# Parts-per notation

## Set of units to describe small values / From Wikipedia, the free encyclopedia

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"Parts per billion" redirects here. For the film, see

*Parts per Billion*.In science and engineering, the **parts-per notation** is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are **parts-per-million** (**ppm**, 10^{−6}), **parts-per-billion** (**ppb**, 10^{−9}), **parts-per-trillion** (**ppt**, 10^{−12}) and **parts-per-quadrillion** (**ppq**, 10^{−15}). This notation is not part of the International System of Units (SI) system and its meaning is ambiguous.