# Per cent mille

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 nuclear reactor engineering as a unit of reactivity.

## Epidemiology

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

## 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]}

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]}

## Related units

- 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

## See also

- InHour (another unit of reactivity)
- Dollar (reactivity)
- Parts-per notation
- Per-unit system
- Percent point function

## Notes

**^**SCALE: A Comprehensive Modelling and Simulation Suite for Nuclear Safety Analysis and Design. Available from Radiation Safety Information Computational Center at Oak Ridge National Laboratory as CCC-785. Oak Ridge, TN: Oak Ridge National Laboratory, June, 2001. Version 6.1. ORNL/TM-2005/39**^**http://cdc.gov/foodnet/reports/data/infections.html**^**http://apps.who.int/gho/data/node.main.VIOLENCEHOMICIDE**^**Merljak, Vid. "Reactivity measurements" (PDF). University of Ljublj. Retrieved September 17, 2017.ana**^**"Reactivity".*nuclear-power.net*. Retrieved September 17, 2017.**^**"percent mille – pcm – unit of reactivity".*nuclear-power.net*. Retrieved September 17, 2017.