Rainwater Harvesting Calculator
Fill in the values and the answer updates as you type.
How the Rainwater Harvesting Calculator works
Water collected (litres) = roof area × rainfall (mm) × efficiency. Each millimetre of rain on one square metre of roof gives one litre. Efficiency drops with first-flush diverters, leaks, and overflow, and 80–90% is typical. Example: with the default values, 60 m² of roof in 50 mm of rain yields 2,550 litres, or 13 barrels of 200 L.
Inputs and results
The Rainwater Harvesting Calculator takes 4 inputs and gives 3 results.
What you enter
- Roof catchment area (m²) — starts at 60.
- Rainfall (mm) — starts at 50.
- Collection efficiency (%) — starts at 85.
- Barrel size (L) — starts at 200.
What you get
- Water collected (litres)
- Water collected (m³)
- Barrels to store it all
Example: Rainwater Harvesting Calculator with the starting values
With roof catchment area 60 m², rainfall 50 mm, collection efficiency 85 % and barrel size 200 L, the calculator returns:
| Water collected (litres) | 2,550 |
|---|---|
| Water collected (m³) | 2.55 |
| Barrels to store it all | 13 |
Change any value above and the results update straight away.
Rainwater Harvesting Calculator FAQ
How do I use the Rainwater Harvesting Calculator?
Enter roof catchment area, rainfall, collection efficiency and barrel size in the fields, then read water collected (litres), water collected (m³) and barrels to store it all in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Rainwater Harvesting Calculator calculate?
Water you can collect from a roof and the barrels to store it. It returns water collected (litres), water collected (m³) and barrels to store it all from the 4 values you provide.
What values does the Rainwater Harvesting Calculator start with?
It opens with roof catchment area 60 m², rainfall 50 mm, collection efficiency 85 % and barrel size 200 L so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Rainwater Harvesting Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Rainwater Harvesting Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.