Rain Fall Harvesting Calculator

Not Reviewed
Calculator / Last modified by KurtHeckman on 2017/08/31 17:10
Failed to execute the [groovy] macro. Click on this message for details.

The Rainwater Harvesting Calculator provides equations useful in planning and harvesting rain water.    rainwater-harvesting.gifRainwater Harvesting


This calculator includes the following equations for rain water harvesting:

Amount of Rain - This equation solves for how much precipitation was collected over the area of a surface or what are called, "catchments".

Annual Rain Runoff - This estimates the amount of rain that can be collected yearly by a rain catchment/container.

Cistern Capacity Needed to Harvest from a Large Storm - calculates the minimum cistern volume needed to capture the roof runoff from the expected rainfall of a storm.

Cistern's One-Time Dollar Price for Storage Capacity - This calculates the price of the storage container per gallon of rainwater saved.

Efficiency of Catchment - calculates how much a certain surface can effectively collect rain water.

Estimated Net Runoff - provides an estimate for how much rainfall can be realistically collected off a surface for an average year.

Pressure Head - calculates the water pressure that can be realized below a tank based on the height of storage, the density of water and the force of gravity.

Rain Fall Harvest - calculates how much rain water has been harvested from a certain catchment surface.

Storage Capacity of a Cylinder - calculates how much liquid a cylindrical container can hold.

Storage Capacity of a Square/Rectangular Tank - calculates how much liquid a square / rectangular container can hold.

Water Storage Capacity Needed for a Household - calculates the amount of liquid a storage container must hold in order to sustain a number of people in a household.

Weight of Stored Water - calculates how much the collected water weighs.

Snow Water Equivalent - calculates of the volume of liquid water contained in snow pack defined by the type of snow, depth and area covered.

Snow Water Density - returns the kg per meters cubed (kgm3) for snow of different types per the US Geological Survey (USGS).

Rainwater Harvesting

Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Its uses include water for garden, water for livestock, water for irrigation, water for domestic use with proper treatment, and indoor heating for houses etc. In many places the water collected is just redirected to a deep pit with percolation. The harvested water can be used as drinking water as well as for storage and other purpose like irrigation.


Rainwater harvesting provides an independent water supply during regional water restrictions and in developed countries is often used to supplement the main supply. It provides water when there is a drought, can help mitigate flooding of low-lying areas, and reduces demand on wells which may enable ground water levels to be sustained. It also helps in the availability of potable water as rainwater is substantially free of salinity and other salts.


(Above) Sample diagram for how a rain water collection setup works. (Click here for more information) abpRainSaucer_single_dblbung_275x310.png Rain Saucer

System setup


Rainwater harvesting systems can be installed with minimal skills. The system should be sized to meet the water demand throughout the dry season since it must be big enough to support daily water consumption. Specifically, the rainfall capturing area such as a building roof must be large enough to maintain adequate flow. The water storage tank size should be large enough to contain the captured water.
(Right) Example of a simple Rain Saucer made from food grade, UV resistant Polypropylene.(Click here for more information)


The concentration of contaminants is reduced significantly by diverting the initial flow of run-off water to waste. Improved water quality can also be obtained by using a floating draw-off mechanism (rather than from the base of the tank) and by using a series of tanks, with draw from the last in series. Pre-filtration is a common practice used in the industry to ensure that the water entering the tank is free of large sediment. Pre-filtration is important to keep the system healthy.

cistern for rain collecting.jpg 

(Above) Example of a cistern for rainwater collecting storage. (Click here to view full image.)

Snow Harvesting:

Snow can also be harvested to create a renewable water supply. To read more, see link below.

See: http://www.harvestingrainwater.com/wind-snow-harvesting/  You can also calculate the amount of water that comes from different types of snow in the following calculator:


Other Calculators from this Author:


This calculator, Rain Fall Harvesting Calculator, is listed in 2 Collections.