The pressure a liquid exerts against the sides and bottom of a container depends on the density and the depth of the liquid. If atmospheric pressure is neglected, liquid pressure against the bottom is twice as great at twice the depth; at three times the depth, the liquid pressure is threefold; etc. Or, if the liquid is two or three times as dense, the liquid pressure is correspondingly two or three times as great for any given depth. Liquids are practically incompressible – that is, their volume can hardly be changed by pressure (water volume decreases by only 50 millionths of its original volume for each atmospheric increase in pressure). Thus, except for small changes produced by temperature, the density of a particular liquid is practically the same at all depths. Atmospheric pressure pressing on the surface of a liquid must be taken into account when trying to discover the total pressure acting on a liquid. The total pressure of a liquid, then, is ρgh plus the pressure of the atmosphere. When this distinction is important, the term total pressure is used. Otherwise, discussions of liquid pressure refer to pressure without regard to the normally ever-present atmospheric pressure. The pressure does not depend on the amount of liquid present. Volume is not the important factor – depth is. The average water pressure acting against a dam depends on the average depth of the water and not on the volume of water held back.
The equation for all points of a system filled with a constant-density fluid is

  
    
      
        
          
            p
            γ
          
        
        +
        
          
            
              v
              
                2
              
            
            
              2
              g
            
          
        
        +
        z
        =
        
          c
          o
          n
          s
          t
        
        ,
      
    
    {\displaystyle {\frac {p}{\gamma }}+{\frac {v^{2}}{2g}}+z=\mathrm {const} ,}
  where:

p, pressure of the fluid,

  
    
      
        
          γ
        
      
    
    {\displaystyle {\gamma }}
   = ρg, density × acceleration of gravity is the (volume-) specific weight of the fluid,
v, velocity of the fluid,
g, acceleration of gravity,
z, elevation,

  
    
      
        
          
            p
            γ
          
        
      
    
    {\displaystyle {\frac {p}{\gamma }}}
  , pressure head,

  
    
      
        
          
            
              v
              
                2
              
            
            
              2
              g
            
          
        
      
    
    {\displaystyle {\frac {v^{2}}{2g}}}
  , velocity head.