Roof Rainwater Catch Calculator
Find out how much water your roof can put in the tank from a storm, a month or a year of rain.
This gives a rainwater catch estimate only. Every 1 mm of rain on 1 square metre of roof is 1 litre of water, before losses. Everything is worked out in your browser. Nothing is saved or sent anywhere.
1. Your roof
Measure the roof's footprint from above (the area it covers on the ground, including eaves), not the sloping surface. Only count the parts with gutters running to this tank.
Along the gutter.
Gutter to gutter, measured flat from above.
For example 50% if only one side's gutters run to the tank.
2. Rain
Rainfall averages for your town are on the Bureau of Meteorology climate statistics pages (bom.gov.au). Use the mean rainfall for a month or a year.
Changes how the results are worded.
For the period above.
Allows for splash, evaporation, first-flush and overflowing gutters. 80% is a common starting point for a metal roof.
From the Water Tank Size Calculator. Leave at 0 to skip.
Results
How it was worked out
Before you rely on it
- This is a catch estimate only. Real catch depends on gutter and downpipe size, how heavy the rain is, how full the tank already is, and the roof material.
- Light showers often don't produce much runoff at all, because the roof has to wet first and the first flush is diverted.
- Rainfall varies a lot from year to year. Planning on an average year leaves you short in a dry one, so check the lower figures in the Bureau of Meteorology data too.
- For drinking water, ask your local council or state health department about roof materials, first-flush diverters and treatment.