International Plumbing Code addresses water pipe sizing in Appendix E in the back of the book. There are several methods for water pipe sizing. There is the segmented loss method, which involves a lot of calculations as pressure losses from pipe, valves and fittings are subtracted.
The more basic method is the Selection of Pipe Size method. In this video Plumber-Tom explains the four basic steps for this method, and then demonstrates the use of the method using several examples.
This video is based on the 2021 International Plumbing Code.
