Humidex Calculator
Combine air temperature and dew point into the humidex — the comfort index Environment Canada uses to describe how hot a humid day really feels.
Two inputs, one comfort number
Enter the air temperature and the dew point in °C and the calculator returns the humidex from Environment Canada's official formula.
Reported only when it's warm
The humidex is meaningful when it exceeds the air temperature — warm, humid air. Below about 20 it is not reported, because dry or cool air barely changes how the temperature feels.
What is a humidex calculator?
Temperature and dew point in, feels-like out
A humidex calculator turns two weather readings — the air temperature and the dew point — into Canada's humidex, a single number describing how hot a humid day actually feels. The dew point measures how much moisture is in the air: the higher it is, the more humid the day. When the air is moist, sweat evaporates slowly and your body cools less efficiently, so a given temperature feels hotter. Environment Canada captures that effect by converting the dew point into a water-vapour pressure and adding a temperature-equivalent correction to the air temperature. The result is the number you hear in Canadian summer forecasts.
Enter the air temperature and dew point to get the humidex index instantly.
The humidex first turns the dew point Td into a vapour pressure e, then adds a scaled correction to the air temperature T.
e = 6.11 · exp(5417.7530 · (1/273.16 − 1/(Td + 273.15))); humidex = T + 0.5555 · (e − 10)The exponential term converts the dew point into the actual water-vapour pressure of the air in hectopascals. Subtracting the reference value of 10 hPa and multiplying by 0.5555 turns that excess moisture into a temperature-equivalent number of degrees, which is added to the real air temperature. More moisture means a larger correction and a higher humidex.
Suppose the air temperature is 30 °C and the dew point is 20 °C.
Vapour pressure from the dew point
e = 6.11 · exp(5417.7530 · (1/273.16 − 1/293.15)) ≈ 23.6 hPa — the moisture content of the air.
Add the moisture correction
0.5555 · (23.6 − 10) ≈ 7.6 — the extra "feels-like" degrees from humidity.
Read the humidex
30 + 7.6 ≈ 38 — it feels like about 38, far hotter than the 30 °C the thermometer shows.
The humidex is a comfort and safety gauge. Environment Canada's rough bands are: below 29 means little discomfort, 30–39 means some discomfort, 40–45 means a great deal of discomfort and caution is advised, and above 45 is dangerous, with heat stroke possible. A humidex of 38, as in the example, sits firmly in the "some discomfort" range and is a signal to slow down, find shade, and drink water. The single most useful insight is that the dew point drives the result: at a fixed air temperature, a higher dew point steadily pushes the humidex up, which is why a muggy 30 °C day is far less comfortable than a dry one. Use the result to decide whether to reschedule outdoor exercise, take more breaks, or stay in air conditioning.
The humidex is the official Canadian index, but a few practical points are worth keeping in mind.
Shade values, and a reporting threshold
The humidex assumes shade air temperature and light wind; direct sunshine makes it feel hotter still. Environment Canada only reports a humidex when it exceeds the air temperature and the value is roughly 20 or above — in cool or dry air the moisture correction is negligible. The humidex is unitless by convention even though it is built from a temperature, so it is not the same as a temperature reading. Finally, it is a population average: age, fitness, clothing, and hydration all change how a given humidex affects you, so treat it as guidance, not a personal medical limit.