Heat Index Converter
Convert heat index values between Fahrenheit and Celsius, and see danger levels based on temperature and relative humidity.
Enter temperature and humidity to calculate heat index, or type a heat index value to convert between °F and °C.
What is the Heat Index?
The heat index, also called the apparent temperature or “feels like” temperature, combines air temperature and relative humidity to determine how hot it actually feels to the human body. When your body sweats, the evaporation of moisture cools you down. High humidity slows this evaporation process, making you feel significantly hotter than the actual air temperature suggests.
The Rothfusz Regression Formula:
The National Weather Service in the United States uses the Rothfusz regression equation to calculate the heat index. This formula was developed in 1990 and remains the standard method used by meteorologists worldwide:
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T is the air temperature in degrees Fahrenheit and R is the relative humidity expressed as a percentage.
Celsius to Fahrenheit conversion for heat index values:
°F = (°C × 9/5) + 32°C = (°F - 32) × 5/9
Heat Index Danger Levels (NWS Categories):
| Heat Index Range | Danger Level | Possible Health Effects |
|---|---|---|
| Below 80°F (27°C) | None | Comfortable conditions for most people |
| 80-90°F (27-32°C) | Caution | Fatigue possible with prolonged exposure and physical activity |
| 91-103°F (33-39°C) | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104-125°F (40-51°C) | Danger | Heat cramps and exhaustion likely, heatstroke possible |
| Above 125°F (52°C+) | Extreme Danger | Heatstroke highly likely, can be fatal |
Practical Examples:
- At 90°F (32°C) with 40% humidity, the heat index is about 91°F (33°C), barely above the actual air temperature.
- At 90°F (32°C) with 90% humidity, the heat index jumps to approximately 122°F (50°C), which is dangerously hot.
- At 100°F (38°C) with 50% humidity, the heat index reaches roughly 120°F (49°C).
- At 85°F (29°C) with 80% humidity, the heat index is about 97°F (36°C).
Important Tips:
- The Rothfusz formula is only accurate for temperatures above 80°F (27°C) and humidity above 40%.
- Direct sunlight can add up to 15°F (8°C) to the calculated heat index value.
- Athletes, outdoor workers, elderly individuals, and children should pay close attention when the heat index exceeds 90°F (32°C).
- Always stay hydrated and seek shade or air conditioning during high heat index conditions.