Humidex vs Relative Humidity
Humidity shows up in weather reports two ways, and they answer different questions. Relative humidity tells you how close the air is to saturation — the number behind comfort ranges, condensation, and mold risk. The humidex tells you how hot it feels — a perceived temperature that combines heat and moisture into a single number. Both the humidex calculator and the relative humidity calculator take the same two readings, air temperature and dew point, and turn them into very different answers. Choosing the right one is really about choosing the question.
This page compares the two so you can match the tool to what you are planning. It is informational, not health guidance: heat-stress and humidity comfort depend on exposure, activity, and individual tolerance.
What each calculator does
The Humidex Calculator computes the Canadian heat-stress index from air temperature and dew point with the Environment Canada formula — H = T + 0.5555 × (6.11 × e^(5417.7530 × (1/273.16 − 1/(273.15 + Td))) − 10) — then maps the result onto the official comfort scale: 20–29 little discomfort, 30–39 some discomfort, 40–45 great discomfort, and 46 and over dangerous. Its worked example, 30 °C with a 25 °C dew point, produces a humidex of 42.34, displayed as 42 in the great-discomfort band. ECCC reports the index only when the air temperature is at least 20 °C and the humidex is at least one degree above the temperature, so the calculator is aimed at hot, humid summer days.
The Relative Humidity Calculator takes the same inputs — air temperature and dew point, in Celsius or Fahrenheit — and estimates the percent saturation using the Magnus-Tetens equation, RH = 100 × exp(17.625 × Td / (243.04 + Td)) / exp(17.625 × T / (243.04 + T)). Its default example, 24 °C with a 15 °C dew point, gives 57.15 percent, inside the page’s 30-to-60 percent comfortable band. The page ties the result to practical consequences: high humidity plus cool surfaces means condensation and mold risk, while very low humidity speeds drying.
Side-by-side comparison
| Feature | Humidex | Relative Humidity |
|---|---|---|
| Question answered | How hot does it feel? | How close is the air to saturation? |
| Inputs | Air temperature and dew point (°C) | Air temperature and dew point (°C or °F) |
| Formula | Environment Canada humidex equation | Magnus-Tetens vapor-pressure ratio |
| Output | ’Feels like’ number with comfort bands (20–29 … 46+) | Percentage, clamped to 0–100, with a 30–60% comfort label |
| Sensitivity | Tracks moisture directly; higher is worse | Falls as temperature rises even when moisture is unchanged |
| Typical use | Outdoor heat planning in hot, humid weather | Indoor comfort, condensation, mold risk, drying |
When to use which
Use the humidex calculator when the day is hot and humid and you need to plan outdoor activity — the answer is a perceived temperature you can compare against the bands, and above 30 ECCC recommends limiting or modifying exercise, above 40 reducing all unnecessary physical activity.
Use the relative humidity calculator when the number you need is the percent itself: an indoor comfort target, a basement or greenhouse moisture check, a condensation question, or a drying estimate. Remember that a single afternoon reading can hide how close the overnight air will get to saturation, and that 60 percent at 10 °C contains far less vapor than 60 percent at 30 °C. For cold weather the humidex is not reported and the wind chill calculator is the relevant index; for the U.S. heat index, which uses relative humidity rather than dew point, see the heat index calculator.
Where to start
- Humidex Calculator — heat-discomfort index with Environment Canada comfort bands.
- Relative Humidity Calculator — percent saturation from temperature and dew point, with comfort and mold-risk guidance.
Informational note: This page is an educational comparison, not health guidance. Heat-stress and humidity effects vary with exposure, activity, clothing, and individual health, and the comfort bands are population-level guidelines.