Calculating Your Rainwater Harvesting Tank Size based on Your Roof Area, Rainfall, and Consumption

Calculating Your Rainwater Harvesting Tank Size based on Your Roof Area, Rainfall, and Consumption
This is a simple graphic calculator that will help you determine the best tank size based on your rainfall and consumption.

To use this calculator, you will need to fill in data in the yellow cells. The numbers that are already in the cells are simply used as a starting point. I recommend that you read through the whole spreadsheet before you change any numbers. Then, type over the numbers in the yellow boxes with numbers that apply to you.

You might want to play around with these numbers to see the impact of different variables, after you've gone through it the first time.
The graph at the bottom of this page, and all of the numbers in black will automatically update each time you make a change in a yellow box.

First; We will estimate your monthly household water usage (exclude gardening and irrigation use - you can add these later if you like, but it takes a lot of water to irrigate)
The amount of water used by each person per day varies greatly

In western countries it's about 30 to 50 gallons per person per day
In non-western wet climates with water piped to a bathroom and kitchen it's about 20 gallons per person per day
In dry areas where water is piped to a faucet in the yard outside the house, it is about 10 gallons per person day per
In dry remote areas where water is carried less than 200 meters it is about 2 gallons per person per day
In dry remote areas where water is carried more than 200 meters it is about 1 gallon per person per day

Second: Entering your roof area and gutter efficiency.
Enter the efficiency of your collection system

If you have a high quality, well maintained gutter system the efficiency is probably about 75 to 90%.
If your gutter system integrity is questionable and you see a lot of water running over the edge during a rain storm, the efficiency is probably about 50 to 70%

Third: Now we'll enter rainfall data for your area of the world and automatically calculate and graph several things simultaneously.
There are a lot of sources of global, monthly rainfall data on the internet (unfortunately, not too much on max intensity). Several links are provided at the right of this page for your convenience. For now, follow the world climate, type in your nearest city, select "Average Rainfall", then fill in the monthly data in inches in the yellow boxes below. The graph will change shape as you enter data. The dark blue line is collected water. The purple line is your monthly consumption, and the yellow line is your tank inventory at the end of each month.

The amount of water you have at the end of each month will depend on which month you plan on emptying your tank for maintenance. This isn't always an obvious choice.

Fourth: Now for the fun part - sizing your tank!

Find the all detailed calculations in attached calculator below.
 

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