Estimates how much rain your roof catchment collects, compares it to household and lawn irrigation demand, and shows how many days your storage tank can cover using the standard area × rainfall × runoff-coefficient method.
Quick Facts
Harvest formula
Volume = Area × Effective rainfall × Runoff coefficient
1 mm of rain over 1 m² of catchment yields 1 liter before losses; a 0.75-0.9 coefficient is typical for a pitched composite-shingle roof.
Rainwater Reserve Outputs
Water Budget
Annual Harvest
0 L
Rain captured off the catchment area per year
Daily Demand
0 L
Household + irrigation use per day
Tank Autonomy
0 days
Days the full tank covers demand in the dry season
Annual Coverage
0%
Share of yearly demand met by harvested rain
Water Budget Components
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How to use this calculator
Enter your roof or catchment area, local average annual rainfall, and an estimated runoff coefficient, then add your storage tank size and typical household and irrigation water use. Click Calculate to see your estimated annual rainwater harvest, daily water demand, tank autonomy, and the share of your yearly water needs that rainwater can cover. Click Reset to restore the default suburban example.
Understanding the inputs
Catchment/roof area is the horizontal footprint feeding your gutters, in square meters. Average annual rainfall is your local yearly total in millimeters — check a nearby weather station or climate normal. The runoff coefficient (0 to 1) is the fraction of rain that actually reaches the tank after losses to roof absorption, evaporation, and gutter overflow, typically 0.75-0.9 for a pitched roof. First-flush diversion is the depth of debris-laden runoff discarded before each rain event, commonly 1-3 mm. Storage tank size is your usable capacity in liters. Household and irrigation water use describe your baseline daily and weekly consumption, and dry-season length estimates how many months of the year irrigation demand rises because rainfall is scarce.
Interpreting the results
Annual Harvest is the rainwater your roof is expected to collect over a year, from Volume = Area × Effective rainfall × Runoff coefficient. Daily Demand combines household and irrigation use into a single daily liter figure. Tank Autonomy shows how many days a full tank could supply that demand during the dry season before running dry. Annual Coverage is the share of your total yearly water demand that harvested rainwater could offset — a figure under 100% means you still need a supplemental water source for part of the year.
Frequently Asked Questions
What formula does this rainwater reserve calculator use?
Annual harvest volume follows the standard rainwater harvesting formula: Volume = Catchment area (m²) × Effective rainfall (mm) × Runoff coefficient. Because 1 millimeter of rain over 1 square meter equals 1 liter, the area and rainfall inputs combine directly into liters, and the runoff coefficient accounts for water lost to evaporation, roof absorption, and overflow before it reaches the tank.
What runoff coefficient should I use?
The runoff coefficient is the fraction of rainfall that actually reaches your storage tank instead of being lost to evaporation, roof absorption, or gutter overflow. Smooth, impervious roofing such as metal, tile, or asphalt shingle typically runs 0.75-0.9; the calculator's 0.82 default is a reasonable mid-range estimate for a typical suburban composite-shingle roof.
Why does first-flush diversion reduce the rainfall total?
The first flush of runoff from any storm carries dust, pollen, and roof debris, so it is normally diverted away from the tank rather than stored. This calculator assumes roughly 12 significant rain events a year (about one per month) and subtracts your first-flush depth for each one before applying the runoff coefficient.
How is tank autonomy calculated?
Tank autonomy is the storage tank's volume divided by an estimated daily demand — household use plus irrigation use converted to a daily rate — with a modest increase applied for the length of your dry season to reflect higher irrigation needs when rain is scarce. It estimates how many days a full tank can supply your household and lawn without additional rainfall.