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Heat Index Calculator

Free heat index calculator using the NWS Rothfusz regression. Enter temperature and relative humidity to get the "feels like" heat index in °F and °C plus the official caution, danger and extreme-danger bands.

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Enter a temperature and humidity.

Deep dive guides

A LITTLE CONTEXT · 3 MIN READHeat Index Explained: What Humidity Does to a Hot Day
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How heat index calculator works

Humid heat feels worse because sweat must evaporate to cool you, and humid air slows that evaporation. The heat index combines air temperature and relative humidity into one apparent temperature. The NWS uses the Rothfusz regression — a multi-term fit to full-body heat-balance model data — with two small adjustments for very dry air (RH under 13%) and very humid air (RH over 85%).

The full regression only applies when conditions are genuinely hot: below an apparent 80°F, the NWS switches to a simple linear estimate averaged with the air temperature. This tool follows that exact sequence, then maps the result onto the NWS safety bands — Caution (80–89°F), Extreme Caution (90–102°F), Danger (103–124°F) and Extreme Danger (125°F and above).

Enter a temperature (°F or °C) and relative humidity in percent. The output shows the heat index in both scales, which band it falls in, and which formula branch ran. Like wind chill, heat index is defined for shade conditions — full sun can add up to 15°F to how hot it actually feels.

Frequently asked questions

Why does 90°F with 70% humidity feel like 106°F?
At high humidity sweat evaporates slowly, so your body's main cooling channel loses capacity while heat production continues. The Rothfusz regression converts that reduced cooling into an equivalent still-air temperature: 90°F at 70% RH works out to a heat index near 106°F, which the NWS rates as Danger territory.
What heat index is dangerous?
The NWS bands: 80–89°F Caution (fatigue possible with prolonged exposure), 90–102°F Extreme Caution (heat cramps or exhaustion possible), 103–124°F Danger (heat exhaustion likely, heat stroke possible), and 125°F+ Extreme Danger (heat stroke highly likely). The calculator names the band for any temperature-humidity pair.
Why does the calculator sometimes use a different formula?
The Rothfusz regression is only appropriate for heat indices of 80°F and above. On cooler days the NWS uses a simpler linear estimate and averages it with the air temperature, which behaves better in mild conditions. The result reports which branch (simple or Rothfusz) produced your answer.
Is heat index measured in the sun or shade?
Shade. The NWS defines heat index for shaded, light-wind conditions; direct sunlight can push the apparent temperature up to 15°F higher. Plan for the shaded value as a floor, and add margin for sunny work or play.
How accurate is the heat index?
It is an empirical fit for typical summer conditions, not a physical constant: expect it to be most reliable between roughly 80–110°F with 40–95% humidity. Individual factors — hydration, clothing, exertion, wind — shift how any specific person experiences the same heat index.

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