Heat Index Calculator
Calculate apparent temperature from air temperature and humidity
Imperial mode uses degrees Fahrenheit and relative humidity percent.
°F
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Heat note: Heat index is an apparent temperature estimate. Follow local forecasts, shade, hydration, and heat advisories for real outdoor decisions.
Heat Index
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Humidity
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Feels Warmer By
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Heat Category
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Formula Mode
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Calculation Steps
How Heat Index Is Calculated
The calculator follows the NWS approach for combining temperature and humidity.
Humidity effect
High humidity slows evaporative cooling, so the same air temperature can feel much hotter.
NWS formula range
A simple apparent-temperature estimate is used first. When the result reaches about 80°F, the Rothfusz regression is applied.
Outdoor planning
Use heat index as one signal alongside sun exposure, activity level, clothing, wind, and local weather alerts.
Heat Index Examples
Compare how humidity changes the feels-like temperature.
Hot and humid
90°F with 70% humidity feels like about 106°F
Metric summer day
32°C with 70% humidity feels like about 41°C
Lower humidity
The same air temperature feels less intense when relative humidity falls.