4 × 3 m slab, 100 mm thick | 1.2 | 1.2 m³ — order ready-mix |
| 2 × 1 m path, 75 mm thick | 0.15 | 0.15 m³, about 17 bags of 20 kg |
| 13 × 10 ft slab, 4 in thick | 1.605 | 1.6 yd³ |
FAQFrequently asked questions
How many 20 kg bags of concrete per cubic metre?
About 100–110: a 20 kg bag of premix yields roughly 10 litres. Check the yield printed on the bag — it varies by product.
When should I order ready-mix?
Above about 0.5 m³ (50 bags) it is cheaper per cubic metre and you avoid mixing 1.2 tonnes by hand. Most suppliers have a minimum load of 0.5–1 m³.
How much waste should I allow?
10% is standard for slabs on prepared ground; allow 15–20% for uneven or hand-dug excavations.
How thick should a slab be?
Paths and patios are typically 75–100 mm; driveways and garage floors 100–150 mm with reinforcement. The thickness needed depends on the load and the ground — ask a builder for anything structural.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.