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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) at the given temperature and pressure:

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

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, M _i is the molecular mass of the ith component and ρ_mixture is the mixture density at the given temperature and pressure.

Ideal gases

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

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