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Rainwater collection tank size
Enter the roof and a yearly rainfall. See litres collected, a suggested tank, and how many days of basic household water that tank holds.
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The one-litre rule
One millimetre of rain on one square metre of roof is one litre of water. Multiply roof length by roof width by yearly millimetres, then by a collection efficiency. The efficiency starts at 80 percent to allow for the first dirty flush, leaks, and overflow.
The suggested tank is the larger of two numbers: 30 days of basic water for the household at 3.8 litres per person per day, and two average months of what the roof collects. It is never larger than a full year of collection. A dry climate and a small roof will suggest a tank you can actually fill. A wet climate will suggest a tank that holds a couple of months, not the whole year.
This is volume only. Roof water used for drinking needs filtration and treatment that this page does not design.
How to size the tank
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Step 1
Measure the footprint, not the slope
Length times width of the area that drains to the tank. The slope does not add area.
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Step 2
Pick a rainfall band or type millimetres
Bands are 400, 800, and 1,400 millimetres a year. Replace them with a local yearly total if you have one.
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Step 3
Compare the tank with 30 days of water
The last line is how long a full tank covers the household at the basic daily amount.
Rainwater questions
Can I drink it?
Not straight from the gutter. The calculator is about how many litres you can catch. Treatment is a separate job.
What is the first flush?
The first rain after a dry spell washes dust and droppings off the roof. Diverting it is why efficiency is not 100 percent.
Does a flat roof count?
Yes, if the water actually reaches the tank. Use the drained area, not the whole building.
Related
A tank is only half of it. You still need a way to keep the water usable.