Basic mass air flow calculation in pipe?

Mass flow in air can be calculated if you know the pressure drop across the pipe. Then it can be calculated using Darcy's Equation for Pressure,which is: P2-P1 = (4fLv*v)/d*2*g where, P2 & P1 are pressures at two points in pipe, f = friction factor, L= length of pipe, v = velocity of fluid, d = diameter of pipe, g = gravity. from this formula we can calculate the velocity and hence the flow rate.