Rainwater Harvest Calculator
Estimate harvestable rainwater from roof area, rainfall, and efficiency.
Quick answer: enter your details below and click Estimate Harvested Water — your result appears instantly, right here in your browser. Nothing you enter is uploaded or stored.
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Enter your details below.
How to use
- Enter your values in the fields provided.
- Click Estimate Harvested Water.
- Read the result and adjust an input if you want a different outcome.
About this tool
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.
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.