Heat Index Calculator
Humidity is what makes heat dangerous. Enter the air temperature and relative humidity to get the heat index — how hot it actually feels — using the exact equation the US National Weather Service uses.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (NWS Rothfusz heat-index regression; NWS heat-risk categories). Last verified July 2026 — our methodology.
Feels like
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- Actual temperature
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- Feels hotter by
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- NWS risk level
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Calculated with the National Weather Service Rothfusz regression, the same equation behind the NWS heat-index chart, including the low-humidity and high-humidity adjustments. Below about 80°F the "feels like" temperature is essentially the air temperature.
How the heat index works
The heat index combines air temperature and humidity into a single "feels like" number. When humidity is high, sweat evaporates slowly, so your body cannot shed heat and the air feels hotter than the thermometer reads. This tool uses theNWS Rothfusz regression, the published equation the National Weather Service uses for its heat advisories, so the result matches the official NWS heat-index table. The four risk bands — Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–124°F), and Extreme Danger (125°F+) — are the NWS's own categories. Indoors, the same physics is why keepinghumidity in the 30–50% rangemakes a warm room feel cooler, and why ahigh dew point feels so oppressive.