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The molar volume (symbol V_m) is the volume occupied by one mole of a substance (chemical element or chemical compound) at a given temperature and pressure. It is equal to the molecular mass (M) divided by the density (ρ):

$V_{\rm {m}}={M \over \rho }$

It has an SI unit of cubic metres per mole (m³/mol). However, molar volumes are often expressed as cubic metres per 1,000 moles (m³/kmol) or cubic decimetres per mol (dm³/mol) for gases and as centimetres per mole (cm³/mol) for liquids and solids.

If a substance is a mixture containing N components, the molar volume is calculated using:

$V_{\rm {m}}={\frac {\displaystyle \sum _{i=1}^{N}x_{i}M_{i}}{\rho _{mixture}}}$

where x _i is the mole fraction of the ith component.

Ideal gases

The ideal gas law equation can be rearranged to give an expression for the molar volume of an ideal gas:

$V_{\rm {m}}={V \over {n}}={{RT} \over {P}}$.