Rainfall Collection Calculator
Rainwater harvesting calculation applies hydrologic mass conservation to determine harvestable water runoff yields from building catchments (Volume = Area × Rainfall × Runoff Coefficient × Filter Efficiency). Whether sizing off-grid residential cistern tanks, planning permaculture irrigation systems, or fulfilling municipal stormwater retention mandates, this calculator models capture volumes and recommended storage capacity.
Horizontal footprint area of the roof catchment surface in square metres (m²)
Total rainfall depth for the target period (e.g., 100 mm monthly or annual precipitation)
Material runoff coefficient accounting for surface texture and initial wetting loss
Efficiency of gutter first-flush diverters and leaf filtration screens (typically 85–95%)
Total net harvestable water yield in Litres (1 mm rain on 1 m² = 1 Litre)
How to Use This Calculator
- Enter the roof footprint catchment area in square metres (use the flat horizontal plane, not the pitched slope area).
- Enter the expected monthly or annual rainfall depth in millimetres.
- Select your roofing material to apply the appropriate runoff coefficient.
- Review total harvestable volume in Litres, US Gallons, and recommended cistern storage capacity.
Formula & Calculation Methodology
One millimetre of rain falling across one square metre of horizontal surface deposits exactly one Litre of water (0.001 m × 1 m² = 0.001 m³ = 1 L). Net recovery scales by the catchment material runoff coefficient and mechanical filtration efficiency.
Worked Example
A 150 m² metal roof receiving 100 mm of rain with a 90% efficient first-flush filter
Frequently Asked Questions
Why is horizontal roof footprint used instead of true sloped roof surface area?↓
Rain falls vertically under gravity. The horizontal projection (footprint) represents the true cross-sectional area intercepting downward vertical raindrops, regardless of roof pitch or slope steepness.
What is a first-flush diverter in rainwater harvesting?↓
A first-flush diverter routes the initial 1 to 2 mm of rainfall containing atmospheric dust, bird droppings, and roof pollen away from the main cistern into a separate drain before clean water is captured.
Can harvested rainwater be used for drinking water?↓
Untreated roof runoff is suitable for landscape irrigation, washing, and toilet flushing. For potable drinking use, it must undergo multi-stage sediment filtration, activated carbon treatment, and UV disinfection or chlorination.
What limitation applies during extreme cloudburst downpours?↓
During intense torrential storm events (e.g. > 50 mm/hour), gutter conveyance capacities and overflow weirs may be overwhelmed, spilling water before it reaches the cistern and temporarily reducing effective collection efficiency.
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Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.