Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
How do you work it out? thanks guys
Don't. It's a trap. Tell the builder to ask the architect and/or engineers who designed the building to provide the figures they need.
If you provide the builder with a figure you've got from an internet forum or a rule of thumb that turns out to be wrong you may end up liable for the remedial work, which for a block of flats could bankrupt you.
Absolutely agree all this should be provided on the design plans size of water main pipe runs location of water tanks etc. Have you seen the drawings for the site ? Have you got a copy ? Do you use a Scale rule to read the drawings ? All this information should be given to you if you are quoting for the work.
Reply to the thread, titled "water usage" which is posted in USA Plumbers Advice on Plumbers Forums.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.