Heat Index Calculator Temperature Tool
Heat Index Calculator
Input Method
Air Temperature
Relative Humidity
Heat Index
Heat Safety Tips
The heat index (feels like temperature) combines air temperature and relative humidity to estimate how hot it feels to the human body. High humidity impairs sweat evaporation, reducing the body's ability to cool itself. This calculator uses the National Weather Service formula to compute the apparent temperature for any temperature/humidity combination.
See also:
How Heat Index is Calculated
The NWS heat index formula applies only when temperature is at or above 80°F (27°C). For lower temperatures, the heat index equals the air temperature. The Steadman formula is a polynomial approximation of human thermoregulation models.
NWS Rothfusz Formula (simplified): HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R² T = temperature (°F), R = relative humidity (%)
Adjustment factors apply for low humidity (<13% at 80-112°F) or high humidity (>85% at 80-87°F).
Heat Index Danger Levels
| Heat Index | Heat Disorder Risk |
|---|---|
| 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; heatstroke possible |
| Above 124°F (51°C) | Extreme Danger: heatstroke highly likely |
Frequently Asked Questions
Why does humidity make it feel hotter?⌄
Your body cools itself primarily through sweat evaporation. Evaporation draws heat from the skin, lowering your temperature. High humidity means the air is already saturated with water vapor, so sweat evaporates much more slowly. Less evaporative cooling means more heat is retained, making you feel hotter despite the same air temperature.
Is there a heat index for temperatures below 80°F?⌄
The heat index formula is only valid above 80°F. Below that temperature, the combination of temperature and humidity does not create dangerously elevated apparent temperatures for the same physiological reasons. The Steadman model becomes less accurate at lower temperatures where other factors dominate.
How is heat index different from wet-bulb temperature?⌄
Wet-bulb temperature measures the lowest temperature achievable through evaporative cooling and is used by climate scientists to assess survivability limits. Heat index is a practical "feels like" estimate for public health warnings. Wet-bulb temperature above 35°C (95°F) is considered the upper survivability limit for humans even in shaded, ventilated conditions.
What is the heat index record?⌄
The highest reliably recorded heat index is around 178°F (81°C), measured in Dhahran, Saudi Arabia in July 2003 (108°F air temperature, 95% humidity). In the United States, heat index values above 130°F have been recorded during severe heat events in the Gulf Coast region where high temperatures meet extreme humidity from the Gulf of Mexico.