Heat Index Calculator
Calculate the feels-like temperature based on air temperature and relative humidity.
Includes danger level warnings for heat-related illness.
Heat Index
Heat index estimates how hot it actually feels when humidity is factored in with the air temperature. The NWS uses the Rothfusz regression equation:
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783×10⁻³T² - 5.481717×10⁻²R² + 1.22874×10⁻³T²R + 8.5282×10⁻⁴TR² - 1.99×10⁻⁶T²R²
Where T = temperature (°F) and R = relative humidity (%).
Danger levels (NWS):
- Below 80°F: No heat index concern
- 80–90°F: Caution — fatigue possible with prolonged activity
- 91–103°F: Extreme Caution — heat cramps and exhaustion possible
- 104–125°F: Danger — heat cramps and exhaustion likely, heatstroke possible
- Above 125°F: Extreme Danger — heatstroke highly likely
Tips:
- Heat index is calculated in shade; direct sunlight can add up to 15°F
- At low humidity (<13%) with high heat (80–112°F), a correction is subtracted
- At high humidity (>85%) with moderate heat (80–87°F), a correction is added