Concrete Slab Calculator
Calculate concrete, mesh and formwork for a floor slab.
Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.
What the Concrete Slab Calculator does
Slab concrete is simply area times thickness plus waste, but the supporting items matter too: mesh reinforcement with overlaps, and formwork around the perimeter to contain the pour.
Formula
Volume = Length × Width × Thickness × (1 + Waste %)Mesh sheets = ⌈Slab area × 1.15 ÷ Sheet coverage⌉Formwork area = Perimeter × Formwork height
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Slab length | number | Yes | Accepts more than 0. |
| Slab length unit | one of 6 options | Yes | — |
| Slab width | number | Yes | Accepts more than 0. |
| Slab width unit | one of 6 options | Yes | — |
| Slab thickness | number | Yes | Accepts more than 0. |
| Slab thickness unit | one of 5 options | Yes | — |
| Waste allowance | % | Yes | — |
| Mesh sheet coverage | m² | Yes | A standard A393 sheet covers 4.8 × 2.4 m. Accepts 0.1 or more. |
| Formwork height | number | Yes | Accepts more than 0. |
| Formwork height unit | one of 5 options | Yes | — |
| Concrete price per m³ | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Slab length unit and Slab width unit.
- Enter Slab length, Slab width, Slab thickness and Waste allowance and 2 more.
- Optionally add Concrete price per m³.
- Select Calculate.
Worked example
A 6 m × 4 m slab at 150 mm, 10% waste, A393 mesh sheets covering 11.16 m².
- Length
- 6 m
- Width
- 4 m
- Thickness
- 150 mm
- Waste
- 10
Area 24 m², net 3.6 m³, order 3.96 m³ weighing 9.5 tonnes. Mesh 27.6 ÷ 11.16 = 3 sheets, formwork 20 m perimeter.
Frequently asked questions
How thick should a concrete slab be?
100 mm for a domestic floor, 150 mm for a garage, and 200 mm or more where vehicles or heavy loads are involved.