WBGT Calculator
Set the air temperature, humidity, sun angle, cloud cover, and wind. The calculator estimates the wet-bulb globe temperature (WBGT) and turns it into one of four risk flags.
Estimated WBGT
°F
–
Near
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Heat index:
This combination is hotter and more humid than conditions ever recorded on Earth.
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 WBGT is
Wet-bulb globe temperature (WBGT) is designed to capture how dangerous outdoor heat conditions are. It combines air temperature, humidity, sun strength, and wind into one number. It is the gold-standard heat stress index, used worldwide for decades to decide when to limit outdoor activity. WBGT can be measured with physical sensors or estimated with mathematical models. This calculator estimates it with the same engine as the Swelter app. Use WBGT when you plan to be active outdoors. It shows how risky the heat is for exertion in direct sun.
How to use this calculator
Set the five sliders to match your conditions. Air temperature, humidity, cloud cover, and wind are the numbers any forecast gives you. For sun angle, think about where the sun sits in the sky: morning and evening sun sits low, and midday summer sun sits high. The result is a WBGT estimate and one of four risk flags, following the American College of Sports Medicine's published thresholds. When a reading sits just below a flag boundary, a Near line names the next flag.
Why not just use the heat index?
The heat index tells you how hot the air feels by combining temperature and humidity. It assumes you are at rest in the shade with a light breeze. WBGT adds sun strength and wind, with thresholds built for outdoor exertion. That is why a humid 88 °F day can be more dangerous than a dry 100 °F one, and why air temperature alone is not a safety number. You can compare the two on the heat index calculator.
Swelter is validated against the world's largest WBGT monitoring network. Read how the engine was validated.