Akmed, YOU are 100% correct.
Unfortunately an apprentice doesn’t get to learn pipe sizing until their 5th year of schooling (hopefully).
The 1st 4 years are just BASIC tools and how to learn to read a ruler and possibly some very basic code and cleaning fittings for the mechanic and carrying tools and unloading trucks delivering materials.
One Master plumber did lose his license for allowing an apprentice to go out alone and wow what trouble this kid caused just doing a simple toilet stoppage.
Knowing that “helpers” do have trouble with basic pipe sizing and do not know about Pi (3.14159265) R sq. length divided by 231 if in inches and multiplied by 7.48 if in feet.
So to KEEP IT SIMPLE (for) STUPID I use the KISS system when dealing with the schleppers in my shop.
So we use D D 6 and this works out the same as we honestly dont want to discourage helpers from tying to learn the very basics.
For example One 2″ pipe will equal the same AREA as four 1″ pipes
1 1 6 = 6 2 2 6 = 24 24 divided by 6 = 4 TA DUM
I have no idea where the 5th pipe came from SO lets try it another way huh?
3.1416 R sq 1″ pipe = .7854
2″ pipe R sq 3.1416 .1416
3.1416 divided by .7854 = 4 FOUR
no place do we find FIVE pipes
Only in a criminal mind do they arbitrarily look into the sky searching for a number.
Under NORMAL job conditions we would be conserative and figure 3 pipes NEVER more then the actual FU demand as we do take not only hydrostatic pressures and friction losses BUT the “Force” to over come the weight of water (in high rise buildings especially)
When a Journeyman plumber is asked to size a buildings water supply most will actually OVER size the piping one comerical size then actually required to allow for future fixture demands . For example if the piping demands are for a 21/2 ” supply we may go to 3″ on the origional install.
NEVER say 4 is good lets figure 5 then a 3rd year apprentice would come along and say HEY LETS make it 7 pipes equil the same volume.
Volume in pipe sizing is not 2 + 2 = 5 it doesnt work that way.
Here this may help http://www.masterplumbers.com/plumbing/plumbviews/1999/watersupply.html