Concrete Slab Calculator
Calculate the volume of concrete required for a slab, including wastage allowance.
Standard residential slabs are typically 100–150mm thick.
5% for machine-poured slabs, up to 10% for hand-poured.
Formula
Concrete Volume Required = length * width * (thickness / 1000) * (1 + wastage / 100)
Slab volume is calculated by multiplying the slab's plan area (length × width) by its thickness. Because thickness is usually specified in millimeters while length and width are in meters, the thickness is first converted to meters by dividing by 1,000. A wastage percentage is then added to account for surface irregularities, formwork spillage, and sub-base variation — without this allowance, on-site pours routinely fall short.
Worked Example
- 1 Convert thickness to meters: 100mm ÷ 1000 = 0.1m
- 2 Calculate raw volume: 5m × 4m × 0.1m = 2.0 m³
- 3 Apply 5% wastage: 2.0 × 1.05 = 2.1 m³
Result: 2.1 m³ of concrete required
Video walkthrough for Concrete Slab Calculator — coming soon
Frequently Asked Questions
How much concrete do I need for a 5m x 4m slab at 100mm thick? +
Approximately 2.1 m³, including a standard 5% wastage allowance.
Should I order extra concrete beyond the calculated volume? +
Most suppliers recommend rounding up to the nearest 0.5 m³ or the nearest full truck increment, since a short pour is far costlier to fix than a small surplus.
Does this calculator account for reinforcement or formwork? +
No — this calculates concrete volume only. Reinforcement (rebar/mesh) and formwork are estimated separately.