What Is the Heat Index and How Is It Calculated?
The heat index is the temperature the human body actually perceives when relative humidity is combined with the air temperature — often called the "feels like" temperature. High humidity slows the evaporation of sweat, so the same air temperature feels hotter as humidity rises.
This calculator uses the Rothfusz regression, developed by Lans P. Rothfusz for the U.S. National Weather Service (NWS) in 1990: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R², where T is air temperature in °F and R is relative humidity in percent. Low-humidity and high-humidity correction terms are applied outside specific temperature/humidity bands, exactly as the NWS specifies.
According to the National Weather Service, this regression is valid for air temperatures at or above 80°F (27°C) and relative humidity at or above 40%; below that combined threshold, a simpler linear approximation is used instead since heat stress is not significant.
How to Use This Heat Index Calculator
Enter the air temperature and relative humidity and the heat index updates instantly. Here's what each field means:
- Air Temperature: The actual (dry-bulb) thermometer reading, in Fahrenheit or Celsius. Switch units with the toggle.
- Relative Humidity: The percentage of moisture in the air relative to what it can hold at that temperature, from 0–100%. Check a local weather report or weather station for a live reading.
This calculator works in both Fahrenheit and Celsius, converting internally to Fahrenheit for the NWS formula and back to your selected unit for display, so results are accurate worldwide, not just in the United States.
What Your Heat Index Result Means
The NWS assigns a health-risk category to each heat index range. These categories assume prolonged exposure and/or physical activity in the shade with light wind — full sun exposure can add up to 15°F (8°C) to the effective heat index.
| Category | Heat Index Range (°F) | Health Effect |
|---|---|---|
| Normal | Below 80°F | No unusual heat stress expected |
| Caution | 80°F – 90°F | Fatigue possible with prolonged exposure or activity |
| Extreme Caution | 90°F – 103°F | Heat cramps and heat exhaustion possible |
| Danger | 103°F – 124°F | Heat cramps or exhaustion likely; heat stroke probable with continued exposure |
| Extreme Danger | 125°F and above | Heat stroke highly likely |
For example, an air temperature of 95°F (35°C) at 65% relative humidity produces a heat index of roughly 121°F (49°C) — squarely in the "Danger" category despite the thermometer reading being 26°F lower.
Frequently Asked Questions
What is a dangerous heat index?
According to the National Weather Service, a heat index of 103°F (39°C) or higher falls into the "Danger" category, where heat cramps or heat exhaustion are likely and heat stroke is possible with continued exposure or physical activity.
How accurate is this heat index calculator?
This calculator implements the NWS Rothfusz regression exactly, including the low-humidity and high-humidity correction terms, so results match official NWS heat index charts to within about 1°F. It assumes shady, light-wind conditions — full sun can raise the effective heat index by up to 15°F.
What is the difference between heat index and temperature?
Air temperature is a direct thermometer reading, while the heat index adds the effect of humidity to estimate how hot the air actually feels to the human body. The two are equal only in dry air; as humidity rises, the heat index climbs well above the actual temperature.
How do I stay safe when the heat index is high?
Drink water regularly, limit strenuous outdoor activity during peak heat, wear light-colored breathable clothing, and take breaks in air conditioning or shade. Check our Water Intake Calculator for daily hydration targets in hot conditions.
