In chemistry and physics, the dimensionless mixing ratio is the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio (see concentration) or mass ratio (see stoichiometry).

In atmospheric chemistry, mixing ratio usually refers to the mole ratio r_i, which is defined as the amount of a constituent n_i divided by the total amount of all other constituents in a mixture:

The mole ratio is also called amount ratio. If n_i is much smaller than n_tot (which is the case for atmospheric trace constituents), the mole ratio is almost identical to the mole fraction.

In meteorology, mixing ratio usually refers to the mass ratio of water ${\displaystyle \zeta }$, which is defined as the mass of water ${\displaystyle m_{\mathrm {H2O} }}$ divided by the mass of dry air (${\displaystyle m_{\mathrm {air} }-m_{\mathrm {H2O} }}$) in a given air parcel:

The unit is typically given in ${\displaystyle \mathrm {g} \,\mathrm {kg} ^{-1}}$. The definition is similar to that of specific humidity.

Two binary solutions of different compositions or even two pure components can be mixed with various mixing ratios by masses, moles, or volumes.

The mass fraction of the resulting solution from mixing solutions with masses m₁ and m₂ and mass fractions w₁ and w₂ is given by:

where m₁ can be simplified from numerator and denominator