By looking at the average rainfall levels in your area, you can figure out how much rainwater you can expect to collect per year and whether that will meet your family's water needs. An average household with newer plumbing fixtures such as 1.6-
gallon-per-flush toilets and 2.5-gallon-per-minute showerheads uses roughly 55 gallons per person per day.
In general, 1,000 square feet of roof will collect 600 gallons of water for every inch of rain that falls. Remember that actual rainfall amounts fluctuate with the seasons, while household needs remain reasonably steady.
If you're depending on rainwater for all or most of your water needs, your tanks have to be big enough to get you through the dry spells. Plus, you'll need a fair amount of space to house those tanks. Be sure to follow municipal restrictions on tank placement.