Rainwater Collection Calculator
Estimate how much rainwater your roof can collect.
Rainwater Collection Calculator
Estimate how much rainwater your roof can collect
Formula
Volume = Roof Area × Rainfall × Efficiency × 0.623How to Use
- 1Enter roof area — Calculate your roof collection area in square feet (length × width).
- 2Enter annual rainfall — Look up your average annual rainfall in inches (check NOAA or local data).
- 3Set collection efficiency — Typical efficiency is 80-90% for hard roofs, less for porous materials.
- 4Get collection estimate — See annual and monthly rainwater collection in gallons.
Frequently Asked Questions
How much rainwater can I collect from my roof?
Every 1 inch of rain on 1,000 sq ft of roof yields about 620 gallons. A 2,000 sq ft roof in an area with 30 inches of annual rain collects ~37,000 gallons/year.
What size rain barrel do I need?
A 55-gallon rain barrel fills in under 1 inch of rain from a 100 sq ft downspout area. For serious collection, consider 250-500 gallon tanks or cisterns.
Is collected rainwater safe to drink?
Not directly. Rainwater needs filtration and UV or chemical treatment before drinking. It is safe for irrigation, toilets, and laundry without treatment.
Is rainwater collection legal?
Laws vary by state. Most states allow it, but some (like Colorado) have restrictions on the amount. Check your local regulations.