Flow through a venturimeter is given by

where
Q= flow rate, ft3/s (m3/s)

c =empirical discharge coefficient dependent on throat velocity and diameter

d₁= diameter of main section, ft (m)

d₂= diameter of throat, ft (m)

h₁= pressure in main section, ft (m) of water

h₂= pressure in throat section, ft (m) of water