{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"rainwater-harvesting"},"result":{"entity_type":"calculator","id":"rainwater-harvesting","calculator_id":"rainwater-harvesting","canonical_url":"https://tttkmbb.com/energy/rainwater-harvesting","name":"Rainwater Harvesting Calculator","title":"Rainwater Harvesting Calculator – Annual Yield from Roof Area, Rainfall and Runoff Coefficient, Tank Days of Supply","category":"energy","category_name":"Home Energy & Electrical","tool_name":"calculate_rainwater_harvesting","featured_mcp_tool":false,"description":"Computes the rainwater a roof can collect per year from its horizontal area, the annual rainfall and a runoff coefficient, the litres captured per millimetre of rain, and, given a daily demand and tank size, the days of supply a full tank covers and the share of demand the yield meets.","use_when":"You are sizing a rain barrel or cistern, or want to know how much garden or toilet-flushing water a roof can supply.","do_not_use_when":"You need a month-by-month storage simulation with overflow and dry spells, or potable-water treatment sizing; the yield is an annual average and ignores first-flush diversion and evaporation beyond the coefficient.","inputs":[{"name":"roof_area_m2","label":"Roof catchment area","type":"number","unit":"m²","required":true,"max":1000000,"exclusive_min":0,"description":"Horizontal (plan) area of the roof draining to the tank, in square metres; slope does not change the catchment.","example":100},{"name":"annual_rainfall_mm","label":"Annual rainfall","type":"number","unit":"mm","required":true,"min":0,"max":15000,"description":"Average yearly precipitation at the site in millimetres (1 mm = 1 L/m²).","example":600},{"name":"runoff_coefficient","label":"Runoff coefficient","type":"number","required":false,"default":0.8,"max":1,"exclusive_min":0,"description":"Fraction of rain that reaches the tank after evaporation, splash, gutter overflow and first flush: 0.8–0.9 metal or tile, 0.7–0.8 asphalt shingle, 0.5–0.6 green roof.","example":0.8},{"name":"daily_usage_liters","label":"Daily water use","type":"number","unit":"L/day","required":false,"max":1000000,"exclusive_min":0,"description":"Optional daily demand the rainwater should cover (e.g. 200 L for garden and toilets).","example":200},{"name":"tank_size_liters","label":"Tank size","type":"number","unit":"L","required":false,"max":100000000,"exclusive_min":0,"description":"Optional storage volume, to report how many days of use a full tank covers.","example":5000}],"outputs":[{"name":"annual_yield_liters","label":"Annual yield","type":"number","unit":"L","decimals":0,"description":"roof_area_m2 × annual_rainfall_mm × runoff_coefficient."},{"name":"annual_yield_m3","label":"Annual yield (m³)","type":"number","unit":"m³","decimals":2,"description":"annual_yield_liters / 1000."},{"name":"annual_yield_gallons","label":"Annual yield (US gallons)","type":"number","unit":"gal","decimals":0,"description":"annual_yield_liters / 3.785411784."},{"name":"yield_per_mm_liters","label":"Yield per mm of rain","type":"number","unit":"L/mm","decimals":1,"description":"roof_area_m2 × runoff_coefficient: litres collected per millimetre of rainfall."},{"name":"monthly_average_yield_liters","label":"Average monthly yield","type":"number","unit":"L","decimals":0,"description":"annual_yield_liters / 12."},{"name":"days_of_supply","label":"Days of supply from a full tank","type":"number","unit":"days","decimals":1,"description":"tank_size_liters / daily_usage_liters; only when both are given."},{"name":"fraction_of_demand_percent","label":"Share of annual demand covered","type":"number","unit":"%","decimals":2,"description":"annual_yield_liters / (daily_usage_liters × 365) × 100, capped at 100; only when daily usage is given."}],"input_schema":{"type":"object","properties":{"roof_area_m2":{"description":"Horizontal (plan) area of the roof draining to the tank, in square metres; slope does not change the catchment. Unit: m².","type":"number","maximum":1000000,"exclusiveMinimum":0,"examples":[100],"x-unit":"m²"},"annual_rainfall_mm":{"description":"Average yearly precipitation at the site in millimetres (1 mm = 1 L/m²). Unit: mm.","type":"number","minimum":0,"maximum":15000,"examples":[600],"x-unit":"mm"},"runoff_coefficient":{"description":"Fraction of rain that reaches the tank after evaporation, splash, gutter overflow and first flush: 0.8–0.9 metal or tile, 0.7–0.8 asphalt shingle, 0.5–0.6 green roof.","type":"number","maximum":1,"exclusiveMinimum":0,"default":0.8,"examples":[0.8]},"daily_usage_liters":{"description":"Optional daily demand the rainwater should cover (e.g. 200 L for garden and toilets). Unit: