Rain Collector Calculator
Discover how much rainwater you could collect from your roof.
Collected from this rainfall
3,400 L
From 80 m² of roof at 85% efficiency.
Monthly estimate
13,600 L
Assuming four comparable rainfalls a month.
Annual estimate
163,200 L
Assuming 48 comparable rainfalls a year.
Water per millimetre of rain
68 L
- Runs entirely in your browser
- No data is stored
- Free forever
What is this calculator?
This calculator estimates the rainwater your roof could capture. Enter your roof area, the rainfall you get and how efficient your collection setup is, and it returns the volume from a single rainfall event plus monthly and annual estimates. Useful for planning a water butt, sizing a tank, or checking whether garden irrigation could run on rain alone.
How does it work?
Roof area in square metres is multiplied by rainfall in millimetres, which gives litres directly, then multiplied by your collection efficiency. Monthly and annual figures scale that rainfall across the period you choose. Gutter losses, splash and evaporation vary, so efficiency is set to 85% by default.
Frequently asked questions
How is rainwater harvest volume calculated?+
One millimetre of rain on one square metre of roof gives one litre of water, so area multiplied by rainfall gives litres before losses.
Why is efficiency set to 85%?+
Gutters overflow, some water evaporates and first-flush diverters discard the dirtiest run-off. Around 85% is a realistic capture rate for a well-kept system.
Do I use roof area or footprint?+
Use the horizontal footprint the roof covers, not the sloped surface. Rain falls vertically, so the shadow the roof casts is what matters.
Is the rainfall figure per storm or per month?+
It is per rainfall event. The monthly and annual results scale that figure using the rainfall totals shown beneath them.
Can I drink the water?+
Not without proper filtration and treatment. Collected rainwater suits gardens, cleaning and flushing without extra work.
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