Rainwater Collection Calculator | How do you Calculate Rainfall on a Roof?

Last Updated : May 05, 2025

A Rainwater Collection Calculator is a tool for measuring the water harvested from rain. For this, we are sharing a calculator that helps you calculate rainwater in a few seconds. You have to just put input values in the tool and click the calculate button and the result is yours.

Choose a State:

Rainwater Catchment Area(sq.ft.):

Runoff Coefficient:

Tank Size(gal.):

Tank Starting Volume(gal.):

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Average Rainfall (inches)
Rainwater Usage (gallons)

Rainwater Collection Calculator: Rainwater is useful in many ways to us. Collecting rainwater is becoming essential these days. We will collect water during rains to save rainwater that will help us for future use. How much water is collected from rain is a big question. In this article, we are providing you with a calculator through which you can calculate the amount of water collected from the rain. For measuring the rainwater we need the amount of rainfall per square foot and the precipitation amount in millimeters, by putting these values in the formula we get the amount of rainfall.

Rainwater Collection Formula

The rainwater calculation is done by the formula through which we can calculate the maximum amount of water harvested. The formula for calculating the rainwater manually is mentioned below:

Maximum Rainwater collected (mm/year) = Rainfall capture (in square mt²). * Precipitation( average rainfall in a year)

How to Use Rainwater Collection Calculator?

Follow the below-listed steps to calculate the rainwater collection easily.

For calculating the rainwater, you have to first fill the rainwater on the roof area in square meters/ft in the calculator.
Fill the average rainfall in your area in millimeters/ inches per year.
And then press the calculate button to get the desired result.
It will show whether the water is collected in gallons or liters.

Average Annual Rainfall in Some Areas

The average rainfall in some areas is mentioned below in the table.

Zone	Average Rainfall per year
Humid Areas	> 2000 mm
Semi-humid area	900 to1500 mm
Semi-Arid areas	Between 500 to 750 mm
Arid Area	Between 250 to 500 mm
Semi Desert Area	Between 100 to 250 mm
Desert Area	Between 0 to 100 mm

Example:

If the average rainfall in Jaipur city is 150 mm and the rainfall on the roof per square mt is 30 mt², then calculate the total possible rainfall captured per year?

Solution:

Rainfall on the roof per square mt = 30 m²

Total possible rainfall in (m³/ year) =?

First change the 150 mm into mt = 150/1000

= 0.150 m

Rainfall collected = Rainfall per square mt * average rainfall in the city

= 30 m² * 0.150 m

= 4.500 m³/ year

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FAQs on Online Rainwater Collection Calculator

1. What is the basic formula for rainwater collection?

The formula for rainwater collection isRainwater collection (m³/ year) = rainfall per square mt * average rainfall per year.

2. On which factors does the rainfall collected through the rooftop depend?

The rainfall collected through the rooftop depends upon the three factors namely: surface runoff coefficient, precipitation in climate region, and area of runoff.

3. How to change 100 square meters into millimeters?

To change 100 square meters into millimeters, you have to multiply 100 square meters * 1000 and the resultant 100000 millimeters is the answer.

4. What is the unit of water collected from rainwater?

The rainwater collection through the rainfall unit is square m³/ year.