Comprehensive Calculator Guide
The concrete a slab needs, in cubic metres, is length × width × thickness, with the thickness converted to metres, plus a waste allowance. A slab 8 × 5 m and 10 cm thick needs 4 m³ net, or 4.2 m³ with 5% waste. Enter the dimensions and allowance to know what to order before calling the ready-mix plant.
Working out the volume
A rectangular slab is a box, so its volume is its three dimensions multiplied. The usual slip is units: thickness is measured in centimetres while length and width are in metres. Pick each dimension's unit in the calculator and it converts for you. Split irregular areas into rectangles and add them up.
Add 5–10% for waste: uneven ground, formwork that bulges, and concrete left in the pump and lines. Running short mid-pour is far worse than a small surplus, because a delay can leave a weak joint in the slab.
Before ordering ready-mix
Plants sell by the cubic metre and trucks usually carry 6 to 10 m³, so ask your supplier about truck size and minimum orders. Confirm the mix and strength required by your structural drawings; this calculator gives volume only, not cement content or reinforcement.
Ribbed (hollow-block) slabs are not solid, so they need less concrete than area × full thickness. Take their quantity from the drawings.
How to Use
- 01Enter length and width, choosing metres, centimetres or feet.
- 02Enter the thickness and its unit, usually centimetres.
- 03Enter a waste allowance, typically 5–10%.
- 04Read the net and total volume in m³, or switch the result to litres or cubic feet.
Formula Used
Net volume = length × width × thickness (in metres) | Total = net × (1 + waste ÷ 100)Practical Examples
Example 1: a yard slab
8 × 5 m at 10 cm with 5% waste: 4 m³ net, 4.2 m³ total.
Example 2: a side path
15 × 1.2 m at 10 cm with 10% waste: 1.8 m³ net, 1.98 m³ total.
Example 3: a warehouse floor
10 × 10 m at 15 cm with 7% waste: 15 m³ net, 16.05 m³ total, at least two 9 m³ trucks.
Important Tips
- Measure thickness at several points on uneven ground and use the average.
- Have formwork and crew ready before the truck arrives; ready-mix has a limited working time.
Common Mistakes to Avoid
- Multiplying centimetres of thickness by square metres of area, which overstates the volume a hundredfold.
- Ordering the net volume with no allowance and running short mid-pour.
Frequently Asked Questions
- How do I calculate concrete in cubic metres?
- Multiply length, width and thickness, all in metres: 6 × 4 m at 12 cm is 6 × 4 × 0.12 = 2.88 m³ before waste.
- How much waste should I allow?
- Usually 5–10%, more for uneven ground, small pours or long pump lines.
- How many bags of cement per cubic metre?
- It depends on the mix and strength specified, so take it from your engineer or drawings rather than a generic figure.
- Does this work for a ribbed slab?
- No. Hollow blocks take up part of the depth, so treating it as solid overestimates the concrete.