The orifice equation describes the rate of flow of liquid through an orifice.

The equation can be represented as:

$ Q = Cd A\sqrt{2gh} $

where Q = flow (cubic metres per second)
C(d) = coefficient of discharge
A = area of orifice (square metres)
g = acceleration from gravity (9.81 m/s^2)
h = head acting on the centreline (m)

For a circular orifice, the equation becomes:
$ Q = Cd (1/4 \pi D^2)\sqrt{2gh} $

Typical values for the coefficient of discharge are:
Sharp orifice: 0.62
Tube: 0.80