In chemistry, the mole fraction or molar fraction, also called mole proportion or molar proportion, is a quantity defined as the ratio between the amount of a constituent substance, n<sub>i</sub> (expressed in unit of moles, symbol mol), and the total amount of all constituents in a mixture, n<sub>tot</sub> (also expressed in moles):
:<math>x_i = \frac{n_i}{n_\mathrm{tot</math>
It is denoted x<sub>i</sub> (lowercase Roman letter x), sometimes (lowercase Greek letter chi). (For mixtures of gases, the letter y is recommended.
When expressed in percent, it is known as the mole percent or molar percentage (unit symbol %, sometimes "mol%", equivalent to cmol/mol for 10<sup>−2</sup>).
The mole fraction is called amount fraction by the International Union of Pure and Applied Chemistry (IUPAC) This nomenclature is part of the International System of Quantities (ISQ), as standardized in ISO 80000-9, which deprecates "mole fraction" based on the unacceptability of mixing information with units when expressing the values of quantities.