The following is based on information from “Making a whole-house reverse osmosis installation” by Jeff Twitchell in the Sept. 2004 print issue of Water Technology magazine.
The usual assumption is that in a household each person will consume about 75 gallons of water per day. Therefore, you would assume a usage need of about 300 gallons per day for a family of four.
The RO unit itself should be sized so that it produces at least triple the daily usage rate. This allows the unit to run only about 8 hours per day and provides a margin for error. Many features can reduce the estimated production. Cold water, high salt rates, and hardness are common variables that lower production rates.
The storage tank should be at least half as large as the daily household consumption.
These are not hard, fast rules. They are merely suggestions.
(whole house & well units)