How do you calculate fire hydrant flow?

Q = 29.84(cd)(d^2)*(sqrt(p))
29.84 is a constant derived from physical laws relating to water velocity, pressure, and conversion factors. In short, this number keeps the answer in GPM.
cd = the coefficient of discharge, which represents friction loss.
d = the actual inside diameter of the hydrant orifice in inches.
p = the pressure in PSI read at the orifice by the pitot gauge. Because this formula takes the square root of p—rather than p itself—large increases in PSI will have a fairly small impact on the final GPM.
Q = a number used to represent the result or discharge in GPM