Rainwater harvesting calculator
Enter your roof area and the rain, get the water you can collect.
How much water can I collect from my roof?
Each inch of rain on each square foot of roof is 0.623 gallons: 144 cubic inches of water, and a gallon is 231. A 1,000 ft² roof in a 1-inch rain yields about 623 gallons.
What about metric units?
Simpler: 1 millimeter of rain on 1 square meter is exactly 1 liter. 10 mm on 100 m² is 1,000 liters.
Which roof area do I use?
The footprint the roof covers — its area seen from above — not the sloped surface, because rain falls vertically.
Will I really catch all of it?
No — some water splashes, evaporates, soaks into the roof or overflows gutters. Set "Water collected" below 100% to allow for that.
How big a rain barrel do I need?
Compare the yield of a typical storm with the barrel size: a standard 55-gallon barrel fills from about a tenth of an inch of rain on a 1,000 ft² roof, 0.088 inch if every drop is caught and 0.104 inch at 85%.
What collection efficiency should I use?
The Texas Water Development Board's rainwater training material applies a collection efficiency factor, generally 0.85, to allow for overflow and splashing out of the gutters. To use it, set “Water collected” to 85%. Your own roof and gutters may do better or worse, so adjust it.
How much water should the first flush divert?
A first-flush diverter sends the first rain, which carries dust, bird droppings, leaves and other residue off the roof, away from the tank. The Texas Water Development Board gives a rough approximation of 10 gallons per 1,000 square feet of roof. That water is not part of the yield.
How the volume is found
One inch of rain over one square foot is 144 cubic inches of water. A US gallon is 231 cubic inches, so the factor is 144 ÷ 231 = 0.6234 gallons, or 2.36 liters. In metric, 1 millimeter over 1 square meter is exactly 1 liter. The area is the roof's footprint, because rain falls straight down onto the horizontal projection of the roof and a steeper pitch does not collect more. The collection percentage then multiplies the total. The Texas Water Development Board rounds the factor to 0.62, so its figures run a half percent below these.
Worked example
The Texas Water Development Board's training example is a 40 ft × 40 ft roof in a 1-inch rain: 1,600 ft² × 0.6234 = 997 gallons at 100% (the board's rounded factor gives 992). With its 0.85 collection factor that becomes 848 gallons (the board shows 843). For a year, a 2,000 ft² roof that gets 30 inches of rain collects 2,000 × 30 × 0.6234 × 0.85 = 31,792 gallons. A first-flush allowance of 10 gallons per 1,000 ft² would take 16 gallons off the 1,600 ft² roof's first storm.
Common mistakes
Measuring the sloped surface of the roof overstates the yield. Treating 100% as realistic overstates it again, which is why the training material applies 0.85. A yearly total is not a usable supply by itself, because rain arrives in storms and the size of the tank and the length of the dry spells decide how much of it can be used. Mixing unit systems is the last trap: an inch is 25.4 millimeters, so switch the calculator to metric before entering square meters and millimeters.
Sources: Texas Water Development Board, “Rainwater Harvesting Training” (2013 slides: 0.62 gallons per square foot per inch, collection efficiency 0.85, first-flush rule of thumb); the factor 144 ÷ 231 follows from the US gallon of 231 cubic inches. Related: gutter calculator, roof calculator, tank volume calculator, pond calculator. How we check our calculators.