Heat Index Calculator
Find the heat index — how hot it feels once humidity is factored in — from temperature and relative humidity, in °F or °C. Calculates in your browser.
Feels like
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Show the math
The NOAA heat index combines temperature and relative humidity:
HI = f(T, RH), NOAA Rothfusz regression
T is in °F and RH a percentage; Celsius inputs are converted internally. The index is meaningful in warm conditions (about 80°F and up) and assumes shade, direct sun can add up to 15°F. All math runs locally in your browser.
What this does
The heat index calculator finds how hot the air actually feels once humidity is taken into account, using the NOAA Rothfusz regression. High humidity slows sweat evaporation, so the felt temperature is higher than the thermometer reads.
How to use it
- Choose Fahrenheit or Celsius.
- Enter the air temperature.
- Enter the relative humidity.
- Read the heat index (feels-like) temperature.
How it works
The full NOAA regression combines temperature T (°F) and relative humidity RH (%), with two standard adjustments for very dry or very humid extremes. Below about 80°F a simpler average is used. Celsius inputs are converted internally.
HI = f(T, RH) — NOAA Rothfusz regression
Understanding your result
The heat index is what drives heat-illness risk — the higher it climbs, the greater the danger of heat exhaustion and heat stroke, especially during exertion. It is most meaningful in warm, humid conditions (about 80°F and up); in cool weather use wind chill instead, and remember direct sun can add roughly 15°F to the felt temperature.
Example
At 90°F with 70% humidity, the air feels like about 106°F.
Sources & methodology
Last updated .
Frequently asked questions
What is the heat index?
It is how hot the air feels once humidity is factored in. Humidity slows the evaporation of sweat, so your body cools less efficiently and the air feels hotter.
When does the heat index apply?
It is meaningful in warm, humid conditions — roughly 80°F (27°C) and above. In cool weather, wind chill is the relevant feels-like measure.
Does direct sun raise it?
Yes — the official heat index assumes shade. Full sunshine can add as much as 15°F to the felt temperature.
Is my data stored?
No. Everything is calculated locally in your browser.
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