Rainwater Harvest Calculator: How Much Rain Can Your Roof Catch?
Every millimetre of rain on a square metre is a litre of water, before losses. This calculator applies your roof area and efficiency to that rule.
What is the Rainwater Harvest Calculator?
A roof catches rain, but gutters and filters lose some of it. Enter roof area, rainfall, and collection efficiency, and this tool estimates the litres you can actually harvest.
How to use it
- Enter your values in the fields provided.
- Click Estimate Harvested Water.
- Read the result and adjust an input if you want a different outcome.
Worked example
A 100 square metre roof in 20 mm of rain at 80 percent efficiency harvests 1,600 litres.
How much water does a 100 square metre roof catch in 20 mm of rain at 80 percent?
One hundred times 20 gives 2,000 litres before losses, and 80 percent of that is 1,600 litres.
Frequently asked questions
What do I enter to get a result?
Fill in every field with your own numbers or choices, then run the tool. Empty number fields count as zero, so check them before you trust the output.
Can I reuse this result elsewhere?
Yes. The result is calculated in your browser from the values you entered, so you can copy the numbers into your own notes or plan.