Rainwater Harvesting Calculator
Calculate annual rainwater yield from your roof, recommended storage tank size, and how many days of household water demand you can offset. Tuned for major Nepali cities or use your own rainfall figure.
How yield is calculated
Annual yield (litres) = roof area (m²) × rainfall (mm) × runoff coefficient. One millimetre of rain falling on one square metre of catchment equals one litre. The coefficient accounts for evaporation, splash loss, and first-flush diversion. RCC slabs and metal roofs perform best at around 0.85–0.9; clay tiles drop to about 0.7.
FAQ
How much rainwater can I harvest from a 100 sq m roof in Kathmandu?
Kathmandu averages about 1,500 mm of annual rainfall. A 100 m² concrete roof with a 0.85 runoff coefficient yields roughly 127,500 litres per year. That covers around 250 days of usage for a 5-person household using 100 litres per person per day.
What size of storage tank should I install?
A practical residential storage tank holds 15 to 30 days of household demand, typically 5,000 to 15,000 litres for a family of four to six. Going larger doesn't help much because excess yield in monsoon overflows quickly, while the tank still empties in dry months.
Is rainwater safe to drink?
Rainwater collected from a clean rooftop and stored in a covered tank is safe for non-potable uses like toilet flushing, gardening, and washing. For drinking, it should be filtered (sediment + activated carbon) and disinfected by boiling, chlorination, or UV treatment. A first-flush diverter is essential.
Is rainwater harvesting mandatory in Nepal?
In several municipalities — Kathmandu Metropolitan City being the most prominent — new building permits require some form of rainwater management for plots above a certain size. Even where it isn't mandatory, KVDA encourages it during the naksa pass process. Confirm current rules with your ward office.