How to use the humidex calculator
- Choose the temperature unit, Celsius or Fahrenheit.
- Choose how you give the moisture: as a dew point or as a relative humidity.
- Enter the air temperature and the dew point or the relative humidity.
- Read the humidex, its value in your unit, the dew point and vapor pressure the page used, and how many degrees the humidex adds to the air temperature.
Formula
Vapor pressure e (hPa) = 6.11 x exp(5417.753 x (1 / 273.16 - 1 / (dew point in C + 273.16)))
Humidex = air temperature in C + 0.5555 x (e - 10)
From relative humidity, the dew point is found first from the air temperature and the humidity
A result in Fahrenheit is converted back from the Celsius value
The humidex is a number, not a temperature. It is written as if it were a temperature in Celsius so people can relate it to how warm the air feels.
30 C with a dew point of 20 C
A dew point of 20 C gives a vapor pressure of 23.63 hPa. The humidex is 30 + 0.5555 x (23.63 - 10) = 37.57. That is 7.57 degrees above the air temperature, so the air feels like about 37.6 C.
The same air with a dew point of 10 C has a vapor pressure of about 12.3 hPa and a humidex of about 31.3, which shows how much moisture adds. The formula is meant for warm, humid weather, and a dry day with a low dew point adds little.
What the number means
Sweat cools you only if it can evaporate, and humid air slows evaporation. The humidex puts the combined effect of temperature and humidity on one scale. Weather services describe ranges of humidex with comfort and caution levels, and these are defined by each service, so check the levels published where you live.
The United States uses a different index. For that, see the heat index calculator. To work out the dew point from a reading, use the dew point calculator.
Limits
The humidex describes how conditions feel in the shade for an average person at rest. Sun, wind, clothing, effort and individual health all change how you feel, so it is not a safety rating. Include wind with the apparent temperature calculator, or see the wind chill calculator for cold days.
Assumptions and limits
- The formula is intended for warm weather. Below a dew point of about 10 C the added value is small or negative.
- Humidity input is relative humidity from 0 to 100 percent, or a dew point at or below the air temperature.
- Results are an index and not a safety limit. Follow the advice of your local weather service in hot conditions.
- The vapor pressure is a standard approximation and not a laboratory value.
Frequently asked questions
What is humidex?
It is an index used in Canada that combines air temperature and humidity into one number that describes how hot the air feels.
How is humidex different from the heat index?
Both combine heat and humidity, but they use different formulas and give different numbers for the same weather. Compare each only with its own scale.
Can I use relative humidity?
Yes. Choose relative humidity as the moisture input, and the page finds the dew point and vapor pressure for you.
Why does a high dew point matter?
The dew point measures the amount of water in the air. A high dew point means sweat evaporates slowly, so the same temperature feels hotter.