Heat Index Calculator

Calculate the heat index

The heat index is how hot it really feels when humidity is combined with the air temperature, because high humidity slows the evaporation of sweat that normally cools the body. This calculator uses the Rothfusz regression equation used by the United States National Weather Service, taking the air temperature in Fahrenheit and the relative humidity. At ninety degrees with seventy percent humidity, it feels like about one hundred six degrees, sixteen degrees hotter than the actual temperature.

The calculator also classifies the risk level, from comfortable up to extreme danger, matching the official heat index categories.

Heat and humidity safety

A high heat index signals a real danger of heat-related illness such as heat exhaustion and heat stroke, because the body struggles to cool itself when sweat cannot evaporate. Weather services issue heat advisories based on the heat index rather than the raw temperature. Knowing it helps you decide when to limit outdoor activity, hydrate and seek shade or air conditioning.

The regression formula is most accurate at higher temperatures and humidities, where heat stress is a concern. The result is shown in Fahrenheit and Celsius. All calculation happens locally in your browser.

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