Rainwater Harvesting Calculator
Annual yield from your roof and a suitable tank size.
6 % of the smaller of yield and demand ≈ 3 weeks of storage (simplified method of DIN 1989-1); filter losses of 10 % included.
Runs entirely in your browser. Nothing is sent anywhere.
How it works
- 1Enter your values.
- 2See the result update instantly as you type.
- 3Change any value to compare scenarios.
About this tool
Rainwater Harvesting Calculator: Annual yield from your roof and a suitable tank size. Type your numbers and the result is calculated instantly — no button to press, nothing to install. All math runs locally in your browser, so the values you enter are never sent anywhere.
Example
| Input | |
|---|---|
| Roof area in plan (m²) | 120 |
| Annual rainfall (mm) | 750 |
| Roof type (runoff coefficient) | Pitched tile roof (0.8) |
| Water use per year (m³, optional) | 40 |
| Result | |
| Usable rainwater per year | 64.8 m³ |
| Suggested tank size (6 % rule) | 2.4 m³ |
| Per month (average) | 5,400 L |
How do I use the Rainwater Harvesting Calculator?
Enter your values. See the result update instantly as you type. Change any value to compare scenarios.
Is the Rainwater Harvesting Calculator free?
Yes. It is completely free with no sign-up, no usage limits and no watermark.
Is my data sent to a server?
No. Everything is calculated locally in your browser. Nothing you enter is uploaded, logged or stored.
Does it work on my phone?
Yes. It works in any modern browser on Android, iPhone, iPad, Windows, Mac and Linux — no app needed.