Heat Index Calculator
Calculate how hot it feels (apparent temperature) using the Rothfusz regression formula with NWS adjustments. Includes danger category classification.
About the Heat Index Calculator
The heat index combines air temperature and relative humidity to estimate how hot it feels to the human body. High humidity reduces the effectiveness of sweat evaporation—the body's primary cooling mechanism—making the air feel much hotter than the actual temperature. The Rothfusz regression formula used here is the same one used by the National Weather Service. For cold-weather conditions, see our Wind Chill Calculator.
The heat index is only meaningful at temperatures at or above 80°F (26.7°C). At lower temperatures, the apparent temperature is close to the actual temperature. Two NWS adjustments are applied: one for low humidity between 80–112°F, and one for high humidity between 80–87°F. To understand moisture in the air more broadly, use our Dew Point Calculator to assess comfort and condensation risks.
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For a related calculation, use Dew Point Calculator to calculate dew point from temperature and relative humidity. Includes comfort level interpretation. Alternatively, use Wind Chill Calculator to calculate wind chill temperature using the NWS formula. Shows frostbite risk thresholds.