These calculators calculate Humindex, index used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. Humidex value is basically an equivalent or apparent temperature (one that human body would feel). It's a standard for Canada, but variations are used around the world. Humidex differs from the heat index used in the United States in being derived from the dew point rather than the relative humidity.
The current formula for determining the humidex was developed by J. M. Masterton and F. A. Richardson of Canada's Atmospheric Environment Service in 1979.
Here, the dewpoint temperature should be given in Kelvin, the magic number 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of vaporization, and the universal gas constant.
This calculator below uses this formula to calculate humindex by air temperature and dew point.
Also, there is another formula which relates humindex to temperature and relative humidity, which value easier to get than dew point value.
This calculator below uses this formula to calculate humindex by air temperature and relative humidity.
How to interpret humindex?
According to Environment and Climate Change Canada - Spring and Summer Weather Hazards humindex has the following meanings:
Also, Canadian Centre for Occupational Health and Safety gives the following recommendations based in humindex reading for "Humindex 1 - Moderate physical work, unacclimatized worker, OR Heavy physical work, acclimatized worker":
Source: Humidex Rating and Work