Why does the same slab area need different concrete volumes for different slab thicknness?

Concrete volume scales directly with thickness, so a 150mm slab needs meaningfully more concrete than a 100mm slab of the identical floor area, and this is one of the most common inputs owners get wrong when estimating quickly by area alone. Slab thickness is determined by your structural engineer based on span, load and reinforcement design, so confirm the correct thickness for your specific slab before entering it into the calculator.