The amount of rainfall that you can collect is governed by the following formula:
Easy to Remember Formula:
1″ of rain x 1 sq. ft. = 0.623 gallons.
1″ of rain x 100 sq. ft. = 62.3 gallons.
1″ of rain x 1000 sq. ft. = 623 gallons.
To calculate the amount of rainwater you can collect, you need to know your annual average precipitation for your area. You can use the precipitation map below to find an approximate amount for your area.
Sitemap & Additional Resources: http://www.rwh.in/sitemap.htm .
Rainfall Data of US/UK/Australia
US : https://www.usclimatedata.com/
UK : https://www.metoffice.gov.uk/research/climate/maps-and-data
Australia : http://www.bom.gov.au/climate/cdo/about/cdo-rainfall-feature.shtml
Blog: https://rainwater.blog/sitemap/
Last updated on 29-Sep-2025.
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3 replies on “So Just How Much Rain Can I Collect?”
The best part of the article is>> Easy to Remember Formula: 1″ of rain x 1 sq. ft. = 0.623 gallons.
Loved reading the blog about easy to remember formula: 1″ of rain x 1 sq. ft. = 0.623 gallons.
The current average rainfall data is available at: https://www.currentresults.com/Weather/US/average-annual-state-precipitation.php
https://www.ncdc.noaa.gov/cdo-web/
https://rainwatersystem.blog/
Along with that don’t forget to read Chapter 4: Water Balance and System Sizing of the PDF book: The Texas Manual on Rainwater Harvesting Page 34
Click to access RainwaterHarvestingManual_3rdedition.pdf