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Heat Index Calculator
Find out how hot it actually feels based on humidity and temperature.
What is a Heat Index Calculator?
A Heat Index Calculator estimates how hot it actually feels to the human body by combining air temperature and relative humidity. This "feels like" temperature is often much higher than the thermometer reading, especially in humid conditions.
How to Use the Heat Index Calculator
- Select your temperature unit (°C or °F).
- Enter the current air temperature and relative humidity.
- Click "Calculate Heat Index" to see the feels-like temperature in both units.
- Check the heat risk category shown below the result.
Formula Behind the Calculation
This calculator uses the Rothfusz regression, the same method used by the US National Weather Service. It combines temperature and humidity in a multi-term equation, with small adjustments for very dry or very humid air. Below roughly 80°F (27°C) a simpler formula is used, because the heat index is close to the air temperature.
The heat index rises as either temperature or humidity rises, because humid air slows sweat evaporation, which is the body's main way of cooling down.
Heat Risk Categories
| Heat Index | Category | Possible Effects |
|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue with prolonged exposure |
| 90–103°F (32–39°C) | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103–125°F (39–52°C) | Danger | Heat exhaustion likely, heat stroke possible |
| 125°F+ (52°C+) | Extreme Danger | Heat stroke highly likely |
Worked Example
At an air temperature of 35°C (95°F) with 70% relative humidity, the heat index is about 50°C (123°F). That falls in the "Danger" category, where heat cramps and heat exhaustion are likely with continued exposure.
Why This Tool Matters
The heat index helps outdoor workers, athletes, and event planners judge real heat risk beyond the thermometer. It shows when extra water and rest breaks are needed. Note that these values assume shade; direct sunshine can make it feel up to 15°F (8°C) hotter.
Frequently Asked Questions
1. Why does humidity make heat feel worse than the actual temperature?
High humidity reduces sweat evaporation, which is the body's primary cooling mechanism, so the air feels hotter than the thermometer shows.
2. What heat index level is considered dangerous?
A heat index above 103°F (39°C) is in the "Danger" category, and 125°F (52°C) or more is "Extreme Danger" for prolonged exposure.
3. Does this calculator work for both Celsius and Fahrenheit?
Yes. Choose your unit from the dropdown and the result is shown in both °F and °C.
4. Is the heat index the same as the actual temperature?
No. The actual temperature is what a thermometer reads. The heat index is how hot it feels to your body once humidity is included.