Heat Index Calculator
Compute apparent temperature from heat + humidity (NWS Rothfusz).
Enter air temperature and relative humidity to compute the heat index — how hot it actually feels. Uses the NWS Rothfusz regression with caution-level warnings.
Common use cases
- Assess heat-stroke risk before outdoor exercise on humid days
- Decide whether to schedule outdoor events during a heat wave
- Understand why 90 °F at 80% humidity feels like 105 °F
- Verify weather app heat index numbers
Frequently asked questions
When does heat index apply?▾
Most accurate for T ≥ 80 °F and RH ≥ 40%. Below that, the formula falls back to a simpler average.
What do the warning levels mean?▾
<80 safe; 80-90 caution; 90-103 extreme caution; 103-125 danger; >125 extreme danger (heat stroke imminent).
Why does humidity matter?▾
High humidity slows sweat evaporation, the body's main cooling mechanism. Hot + dry air feels cooler than hot + humid.