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.

## Epidemiology

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

## Nuclear Reactivity

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]

${\displaystyle \rho =(k_{\text{eff))-1)/k_{\text{eff))}$

where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:[5]

${\displaystyle 1~{\text{pcm))=\rho \cdot 10^{5))$

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]