Garden / Water
Rainwater Harvesting Calculator (Roof Area and Tank Size)
Work out how much rainwater a roof can collect in a year and in one heavy rain, in litres or US gallons, allowing for the roof material, and how many days of your water use that covers.
Rainwater Harvesting Calculator (Roof Area and Tank Size): One millimetre of rain on one square metre gives one litre, and one inch on one square foot gives 0.623 US gallons. Multiplied by the roof's area on the ground and by a runoff coefficient for its material, from 0.95 for metal to 0.4 for a planted roof, that is the water reaching the gutters. A tank that holds one heavy rain's worth catches most of what falls. Runs 100% locally in your browser with zero server file uploads.
- Category
- Home & family tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
Each millimetre of rain on a square metre of roof is a litre of water, and each inch on a square foot is 0.623 US gallons; the roof material decides how much reaches the gutter. Measure the area the roof covers on the ground, not its sloping surface. Rain falls unevenly through the year, so a tank big enough for one heavy rain catches most of it. Check local rules before using rainwater for drinking.
A worked example
A 120 m² tiled roof in a place with 800 mm of rain a year collects 120 × 800 × 0.9 = 86,400 litres. A heavy rain of 25 mm brings 2,700 litres, so a tank of about 2,500 to 3,000 litres would catch most storms.
In US units, a 1,500 ft² shingle roof with 38 in of rain a year collects about 30,200 gallons, and about 800 gallons from an inch of rain.
Planning the system
A first-flush diverter sends the first, dirtiest water from each rain away from the tank, and a screen at the gutter keeps out leaves. The tank needs an overflow led away from the house's foundations.
To work out the roof's footprint, measure it on the ground or from a plan; for the slope itself, the roof pitch calculator converts between pitch and angle.
How to use it
- Enter the roof area as seen from above, and your yearly rainfall.
- Pick the roof material, and the size of a heavy rain where you live.
- Read the water collected a year and per heavy rain, which suggests a tank size.
Privacy & limitations
Your figures stay in your browser and are not sent anywhere.
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Frequently asked questions
Should I use the roof's sloping area?
No: rain falls vertically, so what counts is the area the roof covers on the ground, its footprint including the overhangs.
Where do I find my rainfall?
Your national weather service publishes average yearly and monthly rainfall for your area.
Can I drink the rainwater?
Only after proper filtering and treatment, and where local rules allow; untreated, use it for the garden, toilets, or washing.
Free tool · runs in your browser · no account required