Free interactive tool
Rainwater Harvest Calculator
Roof area × rainfall × 0.9 — see how fast a real roof fills a real tank.
Turn a roof into a reservoir
Roof area times rainfall times real-world losses — the tank fills by formula.
Rain harvesting is honest arithmetic: a modest roof in a modest storm out-collects a rain barrel, which is why overflow planning matters as much as collection.
- Size the roof that fills a 200 L barrel in a 20 mm storm.
- Double the roof and halve the rain — same water?
The formula is honest arithmetic: every millimeter of rain on a square meter of roof is a liter of water, minus about 10% real-world losses.
A modest 60 m² roof in a 20 mm storm out-collects a standard rain barrel five times over — which is why overflow planning matters as much as collection.
How much rain fills a 200 L barrel?
On a 60 m² roof, barely 4 mm of rain — one decent shower.
Is roof water safe for vegetables?
Generally fine for soil irrigation; avoid spraying leaves close to harvest and skip roofs with lead or fresh chemical treatments.