# What is the amount of space occupied by a substance?

Clearly, it's the $\text{volume}$.
All matter, gas, liquid, and solid expresses a volume. For gases, the volume can be altered for a given quantity (i.e. mass) by compression or expansion, and a gas will always fill a vacuum evenly. $\text{Gaseous volumes}$ have typical units of $L$, or ${m}^{3}$.