WeatherInfo

Heat index calculator

The heat index is how hot it feels when humidity is added to the air temperature. Enter both to calculate it the way the National Weather Service does.

Reviewed October 2026

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Heat index calculator

Enter the temperature and humidity.

How it's calculated

We use the National Weather Service's heat index equation:

  1. Start with a simple formula, and average its result with the temperature. If that comes out below 80°F, that's the heat index.
  2. Otherwise, use the full Rothfusz regression, a formula fitted to Steadman's original heat index tables.
  3. Adjust down slightly when humidity is below 13% at 80 to 112°F, and up slightly when humidity is above 85% at 80 to 87°F.

The NWS notes the equation isn't valid for extreme combinations of temperature and humidity beyond the data it was built from.

As a check, the National Weather Service gives 96°F with 65% humidity as a heat index of 121°F.

What the number means

  • It assumes shade and a light wind. The NWS says full sunshine can raise heat index values by up to 15°F.
  • Higher means more danger. As the heat index rises, so does the risk of heat cramps, heat exhaustion and heat stroke, especially with exertion or long exposure.
  • Watch for official heat alerts. Search your ZIP code in the forecast to see any heat advisories or warnings in effect.

Sources