Rainwater harvesting calculator: yearly litres or gallons a roof collects from its area, rainfall and efficiency, and the share of household use it covers.
By AdminDrafted with AI writing toolsUpdated Checked examples: 5
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.
Multiply the roof's footprint by the yearly rainfall and a collection efficiency: liters = m² × mm × efficiency, because 1 mm of rain on 1 m² is 1 liter. A 100 m² roof under 800 mm of rain a year at 0.85 efficiency collects 68,000 L (68 m³), about 186 L a day on average.
In US units, gallons = ft² × inches × 0.623 × efficiency: an inch of rain on a square foot of roof is 0.623 US gallons.
Metric: 100 m² × 800 mm = 80,000 L of rain lands on the roof. At 0.85 efficiency, 68,000 L reach the tank, an average of 5,667 L a month or 186.3 L a day.
US units: 2,000 ft² × 32 in × 0.623377 = 39,896 US gallons of rain. At 0.85, the roof collects 33,912 gallons.
The Texas Water Development Board manual rounds the factor to 0.62, which gives 33,728 gallons for the same roof, 184 fewer.
For quick estimates, each millimeter of rain on a 100 m² roof is 100 L before losses, and each inch on 1,000 ft² is 623 gallons.
Rain falling straight down lands on the roof's horizontal footprint, so a steeper pitch adds no catchment. A roof whose eaves enclose 10 m × 10 m catches rain on 100 m², flat or steep.
Count only the area whose gutters feed the tank. If half the roof drains elsewhere, use half the footprint.
Efficiency, also called the runoff coefficient, is the share of the rain on the roof that reaches the tank. The Texas manual says most installers assume 75% to 90%, a coefficient of 0.75 to 0.90.
Water is lost to first-flush diversion, splash-out from gutters, evaporation, leaks and overflow in heavy storms. Use the low end for composite or asphalt shingle roofs, which the manual puts at about a 10% loss.
A first-flush diverter that discards 1 to 2 US gallons per 100 ft² of roof throws away the first 0.4 to 0.8 mm of every storm, so frequent light rain loses a larger share.
| Scenario | Annual liters | Annual US gallons | Average liters/day |
|---|---|---|---|
| Default | 68000 | 17964 | 186.3 |
| Default roof, 0.75 efficiency | 60000 | 15850 | 164.4 |
| Default roof, 0.90 efficiency | 72000 | 19020 | 197.3 |
| 100 m², 600 mm, 0.80 efficiency | 48000 | 12680 | 131.5 |
| 2000 ft², 32 inches, 0.85 efficiency | 128370 | 33912 | 351.7 |
The first row is the 100 m² roof under 800 mm at 0.85 efficiency. The next two rows change only the efficiency: 0.90 collects 20% more than 0.75 from the same roof and rain.
The last row uses US units: 2,000 ft² under 32 inches of rain collects 128,370 L, or 33,912 US gallons.
Divide the yearly harvest by yearly use. A household using 300 L a day uses 109,500 L a year, so the 68,000 L roof covers 62.1% of it.
The Texas manual puts a water-conserving household at 25 to 50 US gallons, about 95 to 189 L, per person per day. Use the whole household's daily total in the comparison.
Coverage above 100% means a yearly surplus on paper. How much of it you can use depends on storage.
BS EN 16941-1, the European standard for rainwater-use systems, covers non-potable supply. Drinking water is a separate question of treatment.
The yearly figure is an average. Rain arrives in bursts, so a tank overflows in wet months and runs dry in long dry spells.
The Texas manual's rule is to store enough for the longest expected spell without rain. At 300 L a day, a 30-day dry spell needs about 9,000 L of storage; a smaller tank captures less than the yearly figure.
To turn liters into US gallons, use the liters to gallons converter. For another household resource, the carbon footprint calculator adds up yearly emissions.
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
Checked against: 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.
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