620 gallons of water fall on the roof of a building that has a 1000 sf footprint when one inch of rain falls. (0.62 gallons per sqft)

If you have a 2000 sf home and a 500 sf garage, that totals 2500 sf and can produce 1,550 gallons of water. If you get 15" of rain per year, that's 23,250 gallons per year, almost 16 gallons of water per person per day for a family of four. Seattle averages 37" of rain a year, 57,350 gallons.

An above-ground swimming pool with a 15' diameter (4' deep) holds about 4,646 gallons. An oval in-ground pool 18x33' = 12,267 gallons. Keep in mind that you don't need a tank that holds your entire annual water needs, as the rain falls periodically, and you're using it regularly, so you mainly need to supply your family through the longest usual dry spell of the year.

Most people let it rain well for about 15 minutes before they start collecting, to clean off the roof. Rainwater is 20 times cleaner than the cleanest groundwater -- people panic that it hasn't been purifed, but they'll drink water from their well without testing it for anything but a reasonable fecal coliform count (if that).

A family of four with a 2-gallon toilet tank will flush away almost 12,000 gallons of fresh drinking-quality water per year, almost half the total you could collect in SoCal with 2500 sf of roof. If that's not waste, nothing is. If you haven't already, read Joseph Jenkin's detailed free online guide The Humanure Handbook

There are two books by Art Ludwig, one for rainwater collection and one for intelligent greywater use that everyone should read:

Water Storage: Tanks, Cisterns, Aquifers, and Ponds For Domestic Supply, Fire and Emergency Use (includes directions on how to make ferrocement water tanks).

The New Create an Oasis with Greywater: Choosing, Building and Using Greywater Systems — Includes Branched Drains

He has other books on his site: http://oasisdesign.net/

Sue