---
title: "Per cent mille"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Per_cent_mille"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T11:52:18.992940+00:00"
instance: "kb-cron"
---

A per cent mille, or pcm, is a hundred-thousandth, or a thousandth of a percent. 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".


== 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:

  
    
      
        ρ
        =
        (
        
          k
          
            eff
          
        
        −
        1
        )
        
          /
        
        
          k
          
            eff
          
        
      
    
    {\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:

  
    
      
        1
         
        
          pcm
        
        =
        ρ
        ⋅
        
          10
          
            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.


== 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
Parts-per notation including parts-per million, parts-per billion etc


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


== Notes ==