How this heat index calculator works
The heat index — also called the apparent or "feels like" temperature — estimates how hot the air feels to your body once humidity is taken into account. Your body sheds heat mainly by evaporating sweat, and humid air slows that evaporation, so a muggy 90°F day can feel far hotter than a dry one. The heat index turns that combined effect into a single temperature.
This calculator uses the National Weather Service Rothfusz regression. With the air temperature T in °F and the relative humidity RH as a percentage, the core formula is HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH². Small adjustments are subtracted for very dry air (RH below 13% at 80–112°F) and added for very humid air (RH above 85% at 80–87°F). When the regression returns less than 80°F, a simpler average of the temperature and reading is used instead. If you enter °C it is converted to °F before the calculation and the result is converted back so you see both units.
NWS heat index categories
| Heat index (°F) | Heat index (°C) | Category & effects |
|---|---|---|
| 80–90°F | 27–32°C | Caution — fatigue possible with prolonged exposure |
| 90–103°F | 32–39°C | Extreme Caution — heat cramps and exhaustion possible |
| 103–124°F | 39–51°C | Danger — heat exhaustion likely, heat stroke possible |
| 125°F and up | 52°C and up | Extreme Danger — heat stroke highly likely |
Reference note: the heat index is defined for warm, shady, light-wind conditions (about 80°F / 27°C and up). Below that it is not meaningful, so the air temperature is shown instead. In full sun it can feel up to ~15°F (8°C) hotter than the value here.
Frequently asked questions
- What is the heat index?
- The heat index, or "feels like" temperature, combines air temperature and relative humidity to estimate how hot it actually feels. When humidity is high, sweat evaporates more slowly, so the same temperature feels hotter than the thermometer reads.
- How is the heat index calculated?
- This tool uses the NWS Rothfusz regression with T in °F and RH as a percentage, then applies low-humidity and high-humidity adjustments near the edges and a simple average below about 80°F. Celsius values are converted before and after.
- Why does humidity make it feel hotter?
- Your body cools by evaporating sweat. Humid air already holds a lot of moisture, so sweat evaporates slowly, less heat is carried away, and you feel hotter than the air temperature alone suggests — that extra perceived warmth is the heat index.
- What heat index is dangerous?
- 80–90°F (27–32°C) is Caution, 90–103°F (32–39°C) is Extreme Caution, 103–124°F (39–51°C) is Danger with heat exhaustion likely, and 125°F (52°C) and up is Extreme Danger where heat stroke is highly likely.
- What is the difference between heat index and the real temperature?
- The real air temperature is what a thermometer reads in the shade. The heat index adds humidity to estimate how hot it feels. In humid air the heat index is higher than the air temperature; in very dry air it can read slightly lower.
- Does the heat index work in the shade?
- Yes — the NWS heat index is defined for shady, light-wind conditions. In full sun it can feel up to about 15°F (8°C) hotter than the calculated value, so a sunny day is harder on your body than the number shown.