♨️ Heat Index Calculator
Enter the air temperature and relative humidity to get the ‘feels like’ heat index in °F and °C, plus the National Weather Service risk category — so you know how dangerous the heat really is.
🌡️ Temperature Meets Humidity
What is a Heat Index Calculator?
It converts a plain thermometer reading and the humidity into the apparent temperature your body actually experiences. High humidity blunts sweating — your main cooling system — so a 90°F afternoon at 70% humidity can feel like 105°F. The tool applies the National Weather Service equation and sorts the result into a clear risk band.
Use it before a workout, a shift outdoors, or a day at the beach to judge whether it's safe, whether to slow down, or whether to stay in the shade with a fan and plenty of water. It's a documented estimate for general awareness, assuming shade and light wind — not medical advice.
❓ Frequently Asked Questions
What is the heat index?
The heat index — often called the 'feels like' temperature — combines air temperature and relative humidity into a single number that reflects how hot it actually feels to the body. When humidity is high, sweat evaporates more slowly, so your body can't cool itself as well and the air feels hotter than the thermometer reads.
How is the heat index calculated here?
This tool uses the U.S. National Weather Service Rothfusz regression, the multi-term equation behind the official NWS heat-index charts. It takes the air temperature in Fahrenheit and the relative humidity as a percentage and returns the apparent temperature, which we also convert to Celsius.
What do the risk categories mean?
The NWS groups heat-index values into bands: below 80°F is generally safe; 80–90°F is Caution; 91–103°F is Extreme Caution; 104–124°F is Danger; and 125°F or above is Extreme Danger. As you climb the bands, the risk of heat cramps, heat exhaustion, and heatstroke rises, especially with prolonged exposure or physical activity.
Does direct sun make the heat index higher?
Yes. The NWS equation assumes shady, lightly breezy conditions. Standing in full sunshine can raise the effective heat index by roughly 15°F, so treat the figure here as a shade estimate and add a margin when you're out in direct sun. This is general awareness information, not medical advice.