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Humidex Calculator

Find how hot it feels with the humidex.

How it is calculated

Two steps. First the vapour pressure from the dew point: e = 6.11 × exp(5417.753 × (1/273.16 − 1/(Td + 273.16))), with Td the dew point in °C and e in hPa. Then Humidex = T + 0.5555 × (e − 10), where T is the air temperature in °C. Both inputs are in degrees Celsius and the result is a figure on the same scale, rounded to two decimals.

The tool labels the value: below 30 Comfortable, 30 to under 40 Some discomfort, 40 to 45 Great discomfort, above 45 Dangerous. These are the tool's own bands. Note the input is dew point, not relative humidity.

Worked example

Air temperature 30 °C and dew point 20 °C. The vapour pressure comes out at about 23.64 hPa. Subtract 10: 13.64. Multiply by 0.5555: about 7.58. Add the temperature: 30 + 7.58 = 37.58. The tool reports 37.58 °C, category Some discomfort: the air feels like roughly 38 °C even though the thermometer says 30.

Try this example →

Common mistakes

  • Entering relative humidity (such as 65) in the dew point field. The tool needs the dew point temperature in °C; a value of 65 would give an absurd result.
  • Typing temperatures in Fahrenheit. Both fields are in °C.
  • Reading the humidex as an actual air temperature. It is a comfort index on a Celsius-like scale, and a thermometer would still read the real temperature.
  • Swapping the two fields. The dew point can never be above the air temperature in reality; if it is, re-check your inputs.
  • Comparing it with the US heat index. The two use different formulas and give different numbers for the same weather.

About this calculator

Humidex is the index Canadian meteorologists use to describe how hot the air feels when humidity is factored in alongside temperature. It's computed as humidex = T + 5/9 × (e − 10), where T is the air temperature and e is the vapor pressure (in hPa) derived from the dew point — high moisture content pushes e up and makes the air feel hotter than the thermometer alone suggests, because sweat evaporates less efficiently.

Environment and Climate Change Canada uses humidex values to trigger heat-warning thresholds (45+ is considered dangerous, 54+ requires immediate action), and forecasters, outdoor-event organizers, and employers with heat-safety policies use it the same way Americans use the related but differently-calculated heat index. Enter the air temperature and dew point (or relative humidity) to get the perceived-heat value.

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