vc = q / (4 π x2) = vo A / (12 x2) = vo π d2 / (48 x2).
where
A = duct area (m2, ft2)
q = air volume flow (m3/s, ft3/s)
vo = air velocity in the exhaust outlet opening (m/s, ft/s)
vc = capture air velocity in distance x from exhaust outlet (m/s, ft/s)
x = distance from exhaust outlet (m, ft)
d = diameter of exhaust outlet (m, ft)
To larger exhaust outlets it should be modified to
vc = vo A / (A + 10 x2) = q / (A + 10 x2)