{"id":"rainwater-collection-calculator","name":"Rainwater Collection Calculator — Roof Yield and Storage","shortName":"Rainwater Collection Calculator","canonicalUrl":"https://calcurhome.com/calculators/rainwater-collection-calculator","catalogUrl":"https://calcurhome.com/api/calculators","category":{"id":"landscaping","name":"Landscaping","url":"https://calcurhome.com/calculators/category/landscaping"},"language":"en-US","audience":"US homeowners, DIYers and home-project planners","primaryKeyword":"rainwater collection calculator","secondaryKeywords":["rain barrel calculator","roof rainwater harvesting calculator","rainfall gallons calculator","cistern size calculator","rainwater catchment calculator"],"description":"Estimate harvestable rainwater gallons from roof area, rainfall, runoff and collection efficiency. Compare event yield with planned storage capacity.","directAnswer":"One inch of rain on one square foot equals about 0.623 gallon. Multiply roof footprint by rainfall, then apply runoff and collection efficiency. Storage limits the captured amount. This is a planning estimate; potable use, overflow routing, mosquito control, roof materials, and permits need local verification.","inputs":[{"key":"length","label":"Roof footprint length","unit":"ft","defaultValue":40,"min":1,"max":1000,"step":1},{"key":"width","label":"Roof footprint width","unit":"ft","defaultValue":30,"min":1,"max":1000,"step":1},{"key":"rainfall","label":"Rainfall depth","unit":"in","defaultValue":1,"min":0.01,"max":100,"step":0.1},{"key":"runoff","label":"Roof runoff coefficient","unit":"%","defaultValue":90,"min":1,"max":100,"step":1},{"key":"efficiency","label":"Collection efficiency","unit":"%","defaultValue":90,"min":1,"max":100,"step":1},{"key":"storage","label":"Available tank capacity","unit":"gal","defaultValue":500,"min":1,"max":1000000,"step":10}],"formula":{"title":"Roof rainwater yield formula","expression":"Potential gallons = roof footprint (ft²) × rainfall (in) × 0.623 × runoff coefficient × collection efficiency; captured gallons = lesser of yield or available storage.","explanation":"The 0.623 factor converts one inch of water over one square foot to U.S. gallons. Separate coefficients account for roof runoff losses and collection-system losses before comparing the event yield with empty tank capacity."},"measurementGuide":["Measure the horizontal roof footprint feeding the selected gutters, not the sloped roof surface area.","Divide complex roofs into rectangles and include only sections connected to the planned storage system.","Use a local rainfall depth for the design event or period being evaluated, not an annual total by default.","Subtract water already in the tank from total capacity before entering the available storage value."],"buyingTips":["Choose opaque, screened storage designed to exclude sunlight, debris, animals, and mosquitoes.","Size gutters, downspouts, first-flush devices, overflow, and erosion protection for peak flow as well as volume.","Verify that the roof, coating, fittings, and tank are suitable for the intended nonpotable or potable use.","Provide a stable foundation and protected overflow path before filling a large tank that becomes extremely heavy."],"workedExample":{"title":"One-inch rain on a garage roof","scenario":"A 40 ft by 30 ft roof receives 1 in of rain with 90% runoff, 90% efficiency, and 500 gal of empty storage.","result":"Potential yield is about 606 gal; the tank captures 500 gal and roughly 106 gal requires managed overflow."},"commonQuestions":[{"question":"How many gallons can one inch of rain produce?","answer":"One inch falling on one square foot is approximately 0.623 U.S. gallon. Therefore, a 1,000-square-foot catchment has a theoretical yield near 623 gallons. Real collection is lower because of roof wetting, first flush, gutter loss, leaks, splash, and system inefficiency."},{"question":"Should I use roof area or roof footprint?","answer":"Use the horizontal footprint of the roof sections draining to the collection system. Rainfall depth is measured on a horizontal plane, so using sloped surface area overstates yield. Split hips, valleys, additions, and disconnected gutter sections into separate footprint shapes when necessary."},{"question":"Can collected rainwater be used for drinking?","answer":"Do not assume roof runoff is potable. Drinking-water use can require approved roofing materials, treatment, testing, cross-connection protection, permits, and ongoing maintenance. Requirements vary widely, so consult local health and plumbing authorities before designing any system for consumption or indoor connections."},{"question":"How large should a rainwater tank be?","answer":"Tank sizing should balance catchment yield, local rainfall timing, intended demand, available space, structural support, budget, and overflow frequency. Event yield is only one input. A monthly water-balance analysis provides a better storage decision where irrigation reliability or drought carryover matters."}],"sources":[{"name":"USDA Natural Resources Conservation Service — Rainwater Harvesting","url":"https://www.nrcs.usda.gov/sites/default/files/2022-09/Texas_Conservation_in_Your_Backyard_Rainwater_Harvesting.pdf","note":"Federal planning reference for estimating, collecting, storing, and using harvested rainwater."}],"defaultCalculation":{"inputs":{"length":40,"width":30,"rainfall":1,"runoff":90,"efficiency":90,"storage":500},"result":{"value":"500","unit":"US gallons","label":"Rainwater that can be captured","lines":[{"label":"Roof footprint","value":"1,200 sq ft"},{"label":"Potential event yield","value":"606 gal"},{"label":"Estimated overflow","value":"106 gal"}],"summary":"The entered event can yield about 606 gallons; available storage captures up to 500 gallons.","note":"Planning estimate only. Actual yield varies with first-flush diversion, roof wetting, leaks, debris, tank starting level, and rainfall distribution."}},"reviewedOn":"2026-07-24","disclaimer":"Planning estimate only. Verify product coverage, supplier rounding, local codes and structural requirements before purchase or construction."}