General
Humidex calculator.
How hot it feels when heat and humidity combine, using Environment and Climate Change Canada's own humidex formula. Enter air temperature plus either dew point or relative humidity.
Inputs
Humidex
Fill the form and press Calculate.
ECCC comfort categories
- 20 to 29: little discomfort
- 30 to 39: some discomfort
- 40 to 45: great discomfort; avoid exertion
- Above 45: dangerous; heat stroke possible
Environment Canada notes that any value over 40 is extremely high and unnecessary physical activity should be reduced. Extremely high readings are rare except in southern Ontario, Manitoba and Quebec.
The formula
Humidex = Tair + 0.5555 × (e − 10)
e = 6.11 × exp[5417.7530 × (1/273.15 − 1/Tdew,K)]
Tair is air temperature in °C, e is vapour pressure in hPa, and Tdew,K is dew point in kelvins (°C + 273.15). Environment Canada shows hourly humidex values only when air temperature is 20°C or higher and the humidex is at least 1 degree above air temperature.
If you enter relative humidity, the dew point is first estimated with the Magnus approximation (constants a = 17.625, b = 243.04 °C, Alduchov and Eskridge, 1996). That step is an approximation and is not part of Environment Canada's published formula.
Sources, read 29 September 2026: climate.weather.gc.ca — ECCC glossary, Humidex (formula) and canada.ca — Weather glossary, Humidex (degrees of comfort).
Frequently asked questions
What is the humidex?
An index that combines temperature and humidity into one number to describe how hot the weather feels. A humidex of 40 means it feels about like 40°C dry air, even if the thermometer reads 30°C.
Is humidex the same as the heat index?
No. Humidex is the Canadian index and the heat index is the US one. They use different equations, so the same conditions give different numbers. This tool uses the Canadian humidex only.
Why does my humidex show nothing below 20°C?
Environment Canada only shows hourly humidex when air temperature is 20°C or higher and the humidex is at least 1 degree above it. The calculator still computes the number, and tells you when Environment Canada would not display it.
Should I use dew point or relative humidity?
Dew point is what the official formula uses, so it is exact. Relative humidity works too, but the dew point is estimated from it and may differ slightly.
Estimate only
This tool applies Environment Canada's published humidex formula for general information. For official heat warnings, check Environment Canada's weather alerts.