Calculating GPH

[ScubaSteve]

If you want an exact answer, you'd figure out the pressure drop for each section and add it up. For over-designing, just use the 3/4" to figure out the pressure drop and just know that your flow will actually be higher. HOWEVER, you are going to have to take into account those reducers. There is a pretty decent headloss calculator floating around out there that can do all that for you. I can't find the site at the moment but I think it was an excel sheet in an article by Sanjay.