Rainfall in liters
Enter the rainfall in millimeters and the area of the roof or lot. The calculator shows how many liters fall and how many you can collect.
How to useHow many liters of rain fall
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Rainfall in liters: what it is for
1 mm of rain is 1 liter per square meter. Enter the rainfall and the area of the roof or lot, and the calculator shows how many liters fall and how many reach the gutter, using the runoff coefficient you choose (0.8 by default).
The result appears in liters, m³ and 1,000-liter water tanks and, if you enter your daily use, in how many days of use it covers.
How to use
- Enter the rainfall in millimeters.
- Enter the roof area as seen from above.
- Choose the coefficient and see the liters collected.
When to use it
- Sizing a cistern or rain barrel system.
- Knowing how much water fell on a field or backyard.
- Making sense of your local rainfall report.
Frequently asked questions
How much is 1 mm of rain?
1 millimeter of rain equals 1 liter of water per square meter. A 30 mm rain drops 30 liters on every m²; on a 100 m² roof, that is 3,000 liters (about 790 gallons).
What is the runoff coefficient?
It is the share of the rain that actually runs off the roof into the gutter; the rest wets the roof, evaporates or splashes. A value of 0.8 is a common default when nothing has been measured (it is what Brazil's NBR 15527 rainwater standard recommends); clay tiles sit between 0.8 and 0.9, and glazed or metal roofs reach 0.9 to 0.95.
Which roof area should I use?
The projected area, that is, the roof as seen from above (length × width of the covered area), not the sloped area of the roofing.
Where can I see how much it rained?
On your national weather service (in the US, the NOAA/National Weather Service precipitation data) or a local weather station. To plan a cistern, use the average monthly or yearly rainfall for your region.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.