Humidex Calculator

The humidex calculator computes the Canadian heat-stress index from just two inputs — air temperature and dew point — using the Environment Canada formula, then places the result on the official comfort scale from little discomfort to dangerous. It is the number you hear in Canadian weather forecasts on hot, humid summer days, and it answers the practical question: how hot is it actually going to feel?

The humidex exists because humid air feels hotter than dry air at the same temperature: sweat evaporates more slowly when the air is already full of moisture, so the body cools less effectively. The index turns that effect into a single number that behaves like a temperature.

What the inputs mean

• Air temperature — the actual outdoor temperature in °C.
• Dew point — the temperature at which the air would become saturated, a direct measure of moisture content. It comes from weather reports or from a dew point calculator and can never exceed the air temperature.

How it works

The humidex is calculated from the air temperature T and dew point T_d, both in °C:

$H = T + 0.5555 \times \left(6.11 \times e^{\,5417.7530 \times \left(\frac{1}{273.16} - \frac{1}{273.15 + T_d}\right)} - 10\right)$

The exponential term estimates the actual vapour pressure of the air from the dew point: 6.11 hPa is the saturation vapour pressure at 0 °C, and the exponent scales it to the air’s moisture content. The result minus 10, times 0.5555, is the number of degrees the humidity adds to the perceived temperature. Some Environment Canada references write the same equation with 273.15 in the exponent; the two forms differ by only a few hundredths of a degree.

Worked example

Take an air temperature of 30 °C and a dew point of 25 °C. First compute the two reciprocal terms:

$\frac{1}{273.16} = 0.00366086, \qquad \frac{1}{273.15 + 25} = \frac{1}{298.15} = 0.00335402$

The exponent is:

$5417.7530 \times (0.00366086 - 0.00335402) = 5417.7530 \times 0.00030684 = 1.66239$

Raising e to that power and multiplying by 6.11 gives the vapour pressure:

$6.11 \times e^{1.66239} = 6.11 \times 5.2719 = 32.211 \text{ hPa}$

Finally:

$H = 30 + 0.5555 \times (32.211 - 10) = 30 + 0.5555 \times 22.211 = 30 + 12.338 = 42.34$

The calculator displays 42, which falls in the 40–45 great discomfort band.

For comparison, saturated air at 40 °C (dew point 40 °C) gives:

$H = 40 + 0.5555 \times (6.11 \times e^{2.53280} - 10) = 40 + 0.5555 \times 66.917 = 77.17$

That rounds to 77 — dangerous.

Comfort categories

Environment Canada publishes four bands:

The scale is published from 20 upward, which is why ECCC reports the humidex only when the air temperature is at least 20 °C and the humidex is at least one degree above the temperature. The calculator still evaluates the formula below 20 °C for completeness, but the comfort bands should be read as the official scale.

For a humidex above 30, ECCC recommends limiting or modifying outdoor exercise; above 40, reduce all unnecessary physical activity. These are population-level guidelines — children, older adults, people with chronic conditions, and outdoor workers may be affected at lower values.

Tips

• Use the dew point from the same report as the temperature; mixing observations from different times or places adds error.
• Watch the trend: a humidex that climbs through the afternoon is a sign to shift outdoor activity earlier or later.
• On humid days, rely on shade and airflow, not just on the temperature reading.
• If you only know relative humidity, convert to dew point first with the dew point calculator, or use the U.S. heat index calculator instead.
• In cold weather, the humidex is not reported and the wind chill calculator is the relevant index.

Common pitfalls

• Entering relative humidity as if it were the dew point — the two are very different numbers.
• Entering a dew point above the air temperature; that combination is physically impossible.
• Using the humidex in winter or below 20 °C, where ECCC does not report it.
• Treating the category as a personal health forecast; individual tolerance varies widely.
• Confusing the humidex with the U.S. heat index; they are similar but computed differently.

Sources

• Environment and Climate Change Canada, How to use the Humidex — comfort categories and reporting conditions.
• Environment and Climate Change Canada, Wind Chill and Humidex Calculators — official humidex calculator inputs (air temperature and dew point).
• Environment and Climate Change Canada, Glossary — Humidex — humidex formula and definition.