Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Rainwater can be collected from rivers or roofs, and in many places the water collected is redirected to a deep pit (well, shaft, or borehole), a reservoir with percolation, or collected from dew or fog with nets or other tools. Its uses include water for gardens, livestock, irrigation, domestic use with proper treatment, and indoor heating for houses etc. The harvested water can also be used as drinking water, longer-term storage and for other purposes such as groundwater recharge.(source: wikipedia)

This calculator was developed specifically for the Jaffna district in Sri Lanka, but it can be used for any area/region, provided the required parameters are known. This calculator helps in determining the size of the storage tank required based on the rainfall, roof area, roof type and family size.

Days in Month (days)
Monthly Rainfall (mm/month)
Monthly Rooftop Runoff (m3/month)
Monthly Household Water Consumption (m3/month)
Cumulative Tank Water (m3/month)

Monthly Rooftop Runoff = Collection Efficiency * (Monthly Rainfall - Rooftop Absorption Rate) * Rooftop Area

Monthly Household Water Consumption = Number of People * Daily Water Use per Person * Days in Month

Cumulative Tank Water ( ≤ Tank Capacity) = Monthly Rooftop Runoff + Tank Water from Previous Month - Monthly Household Water Consumption

Note: High resolution satellite images can be used to count roof tops and number of RWH system to be installed see the Jaffna example: Identification of Agro-Wells Using High-resolution Satellite Imagery - Jaffna District