Rainwater Tank Size Calculator

Estimate how much rainwater your roof can collect in a year, how much of your water use that could cover, and how big a tank you need to get through a dry spell. Enter your own numbers below. The defaults are only an example.

How we calculate this

1. Water you can collect in a year = yearly rainfall x roof area x collection efficiency x conversion factor.
2. In US units the conversion factor is 0.623 gallons per square foot per inch of rain (7.48 gallons in a cubic foot, divided by 12 inches in a foot). In metric, 1 mm of rain on 1 square meter is exactly 1 liter, so no factor is needed.
3. Yearly demand = people x water use per person per day x 365.
4. Tank size for a dry spell = people x daily use x days of storage.

The tank size is a starting point. If your roof collects less water in a year than you use, a bigger tank will not help: it will simply stay partly empty.

Worked example

A 1,500 sq ft roof in a place that gets 40 inches of rain a year, with 85% collection efficiency: 40 x 1,500 x 0.623 x 0.85 = about 31,773 gallons a year. Four people using 30 gallons a day each need 4 x 30 x 365 = 43,800 gallons a year, so the roof covers about 73% of that use. To cover a 30-day dry spell the tank would need 4 x 30 x 30 = 3,600 gallons.

What this tool does not cover

• Uneven rainfall. Rain rarely falls evenly through the year. A tank sized from yearly totals can still run dry in a long dry season, so check your monthly rainfall pattern.
• First flush. The first rain after a dry spell washes dirt off the roof. Many systems divert that first part of a storm. There is no single agreed amount, so see the Texas manual below.
• Water quality. Untreated rainwater should not be assumed safe to drink. Ask your local health department about treatment for any indoor use.
• Local rules. Rules on collecting rainwater and on tank placement vary by state and city. Check yours before building.
• Evaporation, overflow, freezing and the cost of the tank are also not included.

Sources and further reading

• Texas Water Development Board: The Texas Manual on Rainwater Harvesting (3rd edition). Design guidance on collection, first flush and storage.
• NOAA National Centers for Environmental Information: U.S. Climate Normals. Average monthly and yearly rainfall for US locations.
• American Rainwater Catchment Systems Association. Industry guidance and training.

Last reviewed: October 27, 2026. This calculator gives planning estimates only and is not engineering or health advice.