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 (also called “feels like” temperature or apparent temperature) combines air temperature and relative humidity to represent how hot it actually feels to the human body. At high humidity, sweat cannot evaporate efficiently, reducing the body’s ability to cool itself — making the perceived temperature much higher than the thermometer reading.
Rothfusz regression formula (NOAA standard, valid when T ≥ 80°F and RH ≥ 40%): HI = −42.379 + 2.04901523×T + 10.14333127×RH − 0.22475541×T×RH − 6.83783×10⁻³×T² − 5.481717×10⁻²×RH² + 1.22874×10⁻³×T²×RH + 8.5282×10⁻⁴×T×RH² − 1.99×10⁻⁶×T²×RH²
Where:
- HI = heat index in °F
- T = air temperature in °F
- RH = relative humidity as a percentage (0–100)
Simplified approximation (Steadman, accurate within ±1.3°F): HI ≈ 0.5 × (T + 61.0 + (T−68.0)×1.2 + RH×0.094)
Heat index risk categories (NOAA):
- 80–90°F (27–32°C): Caution — fatigue possible with prolonged exposure
- 90–103°F (32–39°C): Extreme Caution — heat cramps/exhaustion possible
- 103–124°F (39–51°C): Danger — heat cramps/exhaustion likely; heat stroke possible
-
124°F (51°C): Extreme Danger — heat stroke highly likely
Metric conversion: Convert °C to °F first: T(°F) = T(°C) × 9/5 + 32, then convert result back: HI(°C) = (HI(°F) − 32) × 5/9
Worked example: Air temperature: 95°F (35°C). Relative humidity: 75%.
Using simplified formula: HI ≈ 0.5 × (95 + 61.0 + (95−68)×1.2 + 75×0.094) HI ≈ 0.5 × (95 + 61 + 32.4 + 7.05) HI ≈ 0.5 × 195.45 = 97.7°F — use the full Rothfusz formula for precision: ~114°F (45.5°C) — Danger zone. Outdoor activity should be severely limited.