About this calculator

Annual harvest = roof area × rainfall (mm) × 0.8 efficiency. UK avg rainfall ~900 mm/yr; Mediterranean ~500 mm; Atlantic Europe ~1200 mm. Tank size = 3% of annual harvest (rule of thumb).

Frequently asked questions

What does the Rainwater Harvesting Tank Sizing do?

Annual harvest and tank size from roof area and EU rainfall. Annual harvest = roof area × rainfall (mm) × 0.8 efficiency. UK avg rainfall ~900 mm/yr; Mediterranean ~500 mm; Atlantic Europe ~1200 mm. Tank size = 3% of annual harvest (rule of thumb).

Is the Rainwater Harvesting Tank Sizing free to use?

Yes. Every calculator on Calcurly.com is free, works in your browser without sign-up, and can be embedded on your own website.

How accurate is the Rainwater Harvesting Tank Sizing?

Results are calculated in real time using industry-standard formulas. Treat the output as a well-informed estimate — always cross-check with local regulations, product datasheets, or a qualified rainwater harvesting professional before committing to a purchase or a job.

What inputs does the Rainwater Harvesting Tank Sizing need?

Enter values for: Roof area (m²), Rainfall mm/yr. Sensible defaults are pre-filled where possible so you can start with a single change.

Can I embed the Rainwater Harvesting Tank Sizing on my own website?

Yes — copy the iframe snippet at the bottom of this page, or use the Embed Generator to customise the size. The embedded version stays in sync with the live calculator on Calcurly.com.