Rainwater harvesting calculator
This rainwater harvesting calculator estimates how much rain your roof collects in a year, how much water you could use for toilet flushing, laundry and the garden, and how big your tank should be. The Texas Water Development Board puts the yield at about 0.62 gallons per square foot of roof per inch of rain, less 10 to 25% losses.
Source: Texas Water Development Board – The Texas Manual on Rainwater Harvesting, 3rd ed. 2005, ch. 4 (0.62 gal per ft² per inch, efficiency 75–90 %; read 09.10.2026). Updated: .
How it is calculated
A 1,500 ft² roof in an area with 30 inches of rain a year collects about 23,715 gallons at 85% efficiency. Four people flushing toilets use about 13,100 gallons and 1,000 ft² of garden another 2,480 – a tank of roughly 940 gallons holds about three weeks of supply.
How the rainwater harvesting calculator works
- Yield: rainfall (inches) × roof footprint (ft²) × 0.62 gallons × efficiency, as in the Texas Manual on Rainwater Harvesting. The footprint counts, regardless of roof pitch. Most installers assume 75 to 90% efficiency because of first flush, evaporation, splash-out and overflow. In metric units 1 mm of rain on 1 m² is 1 litre.
- Household demand: German Federal Environment Agency (UBA) data show 126 litres per person per day, 27% of it for toilet flushing (34 l) and 12% for laundry (15 l).
- Garden demand: irrigation depth × area × 0.62 gallons; a German municipal guide gives 80 to 200 litres per m² a year (about 3 to 8 inches).
- Tank size: the smaller of yearly yield and yearly demand × 0.06 – about three weeks of supply.
Examples
- 1,500 ft², 30 in, 85%, 4 people (toilets), 1,000 ft² garden at 4 in: 23,715 gal collected, 15,601 gal used → tank about 936 gallons.
- Texas Manual example: 1.97 in of rain on 2,500 ft² at 85% = 2,595 gallons in one month.
- 150 m² roof, 800 mm, 0.8, 4 people (toilets), 100 m² garden: 96,000 l collected, 59,669 l used → tank about 3,580 litres.
To measure a roof, use the roof area calculator – for the yield only the footprint counts. For other volumes see the pool volume calculator.
Limits
The 6% rule is a rule of thumb for homes. Where rain falls in a few months, the Texas manual recommends a month-by-month balance so the tank lasts through the longest dry spell. Local rainfall varies a lot – use figures from your national weather service. Drinking water use needs treatment and is not covered.
Frequently asked questions
How much rainwater can I collect from my roof?
About 0.62 gallons per square foot per inch of rain in theory; with 85% efficiency, 1,000 ft² and 1 inch give about 527 gallons.
What size rainwater tank do I need?
A rule of thumb is the smaller of yearly yield and yearly demand × 0.06, about three weeks of supply. For 4 people using it for toilets plus 1,000 ft² of garden that is roughly 940 gallons.
Does the roof pitch matter?
No. The collection area is the footprint the roof covers, regardless of pitch (Texas Manual on Rainwater Harvesting).
How efficient is rainwater collection?
Most installers assume 75 to 90%. Water is lost to first flush, evaporation, splash-out and overflow when the tank is full.
How much water does toilet flushing use?
German data show about 34 litres (9 gallons) per person per day, 27% of household use. Efficient toilets use less.
How many litres per square metre per mm of rain?
One litre: 1 mm of rain on 1 m² is exactly 1 litre. A 150 m² roof with 800 mm of rain receives 120,000 litres before losses.
Sources and legal basis
- Texas Water Development Board – The Texas Manual on Rainwater Harvesting, 3rd ed. 2005, ch. 4 (0.62 gal per ft² per inch, efficiency 75–90 %; read 09.10.2026)
- Markt Aidenbach – Hinweise zur Regenwassernutzung (Archiv 2021): Nutzvolumen = jährlicher Bedarf × 0,06, Garten 80–200 l/m² (gelesen 09.10.2026)
- Umweltbundesamt – Wassernutzung privater Haushalte, Trinkwasserverwendung im Haushalt 2023 (126 l/Tag, WC 27 %, Wäsche 12 %; Stand 10.09.2024, gelesen 09.10.2026)
- BauNetz Wissen – Regenwassernutzung (Abflussbeiwert geneigter Dächer 0,8–0,9) und Glossar Abflussbeiwert (Gründach 0,3 nach FLL), 2026
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