Concrete Calculator
How much concrete a slab or footing needs — cubic metres, 20 kg bags, and a truck round-up.
Volume of a slab or footing from the length, width, and depth you measure. Bag count is for 20 kg pre-mix, at about 0.01 m³ a bag. A truck is usually ordered in 0.2 m³ steps.
Measure
Length and width default to metres. Depth defaults to millimetres — 100 mm, not 0.1 m.
Waste allowance
Required material
1.26 m³
You need approximately 1.26 m³ including 5% waste. Volume is length × width × depth.
- Base volume
- 1.2 m³
- Waste
- 5% · 0.06 m³
- To order
- 1.26 m³
- Truck (0.2 m³)
- 1.4 m³
- 20 kg bags
- 126
- 4 m × 3 m × 100 mm.
- 5% waste is included.
- A 20 kg bag of pre-mix is taken as 0.01 m³ (100 bags per m³), rounded up.
- If ordering ready-mix, use the cubic metres — not the bag count.
- Confirm the order against how the supplier sells it — truck, bulk bag, or bagged.
Job summary
- Area
- 12 m²
- Depth
- 100 mm
- Volume
- 1.2 m³
- Order
- 1.26 m³
To order1.26 m³
Measurements stay in this browser. Nothing is uploaded.
Worked example
A slab 4 m by 3 m by 100 mm is 1.2 m³. With 5% waste that’s 1.26 m³. A truck is usually ordered in 0.2 m³ steps, so 1.4 m³. At about 0.01 m³ per 20 kg bag, that’s 126 bags.
FAQ
- How is the volume calculated?
- Length × width × depth. Depth is millimetres by default — type 100 for a 100 mm slab, not 0.1.
- How many bags are in a cubic metre?
- This page uses 0.01 m³ per 20 kg bag, so 100 bags per cubic metre, rounded up after waste.
- Is this ready-mix or bagged concrete?
- The cubic metres suit either. The bag count is for pre-mix bags. If you order a truck, use the rounded cubic metres.