How the heat index works
When humidity is high, sweat evaporates more slowly, so your body cannot shed heat as efficiently and the air feels hotter than the thermometer reads. The heat index, or "feels like" temperature, captures that combined effect of heat and moisture.
For mild conditions the calculator uses a simple averaging formula. Once the apparent temperature climbs into the warm range it switches to the full Rothfusz regression, with the low- and high-humidity corrections published by the U.S. National Weather Service. The answer is shown in both Fahrenheit and Celsius.
Reading the result
The higher the heat index relative to the actual temperature, the more the humidity is amplifying the heat. Use it as a rough guide to the strain hot weather places on the body.
- Roughly 27–32°C (80–90°F): fatigue possible with prolonged activity.
- Roughly 32–41°C (90–105°F): heat cramps and exhaustion become likely.
- Above about 41°C (105°F): heat exhaustion or heat stroke risk is high.
Practical tips
The heat index assumes shade and light wind. Direct sunlight can add the equivalent of several extra degrees, so outdoor exposure feels even hotter than the number suggests.
- Drink water before you feel thirsty and take breaks in the shade.
- Plan strenuous outdoor work for the cooler parts of the day.
Caveats and common mistakes
Enter the air temperature in Fahrenheit and humidity as a percentage; mixing up units is the most common error. The formula is intended for warm conditions and is not meaningful at low temperatures, where wind chill is the relevant measure instead.
This is general weather and comfort guidance, not medical advice. If you or someone nearby shows signs of heat illness, seek appropriate care.
Formula
Rothfusz regression (T in °F, RH in %)