Parts-per notation

In science and engineering, the parts-per notation is a set of pseudo-units to describe the 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
  • parts-per-quadrillion - ppq, 10−15

This notation is not part of the International System of Units - SI system and its meaning is ambiguous.

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