Concrete Volume Calculator
Calculate concrete volume for a slab, column, or footing — get cubic metres, bags of cement, and ready-mix cost.
How to calculate concrete volume
Concrete volume depends on the shape of the pour. For slabs and footings (rectangular), volume = length × width × depth. For columns (cylindrical), volume = π × radius² × height. All dimensions should be in metres; the result is in cubic metres (m³).
Worked example — patio slab
A 5m × 4m patio slab at 100mm (0.1m) depth:
| Step | Value |
|---|---|
| Volume (5 × 4 × 0.1) | 2.0 m³ |
| Add 10% wastage | 2.2 m³ |
| Bags of 25kg cement (1:2:3 mix, ~7 bags/m³) | 16 bags |
| Ready-mix cost at £120/m³ | £264 |
Recommended slab thicknesses
| Application | Typical depth |
|---|---|
| Patio or garden path | 100 mm (0.10 m) |
| Shed or garage base | 100–150 mm |
| Driveway (car traffic) | 150 mm (0.15 m) |
| Structural floor slab | 150–200 mm |
| Heavy vehicle or lorry | 200+ mm |
Always add 5–10% to your calculated volume to account for wastage, spillage, and slight ground unevenness. For jobs over ~1 m³, ready-mix delivered by truck is typically more cost-effective and consistent than mixing bags on-site.
Frequently asked questions
- How do I calculate concrete volume for a slab?
- Multiply length × width × depth (all in metres) to get cubic metres. For example, a 4m × 3m slab at 150mm depth = 4 × 3 × 0.15 = 1.8 m³. Always add 5–10% for wastage.
- How thick should a concrete slab be?
- It depends on the load. Patios and garden paths: 100mm. Shed or garage bases: 100–150mm. Driveways for cars: 150mm. Structural floor slabs: 150–200mm. Areas subject to heavy vehicles: 200mm or more.
- How many bags of concrete do I need?
- Using a standard 1:2:3 mix (1 part cement, 2 parts sand, 3 parts aggregate), you need approximately 7 bags of 25kg cement per cubic metre of concrete. The calculator rounds up since you cannot buy a fraction of a bag.
- What does the 1:2:3 mix ratio mean?
- A standard general-purpose concrete mix by volume: 1 part cement, 2 parts sand, 3 parts aggregate (stone). This gives a C20-grade concrete suitable for most domestic applications — paths, patios, shed bases, and foundations.
- Should I buy bagged cement or ready-mix concrete?
- Bagged cement (mixed on-site with sand and aggregate) is usually more economical for small jobs under about 1 m³. Ready-mix delivered by truck is more convenient and consistent for larger pours, but typically has a minimum order of 1–2 m³ and must be poured quickly once it arrives.
- How much extra concrete should I order for wastage?
- Add 5–10% to your calculated volume. The exact allowance depends on how level and prepared the ground is — rough or uneven ground absorbs more concrete. For a simple well-prepared slab, 5% is usually sufficient.
- What is the difference between a slab and a footing?
- A slab is a flat horizontal pour that forms a floor or surface — a patio, driveway, or floor base. A footing (or foundation) is a rectangular pour that distributes the load of a wall or column into the ground, typically narrower and deeper than a slab.
- How do I calculate concrete for a round column?
- Use the cylinder formula: volume = π × radius² × height. Enter the column diameter and height into the calculator and select "Column" — it applies this formula automatically.
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.