Heat Index Calculator
Calculate the felt temperature by combining temperature with relative humidity.
Tip: With a heat index above 40°C, limit physical activity, stay in the shade, and drink water every 15–20 minutes even if you don't feel thirsty.
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About this digital tool
The heat index calculator shows how hot the weather feels when temperature and relative humidity combine. Enter the temperature in °C and the humidity as a percentage, and get the felt temperature together with the risk level.
The tool uses the US National Weather Service regression (Rothfusz), converting the temperature to Fahrenheit and the result back to Celsius. The formula is valid for temperatures above 27°C and humidity above 40%; outside this range the tool marks the result as not valid. The levels run from Caution, Danger, High danger to Extreme danger. It is useful in summer for deciding when to work, exercise or stop in the shade.
How to use
- Enter the current temperature (°C)
- Enter the relative humidity (%)
- See the heat index (the felt temperature)
- See the risk level (low/medium/high/extreme)
- See the health tips
Frequently asked questions
When does the heat index become dangerous?
27–32°C: caution (fatigue possible). 32–41°C: danger (heat stroke possible). 41–54°C: high danger. Above 54°C: extreme danger (heat stroke very likely).
How does humidity affect it?
High humidity hinders the evaporation of sweat — the body's main cooling mechanism. With 40% humidity, 35°C feels like 36°C. With 80% humidity, 35°C feels like 50°C!
Does it apply only in summer?
Mostly yes. The formula is valid when the temperature is ≥ 27°C and humidity ≥ 40%. In other conditions, the real temperature is representative of the felt one.
Why does humidity increase the feeling of heat?
When the air is humid, sweat evaporates more slowly and the body cools less well. That is why 32°C with high humidity feels hotter than 32°C in dry air.