Rainwater harvesting sizing

Rainwater harvesting is the collection of rain water instead of allowing it to run off. A rain water tank is installed to collect rain water from rooftops via gutters. It reduces CWA water consumption and therefor a reduction in the water bill.

The stored water can be used for irrigation, toilets cistern, washing machines, cleaning house or car. It can also be used for drinking by filtering through BIO UV filtration system to kill bacteria, chemicals and most pathogens but is strongly not advisable.



Rainfall Calculations and Potential Water Collection

The first thing to consider when sizing a rainwater tank is how much rain water can available for reuse.

Two main factors to be taken into consideration is the average rainfall report for the region and the roof surface area of the building

Annual Maximum Rainfall that can be harvested calculation:


Water harvested (m3)

= Annual Rainfall (m) x Roof Size (m2) x 0.85


Capacity of a Rectangular Tank

=Height x Length x Width


Capacity of a Round Tank

= π × R² × H


There are three options in calculating the storage tank capacity :

  • Matching the area of the roof.
  • Matching the quantity of water required.
  • Choosing a tank according to cost and resources.



Suppose the system has to be designed for meeting drinking water requirement of a five-member family living in a building with a rooftop area of 100 sq. m. The average annual rainfall in the region is 600 mm (average annual rainfall is 611 mm). Daily drinking water requirement per person (drinking and cooking) is 10 litres.


Design procedure:

Following details are available:

Area of the catchment (A) = 100 sq. m.
Average annual rainfall (R) = 611 mm (0.61 m)
Runoff coefficient (C) = 0.85


  1. Calculate the maximum amount of rainfall that can be harvested from the rooftop:

Annual water harvesting potential = 100 x 0.6 x 0.85 = 51 cu. m. (51,000 litres)

  1. Determine the tank capacity: This is based on the dry period, i.e., the period between the two consecutive rainy seasons. For example, with a monsoon extending over four months, the dry season is of 245 days.
  2. Calculate drinking water requirement for the family for the dry season
    = 245 x 5 x 10
    = 12,250 litres

As a safety factor, the tank should be built 20 per cent larger than required, i.e., 14,700 litres. This tank can meet the basic drinking water requirement of a 5-member family for the dry period. A typical size of a rectangular tank constructed in the basement will be about 4.0 m x 4.0 m x 1.0 m.



Gutter size


Rainwater harvesting systems are of 2 basic types: dry conveyance and wet conveyance.

In a dry conveyance system, the pipe(s) drain water to the storage tank that is normally empty except during periods of rain. It is used in dry region. The tank is installed near the building. It is easier to install.

In a wet conveyance system, the pipe is connected to a storage tank(s) are continuously flooded or partially filled with water especially in humid region. The water storage tank(s) are located at a distance from the building/roof. A subterranean piping is necessary to drain the rainwater to the storage tank(s).

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