Heat Index Calculator
The NWS heat index formula is only officially valid at 80°F (27°C) and above with 40%+ humidity — below that, this shows a simpler approximation, not the full formula.
Uses the NWS Rothfusz regression, which assumes an average body (5'7", 147 lb) in the shade with light wind — actual "feels like" temperature varies by individual and conditions.
Free heat index calculator using the NWS Rothfusz regression — enter temperature and relative humidity to see the "feels like" temperature accounting for humidity.
How it works
- Choose °F or °C
- Enter the air temperature and relative humidity
- See the heat index ("feels like") temperature instantly
Frequently asked questions
Why is heat index only accurate above 80°F?
The National Weather Service's formula is officially valid at 80°F (27°C) and above with 40%+ humidity — below that, this calculator falls back to a simpler approximation rather than extrapolating the full formula outside its tested range.
Why might this not match how hot it actually feels to me?
The formula assumes an average body size in the shade with light wind — actual perceived heat varies with individual body size, clothing, activity level, sun exposure, and actual wind conditions.
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From the blog
Update history
- 2026-09-03 Bounded numeric fields are now sliders instead of number inputs.
- 2026-09-02 Launched: heat index calculator using the NWS Rothfusz regression, with a validity-range fallback for cooler temperatures. Entirely client-side.