The formula is Q = m.Cp.(Tout - Tin). You get Q from the manufacturer, or motor nameplates, or fuel consumption...
If you are exhausting from a room with tempered outdoor air makeup, then the Tin is your tempered air, and the Tout is the maximum temperature you want in the space. Cp = 1.005 kJ/kg and m/1.2 = Volume flow you require.
