Rainwater Harvesting Calculator
Find out how much rain your roof can actually capture, how big a tank you need to get through a dry spell, and what it saves on your water bill.
Based on 0.623 gallons per square foot per inch of rain. Tank sizing covers a four-week dry spell at your stated demand.
How the math works
gallons/year = roof sq ft × rainfall inches × 0.623 × efficiency
Frequently asked questions
How many gallons can I collect from my roof?
Every inch of rain on 1,000 square feet of roof yields about 623 gallons before losses. Real-world collection efficiency is 75 to 90% after first-flush diversion, splash and evaporation, so a 1,500 sq ft roof in a 38-inch rainfall area realistically collects around 30,000 gallons a year.
What size rain barrel or tank do I need?
Size for the dry spell, not the annual total. A practical rule is enough storage to cover your longest typical gap between rains. This calculator sizes for a four-week dry period at your stated garden demand.
Is roof area the footprint or the sloped surface?
Use the footprint — the area the roof covers as seen from above. Pitch does not change how much rain lands on it, so a steep roof and a flat roof with the same footprint collect the same volume.
Is harvested rainwater safe for vegetables?
For soil-level irrigation it is widely used and generally fine. Use a first-flush diverter, keep the tank opaque and sealed against mosquitoes, and avoid overhead watering of leafy greens close to harvest. Do not drink it without proper treatment, and check local regulations — a few states restrict collection.
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