Introduction
The Humidex calculator shows you how hot the air really feels when you combine temperature and humidity. On a humid day, sweat does not dry off your skin as fast, so your body has a harder time cooling down. The humidex number captures that extra heat stress in a single value. It was created by Canadian meteorologists and is widely used to warn people about dangerous heat.
To use this tool, enter the air temperature in Celsius or Fahrenheit and choose a moisture input: either relative humidity or dew point. The calculator instantly returns your humidex value in both °C and °F, shows a comfort level rating, and walks you through every step of the math. A live chart also plots how the humidex changes as humidity rises or falls at your chosen temperature.
This humidex calculator shows how the weather will feel, which helps when you plan outdoor work or exercise.
How to Use Our Humidex Calculator
Enter the air temperature and a moisture value below. The calculator will show you the humidex, a comfort level rating, a step-by-step solution, and a chart.
Air Temperature: Type the current air temperature in either Celsius (°C) or Fahrenheit (°F). When you fill in one field, the other updates on its own.
Moisture Input Mode: Choose how you want to enter moisture. Click "Relative Humidity (%)" to use a humidity value, or click "Dew Point" to use a dew point temperature instead.
Relative Humidity (%): If you picked the humidity mode, type a number from 1 to 100. This is how much moisture is in the air compared to the most it can hold.
Dew Point: If you picked the dew point mode, type the dew point in either Celsius or Fahrenheit. The dew point must be equal to or lower than the air temperature. The other unit fills in on its own.
Calculate & Reset: Press "Calculate" to see your results. Press "Reset" to clear all fields and start over with the default values.
What Is the Humidex?
The humidex is a number used in Canada to describe how hot the air really feels when you combine heat and humidity. It was created by Canadian meteorologists in 1965. The air temperature alone does not tell you the full story. When there is a lot of moisture in the air, sweat cannot evaporate from your skin as fast, so your body has a harder time cooling down. The humidex puts temperature and humidity together into one number that shows the true level of discomfort.
How the Humidex Is Calculated
The humidex formula starts with the air temperature in degrees Celsius and the dew point. The dew point is the temperature at which water vapor in the air begins to turn into liquid. A higher dew point means more moisture in the air. If you only know the relative humidity, the calculator uses the Magnus formula to find the dew point first. Then it finds the vapour pressure from the dew point and plugs it into the humidex equation:
Humidex = Air Temperature + 0.5555 × (Vapour Pressure − 10)
The result is shown in both Celsius and Fahrenheit.
Humidex Comfort Levels
Environment Canada groups humidex values into comfort levels so you know when the heat could be harmful:
- Below 30: Little or no discomfort.
- 30–34: Some discomfort. You may start to feel the heat.
- 35–39: Clear discomfort. Be careful with physical activity.
- 40–45: Intense discomfort. Avoid hard exercise outdoors.
- 46–54: Dangerous. Heat-related illness is likely without precaution.
- Above 54: Heat stroke is very likely. Stay indoors.
Humidex vs. Heat Index
The heat index, used in the United States, also measures how hot it feels. Both combine temperature and moisture, but they use different formulas and scales. The humidex almost always gives a higher number than the heat index for the same conditions. You should not compare them directly. Use the humidex if you are in Canada and the heat index calculator if you are in the United States. For cold weather, the equivalent measure is wind chill.
Why the Humidex Matters
High humidex values warn you about the risk of heat exhaustion and heat stroke. On days when the humidex is 40 or higher, drink plenty of water, stay in the shade or air conditioning, and limit time outdoors. Children, older adults, and people with heart or breathing problems are at the greatest risk. Checking the humidex before you go outside helps you stay safe in hot, humid weather.