The formula: length × width × height
A rectangular pour is a simple rectangular prism, so its volume is length times width times height (or thickness, for a slab). For a 5m × 4m area poured 0.15m thick: 5 × 4 × 0.15 = 3 cubic meters.
A smaller example
The same formula scales down just as easily. A 3m × 3m patch poured 0.1m thick: 3 × 3 × 0.1 = 0.9 cubic meters.
Why the unit is cubic meters
Cubic meters (m³) is the standard unit for ordering ready-mix concrete in India — length and width are measured in meters, and height/thickness is typically a fraction of a meter (a 150mm slab is entered as 0.15m), so the product comes out directly in cubic meters with no unit conversion needed.
What this figure doesn't tell you
This raw volume is exactly what's needed to order ready-mix concrete by the cubic meter, but it says nothing about how much cement, sand, and aggregate would be needed to mix that same volume on-site — that requires a mix ratio and a wastage allowance, covered in the companion articles.