Heat Index Calculator
Set the air temperature and humidity. The calculator gives the heat index using the National Weather Service formula.
Heat index
°F
This combination is hotter and more humid than conditions ever recorded on Earth.
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Categories and thresholds from the U.S. National Weather Service.
Disclaimer
Swelter is a planning tool, not a medical device. Its readings are estimates from weather data, not measurements from a physical device. Conditions can change quickly. When in doubt, choose the cooler option and use your best judgment. Guidance in the app comes from published, publicly available materials, and Swelter is not affiliated with, endorsed by, or sponsored by the organizations behind them.
Use of this site is subject to the Terms of Use.
Get this live for your location
Swelter computes WBGT & heat index for your exact location, hour by hour, from Apple Weather data. It sends alerts before severe heat and finds the best window for outdoor activity today. Available now on the App Store.
What the heat index is
The heat index tells you how hot the air feels by combining temperature and humidity. Your body cools itself mainly by evaporating sweat, and humid air slows that down, so the same temperature is harder on your body when the humidity is high. The heat index assumes you are at rest in the shade with a light breeze. This calculator uses the National Weather Service formulas: the published heat index regression at 80 °F and above, and the simpler NWS formula below that. Use the heat index for how hot the air feels at rest in the shade. For activity in the sun, use WBGT.
What the categories mean
The scale follows the National Weather Service categories: Caution starts at 80 °F, Extreme caution at 90, Danger at 103, and Extreme danger at 125. The categories describe how hard the heat is on a body at rest. The formula is defined for the range of the official NWS chart, so readings past its edge display as 137+.
When to use WBGT instead
Step into direct sun and the load on your body goes up, but the heat index does not move, because sun and wind are not in the formula. WBGT accounts for both, with thresholds built for outdoor exertion. Two days with the same heat index can carry very different risk once you are active in the sun. The WBGT calculator shows the same conditions with sun and wind included.
The engine behind both calculators is validated against the world's largest WBGT monitoring network.