Explanation
Concrete for a slab
A rectangular slab’s volume is its area multiplied by its depth. An explicit allowance lets you plan additional concrete for uneven excavation or losses without hiding that assumption.
Measure the slab and choose an allowance
Measure length, width and depth in metres. Convert centimetres to metres before entering depth: a 10 cm slab is 0.1 m. Set an allowance based on the actual site and supplier; zero gives the geometric volume.
V = L × W × D × (1 + a/100)
| V | Volume to order (m³) |
|---|---|
| L | Length |
| W | Width |
| D | Depth |
| a | Allowance |
A 5 m × 4 m slab, 10 cm deep
| Length | 5 |
|---|---|
| Width | 4 |
| Depth | 0.1 |
| Allowance | 10 |
| Result | 2.2 m³ |
5 m × 4 m × 0.1 m = 2 m³. Add the 10% allowance: 2 × (1 + 10/100) = 2.2 m³.
A 1 m cube without allowance
| Length | 1 |
|---|---|
| Width | 1 |
| Depth | 1 |
| Allowance | 0 |
| Result | 1 m³ |
1 m × 1 m × 1 m = 1 m³. Add the 0% allowance: 1 × (1 + 0/100) = 1 m³.
A small 20 cm pad
| Length | 2 |
|---|---|
| Width | 3 |
| Depth | 0.2 |
| Allowance | 5 |
| Result | 1.26 m³ |
2 m × 3 m × 0.2 m = 1.2 m³. Add the 5% allowance: 1.2 × (1 + 5/100) = 1.26 m³.
Diagram of this result
The drawing exaggerates the depth so it stays visible; the volumes below it are exact. The allowance is extra concrete for an uneven base and losses, not part of the slab.
The slab volume, the allowance volume and the total to order are stated as numbers beside the drawing.
Common mistakes
Ten centimetres is 0.1 metres, not 10 metres. Do not use this volume calculation to determine structural thickness or reinforcement.
Assumptions and limits
Rectangular solid slab only. Positive dimensions no larger than one million metres; allowance 0–100%. No footings, voids, reinforcing displacement, dry-bag yield, delivery rounding or structural design is inferred. Material estimates only; verify measurements, product guidance and site requirements before ordering.
How many ready-mix bags are needed?
Divide the volume by the stated wet yield per bag and round up. Yield varies by product; this calculator deliberately gives volume rather than assuming a bag size.
Method and references
Volume of a rectangular solid, length × width × depth, scaled by an explicit percentage allowance. Product yields and site allowances must come from the chosen supplier; no standard product yield is assumed.