Your home’s (or other building’s) footprint determines its rainwater collection potential, not the square footage of its roof, because only horizontal area matters. For 1 inch of rain, 623 gallons of water can potentially be collected from a 1,000-square-foot horizontal roof area. But some loss occurs in even the best of systems, so 90% efficiency is considered a maximum.
First, consider whether it is possible, given your family’s water consumption (look at a water bill or make an estimate based on plumbing fixtures), to live off your area’s annual rainfall. Two water-conscious people will consume about 70 gallons per day. If they were living in a home with 1,000 square feet of horizontal roof area, they would need about 1 inch of rain per week (7 days x 70 gallons = 490 gallons per week consumed versus 561 gallons collected). It’s possible to increase collection area by harvesting from nearby outbuildings as well.
Of course, all of this rain might not fall evenly throughout the year. In many areas, much of the rainfall comes only during a few months, so even if there is enough annual rainfall overall, collecting it and having enough storage capacity can be a challenge. Generally, you need to have a tank that is large enough to tide you over through your climate’s three driest months. To calculate this, find your area’s median rainfall amounts (from the National Oceanic and Atmospheric Administration’s website) for the three driest months and assume two-thirds of that rainfall.
In three months, at 35 gallons per day each, two adults will consume 6,300 gallons of water. If supply during the dry months could be as low as 1,000 gallons, for example, then a 5,300-gallon tank would be the minimum you should consider installing. More storage means less risk of running out of water. Assuming two-thirds of normal rainfall might not be sufficient—Austin received only one-third of its median rainfall during its recent drought. So, the more people in a household and the longer the dry spells in your climate, the larger the tank. The more efficient your plumbing fixtures (and water use) and the greater your collection area, the smaller the tank can be. Polyethylene tanks are the least expensive and are used in most whole-house rainwater systems.