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.

Visualisation of 1%, 1‰, 1‱, 1 pcm and 1 ppm as fractions of the large block (larger version)


Statistics of crime rates, mortality and prevalence of disease 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 keff 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 unitsEdit

See alsoEdit


  1. ^ 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
  2. ^
  3. ^
  4. ^ Merljak, Vid. "Reactivity measurements" (PDF). University of Ljublj. Retrieved September 17, 2017.ana
  5. ^ "Reactivity". Retrieved September 17, 2017.
  6. ^ "percent mille – pcm – unit of reactivity". Retrieved September 17, 2017.