A **per cent mille** or **pcm** is one one-thousandth of a percent.^{[1]} It can be thought of as a "milli-percent". It is commonly used in epidemiology, and in nuclear reactor engineering as a unit of reactivity.

Statistics of crime rates, mortality and disease prevalence in a population are often given in "per 100 000".^{[2]}^{[3]}

In nuclear reactor engineering, a per cent mille is equal to one-thousandth of a percent of the reactivity, denoted by Greek lowercase letter rho. Reactivity is a dimensionless unit representing a departure from criticality, calculated by:^{[4]}

where k_{eff} denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:^{[5]}

This unit is commonly used in the operation of light-water reactor sites because reactivity values tend to be small, so measuring in pcm allows reactivity to be expressed using whole numbers.^{[6]}

- Percentage point difference of 1 part in 100
- Percentage (%) 1 part in 100
- Per mille (‰) 1 part in 1,000
- Basis point (bp) difference of 1 part in 10,000
- Permyriad (‱) 1 part in 10,000
- Parts-per notation including parts-per million, parts-per billion etc