L/day.","type":"number","maximum":1000000,"exclusiveMinimum":0,"examples":[200],"x-unit":"L/day"},"tank_size_liters":{"description":"Optional storage volume, to report how many days of use a full tank covers. Unit: L.","type":"number","maximum":100000000,"exclusiveMinimum":0,"examples":[5000],"x-unit":"L"}},"additionalProperties":false,"required":["roof_area_m2","annual_rainfall_mm"]},"output_schema":{"type":"object","properties":{"annual_yield_liters":{"description":"roof_area_m2 × annual_rainfall_mm × runoff_coefficient. Unit: L.","type":"number","x-unit":"L"},"annual_yield_m3":{"description":"annual_yield_liters / 1000. Unit: m³.","type":"number","x-unit":"m³"},"annual_yield_gallons":{"description":"annual_yield_liters / 3.785411784. Unit: gal.","type":"number","x-unit":"gal"},"yield_per_mm_liters":{"description":"roof_area_m2 × runoff_coefficient: litres collected per millimetre of rainfall. Unit: L/mm.","type":"number","x-unit":"L/mm"},"monthly_average_yield_liters":{"description":"annual_yield_liters / 12. Unit: L.","type":"number","x-unit":"L"},"days_of_supply":{"description":"tank_size_liters / daily_usage_liters; only when both are given. Unit: days.","type":"number","x-unit":"days"},"fraction_of_demand_percent":{"description":"annual_yield_liters / (daily_usage_liters × 365) × 100, capped at 100; only when daily usage is given. Unit: %.","type":"number","x-unit":"%"}}},"formula":"annual_yield_liters = roof_area_m2 × annual_rainfall_mm × runoff_coefficient; yield_per_mm_liters = roof_area_m2 × runoff_coefficient; days_of_supply = tank_size_liters / daily_usage_liters; fraction_of_demand_percent = min(100, annual_yield_liters / (daily_usage_liters × 365) × 100)","method":"Standard catchment formula (yield = area × rainfall × runoff coefficient), 1 mm of rain on 1 m² = 1 litre. Actual capture depends on rainfall timing and tank overflow, so a tank sized for 2–4 weeks of demand is a common starting point.","sources":[{"name":"Wikipedia – Rainwater harvesting","url":"https://en.wikipedia.org/wiki/Rainwater_harvesting","type":"reference","retrieved_at":"2026-09-24"},{"name":"U.S. EPA – Soak Up the Rain: Rain Barrels","url":"https://www.epa.gov/soakuptherain/soak-rain-rain-barrels","type":"government","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"100 m² roof, 600 mm/year, 0.8, 200 L/day, 5,000 L tank","inputs":{"roof_area_m2":100,"annual_rainfall_mm":600,"runoff_coefficient":0.8,"daily_usage_liters":200,"tank_size_liters":5000},"expected":{"annual_yield_liters":48000,"annual_yield_m3":48,"yield_per_mm_liters":80,"days_of_supply":25,"fraction_of_demand_percent":65.75},"url":"https://tttkmbb.com/api/v1/calculate/rainwater-harvesting?roof_area_m2=100&annual_rainfall_mm=600&runoff_coefficient=0.8&daily_usage_liters=200&tank_size_liters=5000"},{"name":"150 m² metal roof, 1,200 mm/year, 0.9","inputs":{"roof_area_m2":150,"annual_rainfall_mm":1200,"runoff_coefficient":0.9},"expected":{"annual_yield_liters":162000,"annual_yield_m3":162,"yield_per_mm_liters":135,"annual_yield_gallons":42796},"url":"https://tttkmbb.com/api/v1/calculate/rainwater-harvesting?roof_area_m2=150&annual_rainfall_mm=1200&runoff_coefficient=0.9"}],"faq":[{"q":"Do I use the sloped roof area or the footprint?","a":"The horizontal footprint: rain falls vertically, so a sloped roof catches the same amount as its plan area."},{"q":"Which runoff coefficient should I use?","a":"0.8–0.9 for smooth metal or glazed tile, 0.7–0.8 for asphalt shingles or concrete tile, 0.5–0.6 for green roofs; subtract about 0.05 if a first-flush diverter is fitted."},{"q":"How big should the tank be?","a":"Enough for 2–4 weeks of demand is a typical rule; larger tanks help in climates with long dry spells but overflow unused capacity in wet ones."}],"tags":["rainwater harvesting","rain barrel calculator","roof rainwater yield","rainwater tank size","litres per mm rain"],"related":[{"calculator_id":"volume","reason":"Convert litres, m³ and gallons."},{"calculator_id":"area","reason":"Convert the roof area from ft² to m²."},{"calculator_id":"pool-volume","reason":"Water volume of a pool the tank might top up."}],"links":{"html":"https://tttkmbb.com/energy/rainwater-harvesting","markdown":"https://tttkmbb.com/energy/rainwater-harvesting.md","json":"https://tttkmbb.com/energy/rainwater-harvesting.json","api":"https://tttkmbb.com/api/v1/calculate/rainwater-harvesting","schema":"https://tttkmbb.com/api/v1/calculators/rainwater-harvesting","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:53:28Z","next_actions":[{"tool":"run_calculator","calculator_id":"rainwater-harvesting","reason":"Run Rainwater Harvesting Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/energy/rainwater-harvesting.md"}}