Assess rainwater harvesting potential

Work out how much water your roofs can actually deliver, month by month, before you design gutters or tanks.

How

Measure the roof footprint (plan area as seen from above), not the sloped roof surface. Only areas drained by gutters count.
Get long-term monthly rainfall for the nearest weather station from your national meteorological service. The Texas manual advises using median rather than average monthly rainfall, because a few big storms push averages up.
Apply the rule: 1 mm of rain on 1 square metre gives about 1 litre (the Texas manual gives the US equivalent of 0.62 gallons per square foot per inch).
Multiply by a collection efficiency. Installers typically assume 75 to 90 percent because of first flush, splash, evaporation and overflow. Textured tile and shingle roofs lose roughly 10 percent more than smooth metal.
Use the rainwater harvest calculator on this page, then compare each month against your monthly demand from water-2.
Note the longest run of dry months. That gap sets your storage size.