The rate 
  
    
      
        
          Φ
          
            N
          
        
      
    
    {\displaystyle \Phi _{N}}
   at which a gas of molar mass 
  
    
      
        M
      
    
    {\displaystyle M}
   effuses (typically expressed as the number of molecules passing through the hole per second) is then

  
    
      
        
          Φ
          
            N
          
        
        =
        
          
            
              Δ
              P
              A
              
                N
                
                  A
                
              
            
            
              2
              π
              M
              R
              T
            
          
        
        .
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.