The formula
One millimetre of rain on one square metre is exactly one litre. The runoff coefficient allows for water that splashes off, evaporates or soaks in.
Worked example
A 20 m² metal shed roof with 800 mm of rain a year: 20 × 800 × 0.9 × 0.9 = about 13,000 L a year, or roughly 1,080 L a month on average.
Sizing the tank
A common rule of thumb is about 5% of annual yield, or roughly 18 days of average supply. Larger tanks help where rain is seasonal. Untreated rainwater isn't safe to drink without proper filtration and disinfection. Check local rules on collection, which vary by US state.
Frequently asked questions
How much rainwater can I collect from my roof?
About 1 litre per square metre per millimetre of rain, less 10–30% losses. In US units: about 0.62 US gallons per square foot per inch of rain, before losses.
How big should my rainwater tank be?
Roughly 5% of the annual yield is a common starting point, or enough for 2–3 weeks of your use.