Heat index and humidex are both heat-stress apparent temperatures: they combine air temperature with humidity into a single number that behaves like a temperature, answering the question “how hot does it actually feel?” The difference is regional and methodological. The heat index is the National Weather Service index used in the United States, computed from air temperature and relative humidity (or dew point) and reported in Fahrenheit. The humidex is the Environment Canada index used in Canadian forecasts, computed from air temperature and dew point in Celsius and reported as a number with official comfort categories. They are close cousins rather than rivals, and the one you should use depends on which index your region and weather report use.
What each calculator does
The heat index calculator takes air temperature and either relative humidity or dew point; a dew point entry first derives relative humidity with the separate NWS approximation. Temperature is converted exactly to Fahrenheit. The NWS procedure calculates a simple formula, averages it with air temperature, and applies the nine-term Rothfusz regression only when that average is 80°F or above. It subtracts a low-humidity adjustment only below 13% humidity between 80°F and 112°F, and adds a high-humidity adjustment only above 85% humidity between 80°F and 87°F. The displayed heat index is rounded to the nearest whole degree Fahrenheit and reflects shaded, light-wind conditions.
The humidex calculator takes air temperature and dew point, both in Celsius, and applies the Environment Canada formula:
The exponential term estimates actual vapour pressure from the dew point, and the result minus 10, times 0.5555, is the number of degrees the humidity adds to the perceived temperature. The displayed humidex rounds to the nearest integer and is placed on the official Environment Canada scale: 20–29 little discomfort, 30–39 some discomfort, 40–45 great discomfort (avoid exertion), and 46 and over dangerous.
Side-by-side
| Heat index calculator | Humidex calculator | |
|---|---|---|
| Issuing body | U.S. National Weather Service | Environment and Climate Change Canada |
| Inputs | Air temperature plus relative humidity or dew point | Air temperature and dew point (Celsius) |
| Formula basis | NWS simple formula plus nine-term Rothfusz regression with humidity adjustments | ECCC vapour-pressure equation from temperature and dew point |
| Temperature scale | Fahrenheit (inputs converted exactly) | Celsius |
| Output | Apparent temperature in whole degrees Fahrenheit | Humidex number rounded to an integer plus comfort category |
| Reporting scope | Full regression only when the simple-formula average is 80°F or above | Reported at 20°C or above when the humidex is at least 1 degree above temperature |
| Comfort guidance | None; the page does not assign a risk category | Official bands from little discomfort to dangerous |
| What it rejects | Humidity outside 0%–100%, impossible dew point, temperatures below absolute zero | Dew point above air temperature |
When to use which
Use the heat index calculator when you want the U.S. NWS index — for example, reading American forecasts or comparing against NWS heat-safety guidance. You can supply either relative humidity or dew point, whichever you measured. Use the humidex calculator when you want the Canadian index, such as when a Canadian forecast reports the humidex or you are planning outdoor activity using Environment Canada’s comfort bands. If you only know relative humidity but need the humidex, convert to dew point first with the dew point calculator.
Both tools answer the same underlying question, and their numbers are close but not identical because the formulas and units differ. Neither page is a personalized health forecast: heat-stress risk depends on sun, wind, exertion, clothing, hydration, and individual tolerance. Treat either index as context, and check current heat-safety guidance and local warnings rather than relying on a single number.
Limits and disclaimer
Both pages are educational tools, not medical advice. The NWS warns that the Rothfusz regression is not valid for extreme temperature and humidity combinations outside its underlying data, and the humidex comfort categories are population-level guidelines — children, older adults, people with chronic conditions, and outdoor workers may be affected at lower values. Neither calculator monitors conditions or alerts, and neither estimates personal heat-illness risk. In cold weather, the humidex is not reported and the wind chill calculator is the relevant index.