Every substance an organism needs must cross a surface to enter, and every waste substance must cross a surface to leave. Surface area is the amount of exchange surface available, while volume is the amount of living material that needs supplying.

For example, a cube with side length $5cm$ has a surface area of $6\times 5^2=150{cm}^2$ and a volume of $5^3=125{cm}^3$. Its surface area to volume ratio is $150:125$, or $1.2:1$. If $125$ unit cubes are packed together as one block, their internal faces are not exposed, so the external surface area remains $150{cm}^2$. If the unit cubes are separated, each cube's surface is exposed and their combined surface area is $125\times 6=750{cm}^2$.