Rainwater harvesting calculator: yearly litres or gallons a roof collects from its area, rainfall and efficiency, and the share of household use it covers.
A 100 m² roof with 800 mm of rain a year yields about 68,000 litres (68 m³) at 85% efficiency — 186 litres a day on average.
One millimetre of rain on one square metre is one litre.
The yearly harvest is roof footprint area × rainfall × collection efficiency. One millimetre of rain on one square metre is one litre, and one inch on one square foot is 0.623 US gallons. The efficiency, or runoff coefficient, removes the water lost to first-flush diversion, splash-out from gutters, evaporation and overflow.
Homeowners use it to size a rain barrel or tank and to see how far the water goes. The default roof of 100 m² under 800 mm of rain at 0.85 efficiency collects 68,000 litres a year, 186 litres a day on average, which is 62% of a household using 300 litres a day.
The result is a yearly average. Rain arrives in bursts, so a full tank overflows in wet months and runs dry in long dry spells; tank size decides how much of the average you can actually use.
Checked against: 1 mm on 1 m² = 1 L, so 100 × 600 × 0.8 = 48 000 L by hand
Checked against: TWDB manual: about 0.62 gal per ft² per inch; exact 1000 × 0.09290304 m² × 25.4 mm / 3.785411784 L = 623.377 gal (Python decimal)
Checked against: Python decimal: 2000 ft² × 32 in × 0.85 = 33 911.7 US gal (the TWDB 0.62 rule of thumb gives 33 728)
Checked against: Definition: V = A × R × 0
Multiply the roof footprint by the rainfall and a collection efficiency. Each millimetre of rain on a square metre gives one litre, and each inch on a square foot gives 0.623 US gallons, which the Texas Water Development Board rounds to 0.62. A 2,000 ft² roof under 32 inches of rain at 85% efficiency collects about 33,900 US gallons a year.
Between 0.75 and 0.90 for most systems: the Texas Water Development Board manual says most installers assume 75 to 90%. Use the lower end for composite or asphalt shingle roofs, which the manual puts at about a 10% loss, and where a first-flush diverter discards the first 1 to 2 US gallons per 100 ft² of roof in every storm.
The footprint: the horizontal area the roof covers, length × width from eave to eave, whatever the pitch (Texas Water Development Board). A steep roof whose eaves enclose 10 × 10 m has a 100 m² catchment, the same as a flat one. If only some gutters feed the tank, count only the area they drain.
The Texas Water Development Board puts a water-conserving household at 25 to 50 US gallons (95 to 189 litres) per person per day, and cites a 1999 American Water Works Association study in which water-conserving homes averaged 49.6 gallons. Enter your household total under More options: a 300 litre-a-day household gets 62% of its use from the default 100 m² roof.
Enough to carry the household through the longest expected spell without rain, which is the rule in the Texas Water Development Board manual. At 300 litres a day, a 30-day dry spell needs about 9,000 litres of storage. A tank much smaller than that spills in wet months and runs dry between storms, so it captures less than the yearly figure shown.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “100 m² roof, 600 mm of rain, 0.8 efficiency” is checked against 1 mm on 1 m² = 1 L, so 100 × 600 × 0.8 = 48 000 L by hand.
Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd ed. (2005), ch. 4 — 0.62 gal per ft² per inch; 75–90% efficiency; BS EN 16941-1:2018 — On-site non-potable water systems, part 1: systems for the use of rainwater.
5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
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