Humidex Calculator
The humidex is an index used in Canada to describe how hot the weather feels to a person when humidity is taken into account. High humidity slows the evaporation of sweat, so humid heat feels more oppressive than the same temperature in dry air. The humidex combines the air temperature with the dew point, which measures moisture, into a single comfort number on the same scale as temperature. This tool applies the official Environment Canada formula. Enter the air temperature and the dew point, both in degrees Celsius, to get the humidex value and the comfort band it falls in.
Humidex formula
e = 6.11 * exp(5417.7530 * (1/273.16 - 1/(dew + 273.15)))
humidex = temperature + 0.5555 * (e - 10)
First the vapour pressure e in hectopascals is found from the dew point using the Clausius-Clapeyron relation with Environment Canada's constants. The humidity contribution 0.5555 * (e - 10) is then added to the air temperature. When the dew point is low the vapour pressure is small and the added term can be negative or zero, so the humidex equals the air temperature; when moisture is high the term grows and the humidex rises above the temperature.
Worked example
With an air temperature of 30 degrees Celsius and a dew point of 25 degrees, the vapour pressure is e = 6.11 * exp(5417.7530 * (1/273.16 - 1/298.15)) = 31.69 hPa. The humidity term is 0.5555 * (31.69 - 10) = 12.05. So humidex = 30 + 12.05 = 42.05, well into the band where Environment Canada warns of great discomfort and recommends caution with physical activity. (Using rounded constants the figure is near 42.)
Frequently asked questions
What is the humidex?
The humidex is a Canadian index that expresses how hot humid air feels by combining air temperature and humidity into one number on the temperature scale. Environment Canada introduced it so forecasts could convey the discomfort of muggy heat. It is broadly analogous to the heat index used in the United States, though the two use different formulas.
How do I read the humidex value?
Environment Canada describes humidex 20 to 29 as comfortable, 30 to 39 as some discomfort, 40 to 45 as great discomfort with caution advised, and above 45 as dangerous, with heat stroke possible above about 54. The value has no units; it is read on the same scale as temperature in degrees Celsius.
Why use dew point instead of relative humidity?
The dew point directly measures the amount of moisture in the air, which is what governs how readily sweat evaporates. The official humidex formula is written in terms of the vapour pressure derived from the dew point. If you only have relative humidity, convert it to a dew point first, then enter that here.
Does the humidex consider wind or sun?
No. Like the heat index, the humidex assumes shade and light wind. Direct sunlight can make conditions feel several degrees hotter, and a breeze can ease the discomfort by aiding evaporation. Treat the humidex as a baseline for humid heat in the shade, not an account of every factor affecting comfort.
Official sources
- Environment and Climate Change Canada: humidex definition and formula.
- Government of Canada: Environment and Climate Change Canada.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.