Heat Index Calculator

The heat index, or apparent temperature, is how hot it really feels when relative humidity is combined with the actual air temperature. High humidity slows the evaporation of sweat, which is how your body cools itself, so a humid 90°F day feels far hotter and is more dangerous than a dry one.

This calculator uses the official National Weather Service Rothfusz regression, a multi-term equation of temperature and relative humidity, with the standard low-humidity and high-humidity adjustments. Below about 80°F the NWS uses a simpler averaging formula, which this tool applies automatically.

The NWS thresholds are: 80–90°F caution (fatigue possible), 91–103°F extreme caution (heat cramps and exhaustion possible), 103–124°F danger (heat exhaustion likely, heat stroke possible), and 125°F or above extreme danger (heat stroke highly likely). These assume shade — full sun can add up to 15°F.

No. The official heat index is calculated for shady conditions. Direct sunlight can raise the apparent temperature by as much as 15°F (8°C), so exposure in full sun is more dangerous than the heat index alone suggests. Strong wind blowing over hot, humid skin can also make it feel worse.

They are the warm- and cold-weather counterparts of "feels like" temperature. The heat index combines heat and humidity to show how hot it feels in summer, while wind chill combines cold and wind to show how cold it feels in winter. Heat index applies at high temperatures; wind chill applies at or below 50°F.