Check out Ductulator Pro at:
https://play.google.com/store/apps/details?id=com.welmurr.ductulator2

s can change between I-P and S-I units by tapping the units, on the right.
Duct sizes can be obtained based on either maximum friction or velocity. Tap on the values for velocity or friction to switch between calculation methods.

Calculations are based on standard air 0.075 lb/ft3 (1.2 kg/m3) in clean galvanized steel duct.

Your is much appreciated.