Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and humidity.
Includes danger thresholds from Caution to Extreme Danger in F and C.
The heat index (also called “feels like” temperature) combines air temperature and relative humidity to reflect the actual thermal stress the human body experiences. High humidity prevents sweat from evaporating efficiently, reducing the body’s ability to cool itself — making hot, humid air feel significantly hotter than the thermometer shows.
Formula (Rothfusz regression equation, used by NOAA): HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²×RH + 0.00085282T×RH² − 0.00000199T²×RH²
Where T = temperature in °F and RH = relative humidity in %.
Simplified formula (accurate within ±1.3°F for most conditions): HI ≈ 0.5 × (T + 61.0 + ((T − 68.0) × 1.2) + (RH × 0.094))
When heat index applies: The heat index formula is meaningful only when temperature ≥ 80°F (27°C) and relative humidity ≥ 40%. Below these thresholds, humidity has little perceptible effect.
Worked example: Air temperature: 95°F. Relative humidity: 65%. Using the simplified equation: HI ≈ 0.5 × (95 + 61.0 + ((95 − 68) × 1.2) + (65 × 0.094)) = 0.5 × (95 + 61 + 32.4 + 6.11) = 0.5 × 194.51 = 97.3°F (simplified) Full Rothfusz calculation gives: ~114°F feels-like temperature.
NOAA Heat Index categories:
| Feels Like | Heat Category | Risk |
|---|---|---|
| 80–90°F | Caution | Fatigue possible |
| 91–103°F | Extreme Caution | Heat cramps/exhaustion possible |
| 103–124°F | Danger | Heat cramps/exhaustion likely |
| 125°F+ | Extreme Danger | Heat stroke imminent |
Celsius conversion: Subtract 32, multiply by 5/9 to convert the result to °C.
Protective measures: Stay hydrated (drink before you’re thirsty), seek shade or air conditioning during peak hours (10 AM–4 PM), wear light-colored loose clothing, and never leave children or pets in parked vehicles.