Rainwater Harvesting Calculator
How much water your roof can collect, how long it lasts, and what size of barrel or tank you need.
Part of the 🌾 Farmers toolkit, with more tools made for the job.
Measure on the ground, including overhangs. Ignore the slope.
For one rain, or a month's total.
Leave empty for 90%. Lower for rough roofs or long gutters.
Gives the tank size you need.
Type the roof area and the rainfall, and the water you can collect appears here.
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Enter the area of your roof and the rainfall, and this shows how much water you can collect. Add how much water you use a day to see how many days it lasts, and the dry spell you want to cover to see the tank size.
One millimetre of rain on one square metre of roof is one litre. One inch of rain on one square foot is about 0.623 US gallons. Some water is always lost to splashing, gutters and the first flush, so the calculation allows for an efficiency, often around 90% for a good roof and gutters. You can change it.
How it’s worked out
- Metric: roof area in square metres times rainfall in mm gives litres. Imperial: square feet times inches times 0.623 gives gallons.
- Multiply by the efficiency to allow for losses. Leave it empty to use 90%.
- Days it lasts is the water collected divided by your daily use.
- Tank for a dry spell is your daily use times the number of dry days.
A 40 square metre roof and 25 mm of rain at 90% efficiency collects 900 litres. At 100 litres a day that lasts 9 days.
Common mistakes
- Measuring the sloping roof. Measure the flat area it covers on the ground, including overhangs, because rain falls straight down.
- Using the yearly rainfall for a single tank. Rain comes in bursts, and a tank can only hold what its size allows.
- Assuming all of it is safe to drink. Roof water needs treatment before drinking. This tool is for watering plants and general use.
- Forgetting the first flush. The first rain washes off dirt, and many systems divert it.
Questions
Where do I find rainfall for my area?
Your national weather service or local council publishes average monthly rainfall. Use a dry-month figure if you want to be safe.
What efficiency should I use?
Often around 90% for a smooth roof with good gutters, lower for rough surfaces, leaves or long gutter runs. Leave it empty to use 90%.
Is the water safe to drink?
Not without treatment, and the rules vary. This tool is for sizing a barrel or tank for watering the garden and animals.
Does the roof material matter?
It matters for losses and for what ends up in the water, but not for the volume that falls on it.
Where the method comes from
Estimates only
These are estimates and general information, not agronomic, financial or professional advice. Yields, laying rates, feed amounts, prices and rainfall vary a lot, so use your own numbers and local advice.
Rain stopped play. Officially research.