Parts-per notation

In science and engineering 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.