There are several formulae. Some are more/less convenient depending on the data you have handy. Here is one:
mass flow rate = P*A*M*(gamma/(R*T))^0.5
where
P is static pressure
A is area
M is mach number
gamma is ratio of specific heats (1.4 for air)
R is gas constant
T is static temperature
Make sure you use consistent units and make sure that P, A, M, and T are values from the same cross section of the flow.
If your flow is choked, the formula can be simplified.
Haf