Δ
        P
        ≪
        
          P
          
            
              a
              v
              g
            
          
        
      
    
    {\displaystyle \Delta P\ll P_{\rm {avg}}}
  ), it is possible to express effusion flow as a volumetric flow rate as follows:

  
    
      
        
          Φ
          
            V
          
        
        =
        
          
            
              Δ
              P
              
                d
                
                  2
                
              
            
            
              P
              
                
                  a
                  v
                  g
                
              
            
          
        
        
          
            
              
                π
                
                  k
                  
                    B
                  
                
                T
              
              
                32
                m
              
            
          
        
      
    
    {\displaystyle \Phi _{V}={\frac {\Delta Pd^{2}}{P_{\rm {avg}}}}{\sqrt {\frac {\pi k_{B}T}{32m}}}}
  or

  
    
      
        
          Φ
          
            V
          
        
        =
        
          
            
              Δ
              P
              
                d
                
                  2
                
              
            
            
              P
              
                
                  a
                  v
                  g
                
              
            
          
        
        
          
            
              
                π
                R
                T
              
              
                32
                M
              
            
          
        
      
    
    {\displaystyle \Phi _{V}={\frac {\Delta Pd^{2}}{P_{\rm {avg}}}}{\sqrt {\frac {\pi RT}{32M}}}}
  where 
  
    
      
        
          Φ
          
            V
          
        
      
    
    {\displaystyle \Phi _{V}}
   is the volumetric flow rate of the gas, 
  
    
      
        
          P
          
            
              a
              v
              g
            
          
        
      
    
    {\displaystyle P_{\rm {avg}}}
   is the average pressure on either side of the orifice, and 
  
    
      
        d
      
    
    {\displaystyle d}
   is the hole diameter.
Effect of molecular weight
At constant pressure and temperature, the root-mean-square speed and therefore the effusion rate are inversely proportional to the square root of the molecular weight. Gases with a lower molecular weight effuse more rapidly than gases with a higher molecular weight, so that the number of lighter molecules passing through the hole per unit time is greater. Grahams law
Scottish chemist Thomas Graham (1805–1869) found experimentally that the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles. In other words, the ratio of the rates of effusion of two gases at the same temperature and pressure is given by the inverse ratio of the square roots of the masses of the gas particles.