What are basic creterial in design of water supply ?

Based on fixture units you add up the total fixture units then based on diversity factor you find out the gpm or flow rate and then size the pipe and find the friction factor for material then you find out frction loss in fittings and find the head,for sizing reserviour you find out the number of user and based on uniform plumbing code book you find out the gallons required for the same. and for sizing pipe please refer uniform plumbing book code