Water supply line – GreenBuildingAdvisor

To size your water supply:

Go to the international plumbing code: https://codes.iccsafe.org/content/IPC2021P3

See Appendix E.

Add up your fixture units for your house from Table E103.3(2). Either convert the wsfu to GPM using Table E103.3(3) and then use one of Figures E103.3(2) thru (7) to determine the minimum pipe size by keeping the velocity at 5fps or less Or use Table E104.1. These sections explain how to do the sizing in more detail.

Usually doesn’t hurt to go a size larger than what you need but it will cost more money.

If you’re hiring a plumber they will usually do this kind of thing for you. When I size a system I detail out every item and pressure drop in an excel spreadsheet that matches with the design. Of course you almost never go with a smaller pipe size for obvious reasons.