In chemistry, the **mass fraction** of a substance within a mixture is the ratio (alternatively denoted ) of the mass of that substance to the total mass of the mixture.^{[1]} Expressed as a formula, the mass fraction is:

Because the individual masses of the ingredients of a mixture sum to , their mass fractions sum to unity:

Mass fraction can also be expressed, with a denominator of 100, as **percentage by mass** (in commercial contexts often called *percentage by weight*, abbreviated *wt%*; see mass versus weight). It is one way of expressing the composition of a mixture in a dimensionless size; mole fraction (percentage by moles, mol%) and volume fraction (percentage by volume, vol%) are others.

When the prevalences of interest are those of individual chemical elements, rather than of compounds or other substances, the term *mass fraction* can also refer to the ratio of the mass of an element to the total mass of a sample. In these contexts an alternative term is *mass percent composition*. The mass fraction of an element in a compound can be calculated from the compound's empirical formula^{[2]} or its chemical formula.^{[3]}

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*Percent concentration* does not refer to this quantity. This improper name persists, especially in elementary textbooks. In biology, the unit "%" is sometimes (incorrectly) used to denote mass concentration, also called *mass/volume percentage*. A solution with 1 g of solute dissolved in a final volume of 100 mL of solution would be labeled as "1%" or "1% m/v" (mass/volume). This is incorrect because the unit "%" can only be used for dimensionless quantities. Instead, the concentration should simply be given in units of g/mL. *Percent solution* or *percentage solution* are thus terms best reserved for *mass percent solutions* (m/m, m%, or mass solute/mass total solution after mixing), or *volume percent solutions* (v/v, v%, or volume solute per volume of total solution after mixing). The very ambiguous terms *percent solution* and *percentage solutions* with no other qualifiers continue to occasionally be encountered.

In thermal engineering, **vapor quality** is used for the mass fraction of vapor in the steam.

In alloys, especially those of noble metals, the term **fineness** is used for the mass fraction of the noble metal in the alloy.

The mass fraction is independent of temperature until phase change occurs.

The mixing of two pure components can be expressed introducing the (mass) mixing ratio of them . Then the mass fractions of the components will be

The mass ratio equals the ratio of mass fractions of components:

due to division of both numerator and denominator by the sum of masses of components.

The mass fraction of a component in a solution is the ratio of the mass concentration of that component *ρ _{i}* (density of that component in the mixture) to the density of solution .

The relation to molar concentration is like that from above substituting the relation between mass and molar concentration:

where is the molar concentration, and is the molar mass of the component .

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**Percentage by mass** can also be expressed as *percentage by weight*, abbreviated *wt%*, or weight–weight percentage.

The mole fraction can be calculated using the formula

where is the molar mass of the component , and is the average molar mass of the mixture.

Replacing the expression of the molar-mass products,

In a spatially non-uniform mixture, the mass fraction gradient gives rise to the phenomenon of diffusion.