Rainfall Intensity Calculator
Turn a rain gauge reading into an average rainfall intensity in millimetres per hour, and see whether the fall counts as light, moderate, heavy or violent rain.
Enter Values
How to use this calculator
- Enter the rainfall depth in millimetres — the total that fell during the period you are measuring.
- Enter how long the rain fell for, in minutes.
- Read the intensity in mm/hr, its descriptive class and the equivalent depth over one hour.
How it works
Intensity is just depth spread over time: the duration is converted from minutes to hours (hours = minutes ÷ 60) and the depth is divided by that to give millimetres per hour. The result is then compared with the standard descriptive bands — Light below 2.5 mm/hr, Moderate 2.5 to 7.6, Heavy 7.6 to 50 and Violent above 50 mm/hr.
Worked example
Worked example. 15 mm of rain over 30 minutes is 30 minutes ÷ 60 = 0.5 hours, so the intensity is 15 ÷ 0.5 = 30 mm/hr, which falls in the Heavy band.
Common mistakes
- Entering the duration in hours instead of minutes — this tool expects minutes and converts for you.
- Treating this single average as a storm peak; short bursts within an event can be far more intense than the whole-event average.
- Using this average intensity for drainage or culvert design instead of the Bureau of Meteorology's intensity–frequency–duration (IFD) data.
Frequently asked questions
What counts as heavy rain?
By the bands used here, heavy rain is between 7.6 and 50 mm/hr. Below 2.5 mm/hr is light, 2.5 to 7.6 is moderate, and anything above 50 mm/hr is classed as violent.
Can I use this for stormwater design?
No — it gives one averaged intensity for the event you measured. Design storms use IFD data from the Bureau of Meteorology, matched to the site, storm duration and the chosen annual exceedance probability.
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Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.