I have a 40 gallon H/W tank that I would like to change out to a tank less system. All Tank less systems seem to give its max GPM ratio but not being a pro in the plumbing field I can not figure out my max GPM usage. I rarely run out of hot water even when the tank is supplying multiple appliances.

I have a one inch copper supply line feeding off of the City of Chicago supply. I also know that Chicago is a "High volume Low pressure" system with main pressures ranging form 20 psi to 60 psi with most around 30 psi.

What I would like to know is:

How to figure out the gpm flow of my one inch supply.

Which will help me in pricing out the right Tank less system.