text: what is the formula to calculate flow of liquid?   flow rate q is given by  q=av  where a is the cross sectional area of the pipe through which the fluid is flowing, and v is the velocity of the fluid in the pipe.  if you have a flow where the pipe diameter changes, conservation of flow states that  a1v1=a2v2  so if the pipe diameter decreases (and this the csa gets smaller) the velocity of the fluid increases, and vice versa.