Heat Index Calculator
Calculate the apparent "feels like" temperature from air temperature and relative humidity using the NWS Rothfusz equation. Shows heat danger levels.
Temperature & Humidity
Heat index applies above 80 °F
Apparent Temperature
Enter temperature and humidity to calculate the heat index.
Heat Index Risk Levels (NWS)
| Level | Heat Index Range |
|---|---|
| Caution | 80–90 °F (27–32 °C) |
| Extreme Caution | 90–103 °F (32–39 °C) |
| Danger | 103–124 °F (39–51 °C) |
| Extreme Danger | ≥ 125 °F (≥ 52 °C) |
About the Heat Index Formula
The heat index (also called the "feels like" temperature or apparent temperature) is calculated using the Rothfusz regression equation, the standard formula used by the U.S. National Weather Service (NWS). It combines air temperature and relative humidity to estimate how hot it feels to the human body.
The formula is valid for temperatures above 80 °F (27 °C) and relative humidity above 40 %. Below these thresholds the heat index equals the actual air temperature.
Frequently asked questions
What does Heat Index Calculator do?
Calculate the apparent "feels like" temperature from air temperature and relative humidity using the NWS Rothfusz equation. Shows heat danger levels.
What should I check in the result?
Check units, rates, dates and assumptions. Compare a simple case by hand before using the result for a decision.