Rainfall Intensity Calculator
Rainfall intensity is the rate at which rain falls: the depth of water collected divided by the time it took to fall. It is central to drainage design, flood assessment, and erosion control, and it is what separates a steady soaking from a flash-flood downpour. This tool divides a measured rainfall depth by its duration to give intensity in depth per hour, and classifies the rate against the common light, moderate, heavy, and violent bands. Enter the rainfall depth and the duration in minutes; the depth units you use (millimetres or inches) carry straight through to the per-hour result.
Rainfall intensity formula
intensity per hour = depth / (minutes / 60)
intensity per minute = depth / minutes
Intensity is simply depth divided by time, expressed per hour by converting the duration from minutes to hours. The output keeps whatever depth unit you enter: millimetres in give millimetres per hour, inches in give inches per hour. The World Meteorological Organization classifies rain as light below 2.5 mm/h, moderate from 2.5 to 10 mm/h, heavy from 10 to 50 mm/h, and violent above 50 mm/h.
Worked example
A gauge collects 25 mm of rain over 120 minutes. In hours that is 120 / 60 = 2 hours, so intensity = 25 / 2 = 12.50 mm/h, which falls in the heavy band. Per minute the rate is 25 / 120 = 0.21 mm/min. The same 25 mm falling in just 30 minutes would be 50.00 mm/h, at the threshold of violent rainfall.
Frequently asked questions
How is rainfall intensity defined?
Rainfall intensity is the depth of rain divided by the time over which it fell, normally expressed in millimetres or inches per hour. It describes how hard the rain is falling at a point in time, as distinct from total rainfall, which is just the accumulated depth regardless of how long it took.
What counts as heavy rain?
Using the World Meteorological Organization bands, rain is light below 2.5 mm/h, moderate from 2.5 to 10 mm/h, heavy from 10 to 50 mm/h, and violent above 50 mm/h. These are general thresholds; some national services use slightly different cut-offs, so check your local definitions for official warnings.
Why does intensity matter for drainage?
Drains, culverts, and detention basins are sized to handle a design storm of a given intensity and return period. If real intensity exceeds the design value, systems surcharge and surface flooding follows. Engineers use intensity-duration-frequency curves, which this kind of calculation underpins, to choose the right design rate.
Can I use inches instead of millimetres?
Yes. The calculator divides depth by time regardless of the unit, so if you enter the depth in inches the result is inches per hour. Just keep your depth unit consistent and read the output in the same unit. The WMO intensity bands above are stated in millimetres per hour.
Official sources
- NOAA National Weather Service: precipitation and rainfall.
- NOAA Hydrometeorological Design Studies Center: Precipitation Frequency Data Server.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.