About the Humidex Calculator
This humidex calculator works out the humidex — the index Environment and Climate Change Canada (ECCC) uses to describe how hot and humid weather feels to the average person. Enter the air temperature and either the dew point or the relative humidity, and it returns the humidex value, the matching Government of Canada comfort category, the water-vapour pressure, and how many degrees hotter than the thermometer reading it feels.
It is useful for Canadians reading summer forecasts, outdoor workers and coaches planning activity on hot days, event organisers and students comparing humidex with the US heat index. A chart shows how the humidex climbs with humidity at the current temperature.
The calculation uses the exact formula published in ECCC’s climate glossary, which is based on the dew point. If you enter relative humidity instead, the dew point is first estimated with the Magnus equation, which is accurate to within a fraction of a degree in normal weather. ECCC only reports humidex when the air temperature is at least 20 °C and the humidex is at least 1 degree higher than the temperature; the calculator tells you whether that condition is met. Formula, reporting rule and comfort bands were checked against the current ECCC and Government of Canada pages in October 2026.
With the default inputs, the humidex is 39.3. Change any value above to recalculate instantly.
How to use the humidex calculator
- 1Enter the air temperature in °C.
- 2Choose whether you have the dew point or the relative humidity.
- 3Enter the dew point or humidity from your forecast or weather station.
- 4Read the humidex and its comfort category.
- 5Check the chart to see how much more humidity would raise the humidex.
Formula and method
Environment and Climate Change Canada defines humidex as the air temperature T (°C) plus a humidity term h = 0.5555 × (e − 10), where e is the water-vapour pressure in hectopascals calculated from the dew point Td expressed in kelvin (K = °C + 273.15). When the vapour pressure is 10 hPa (a dew point of about 7 °C), humidex equals the air temperature; more moisture makes it higher.
If you enter relative humidity, the dew point is estimated with the Magnus formula Td = b·γ / (a − γ), where γ = ln(RH/100) + aT/(b + T), a = 17.625 and b = 243.04 °C. The comfort categories are those published by the Government of Canada: 20–29 little discomfort, 30–39 some discomfort, 40–45 great discomfort (avoid exertion), and above 45 dangerous (heat stroke possible).
- T
- Air temperature (°C)
- Td
- Dew point temperature (K in the formula)
- e
- Water-vapour pressure (hPa)
- RH
- Relative humidity (%)
Worked examples
Humid summer afternoon: 30 °C with a 22 °C dew point
e = 6.11 × exp[5417.753 × (1/273.15 − 1/295.15)] = 26.80 hPa. Humidex = 30 + 0.5555 × (26.80 − 10) = 39.3, which rounds to 39 — the top of the “some discomfort” range.
32 °C at 60% relative humidity
The Magnus formula gives a dew point of 23.3 °C and a vapour pressure of 29.0 hPa, so humidex = 32 + 0.5555 × 18.98 = 42.5. That falls in the 40–45 band: great discomfort, avoid exertion.
Pleasant day: 25 °C with a 10 °C dew point
Vapour pressure is 12.31 hPa, only slightly above the 10 hPa neutral point, so humidex = 25 + 0.5555 × 2.31 = 26.3 — little discomfort.
Frequently asked questions
What is the humidex?+
Humidex is a Canadian index that combines temperature and humidity into one number describing how hot the weather feels. A humidex of 40 means 30 °C humid air feels roughly like 40 °C dry air.
How is humidex calculated?+
Environment Canada uses Humidex = T + 0.5555 × (e − 10), where T is air temperature in °C and e is the vapour pressure in hPa from the dew point: e = 6.11 × exp[5417.7530 × (1/273.15 − 1/Td)], with Td in kelvin.
What humidex level is dangerous?+
The Government of Canada scale says 20–29 is little discomfort, 30–39 some discomfort, 40–45 great discomfort (avoid exertion), and above 45 is dangerous, with heat stroke possible.
What is the difference between humidex and heat index?+
Both express “feels like” heat from temperature and humidity. Humidex is used in Canada and is based on dew point in °C; the US heat index uses relative humidity in °F and a different regression, so the two give different numbers.
Why is humidex not reported on cooler days?+
ECCC only shows hourly humidex when the air temperature is 20 °C or more and the humidex is at least 1 degree above the temperature, because humidity has little effect on comfort in cooler conditions.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.