What Is a Heat Index Calculator?
Heat index (sometimes called the "feels like" temperature) combines air temperature and relative humidity to estimate how hot it actually feels to the human body — since high humidity reduces the effectiveness of sweat evaporation, making heat feel more intense.
How to Use the Heat Index Calculator
- Enter the air temperature.
- Enter the relative humidity.
- View the calculated heat index and general risk level.
Heat Index Formula
This calculator uses the National Weather Service's Rothfusz regression formula, a polynomial equation based on temperature and humidity, valid for temperatures at or above 80°F.
Heat Index Risk Categories (NWS Reference)
| Heat Index | Risk Level |
|---|---|
| 80–90°F | Caution |
| 90–103°F | Extreme Caution |
| 103–124°F | Danger |
| 125°F+ | Extreme Danger |
Worked Example
Heat index: approximately 105°F
Risk category: Danger
Understanding Your Results
Higher humidity at the same temperature increases the heat index because sweat evaporates less efficiently, reducing the body's ability to cool itself. This is why humid heat often feels more oppressive than dry heat at the same air temperature.
Heat Safety Tips
- Stay hydrated and take breaks in shade or air conditioning during high heat index conditions.
- Limit strenuous outdoor activity during "Danger" or "Extreme Danger" periods.
- Watch for signs of heat exhaustion or heat stroke, which require immediate attention.
Frequently Asked Questions
A measure of how hot it actually feels to the human body when relative humidity is combined with air temperature, since humidity reduces the cooling effect of sweat evaporation.
High humidity slows sweat evaporation, which is the body's primary cooling mechanism, making the same air temperature feel hotter and increasing heat-related health risks.
The National Weather Service generally considers heat index values above 103°F as 'Danger' and above 125°F as 'Extreme Danger,' with increased risk of heat-related illness.
No — heat index represents the perceived temperature accounting for humidity, which is often higher than the actual measured air temperature in humid conditions.
Stay hydrated, limit strenuous outdoor activity, seek shade or air conditioning, and watch for signs of heat exhaustion or heat stroke, which require prompt attention.