Weather calculators

Humidex Calculator

Updated Oct 8, 2026 By Infinity Calculator
Rate Formulas Sources

Inputs

Air Temperature
Enter either unit — the other updates automatically.
Moisture Input
A value between 0 and 100.
Derived Dew Point: —

Results

Intense discomfort
Humidex 40–45°C
Humidex (°C) 40.9°C
Humidex (°F) 105.6°F

Humidex vs. Relative Humidity (at current air temperature)

Step-by-Step Solution

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 Environment and Climate Change Canada includes it in current conditions, weather forecasts and heat warnings.2

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. 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 combines the air temperature and humidity into one number that reflects the perceived temperature.2

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 to which air must be cooled to reach saturation.4 A higher dew point means more moisture is present in the air.4 If you only know the relative humidity, the calculator uses the Magnus formula to find the dew point first.3 Then it finds the vapour pressure from the dew point and plugs it into the humidex equation:

Environment and Climate Change Canada's standard formula is Humidex = Air Temperature + 0.5555 × (Vapour Pressure − 10), with the vapour pressure in hectopascals worked out from the dew point.1

The result is shown in both Celsius and Fahrenheit.

Humidex Comfort Levels

Environment and Climate Change Canada groups humidex readings into four ranges, from the lowest level of discomfort to the highest:

  • 20 to 29: little discomfort.2
  • 30 to 39: some discomfort.2
  • 40 to 45: great discomfort; avoid exertion.2
  • 46 and over: dangerous; heat stroke possible.2

Some people are more affected by the humidex than others.2

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. They can give quite different numbers for the same conditions. At 30°C and 70% humidity, for example, the humidex is 41.2 The NWS heat index for the same conditions works out to about 35°C. 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. Environment and Climate Change Canada calls any humidex over 40 an extremely high reading, and says to reduce all unnecessary physical activity at that point.2 Drink plenty of water, stay in the shade or air conditioning, and limit time outdoors. Children, older adults, and people with chronic medical conditions are at higher risk.5 Checking the humidex before you go outside helps you stay safe in hot, humid weather.

Formulas used

Magnus gamma parameter 3
\gamma = \ln\!\left(\frac{RH}{100}\right) + \frac{17.625 \times T}{243.04 + T}
Dew point from gamma (Magnus formula) 3
T_d = \frac{243.04 \, \gamma}{17.625 - \gamma}
Relative humidity from dew point 3
RH = 100 \, e^{\left(\frac{17.625 \times T_d}{243.04 + T_d} - \frac{17.625 \times T}{243.04 + T}\right)}
Vapour pressure from dew point 1
e = 6.11\,\exp\!\left[5417.7530\left(\frac{1}{273.15} - \frac{1}{T_d + 273.15}\right)\right]
Humidex formula 1
H = T + 0.5555 \times (e - 10)
Celsius to Fahrenheit conversion 6
T_{^\circ F} = \frac{9}{5} \times T_{^\circ C} + 32

Frequently asked questions

What is the difference between humidex and air temperature?

Air temperature is the actual heat measured by a thermometer. The humidex adds the effect of humidity to that number. When the air holds a lot of moisture, sweat dries slowly and your body feels hotter. The humidex shows that "feels like" temperature. It is higher than the air temperature whenever the vapour pressure is above 10 hPa, because the formula adds 0.5555 × (vapour pressure − 10) to the temperature.1

Should I use relative humidity or dew point as my moisture input?

Use whichever value you have. Most weather apps and home weather stations show relative humidity, so that is the easier choice for most people. If you have a dew point reading, switch to dew point mode. The calculator gives the same humidex result either way.

Where do I find the current relative humidity or dew point for my area?

Check your local weather service website, a weather app on your phone, or a home weather station. Most of these sources list both relative humidity and dew point. You can enter either one into the calculator.

Why is my humidex value the same as the air temperature?

This happens when the humidity is very low. If the vapour pressure is close to 10 millibars, the humidity adds almost nothing to the temperature. In dry air, the humidex will equal or even be slightly below the air temperature. Environment and Climate Change Canada only shows hourly humidex values when the air temperature is 20°C or greater and the humidex is at least 1 degree greater than the air temperature.1

Can the humidex be lower than the air temperature?

Yes, in very dry conditions. When the vapour pressure drops below 10 millibars, the formula subtracts a small amount from the air temperature.1 This can make the humidex a fraction of a degree lower than the actual temperature.

Why can't my dew point be higher than the air temperature?

The dew point is the temperature to which air must be cooled to reach saturation.4 Air that has already cooled to its dew point is saturated, so the dew point cannot be higher than the air temperature. When the dew point equals the air temperature, the relative humidity is 100%.3

Is the humidex used outside of Canada?

The humidex was created in Canada and is mainly used there. Other countries use different scales. The United States uses the heat index, and some countries use the wet bulb globe temperature. Each measures heat stress in a slightly different way.

What should I do when the humidex is above 40?

Environment and Climate Change Canada calls any humidex over 40 an extremely high reading, and says you should reduce all unnecessary physical activity.2 Drink plenty of water, stay in shade or air conditioning, and avoid hard exercise outdoors. Take frequent breaks if you must be outside. Watch for signs of heat illness such as dizziness, nausea, or a fast heartbeat. Children, older adults, and people with health conditions face the highest risk.

Does wind affect the humidex value?

No. The humidex only uses air temperature and moisture.1 Wind can help sweat evaporate and make you feel cooler, but it does not change the humidex number. Think of the humidex as a calm-air "feels like" temperature.

Does the humidex work in cold weather?

The humidex is designed for warm weather. Environment and Climate Change Canada includes it in 24-hour forecasts only when the temperature reaches 25°C or higher, and in current conditions only when it is 20°C or higher, in both cases when the humidex is at least one degree above the temperature.2 For cold weather, wind chill is the better measure because wind strips heat from your body much faster in the cold.

Sources

  1. Glossary: Humidex. Environment and Climate Change Canada. Humidex. Accessed September 29, 2026.
  2. How to use the Humidex. Environment and Climate Change Canada. Understanding Humidex readings; Tables 1-3. Accessed September 29, 2026.
  3. Lawrence MG. The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications. Bulletin of the American Meteorological Society. 2005;86(2):225-233. doi:10.1175/BAMS-86-2-225. Accessed September 29, 2026.
  4. Glossary: Dew Point. NOAA National Weather Service. Accessed September 29, 2026.
  5. Heat and Your Health: Risk Factors. Centers for Disease Control and Prevention. Accessed September 29, 2026.
  6. NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. National Institute of Standards and Technology. Special Publication 811, App. B.8 (degree Fahrenheit). Accessed September 29, 2026.

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