Heat Index Calculator
Calculate how hot it feels based on temperature and humidity (Rothfusz equation)
Air Temperature
°C
Relative Humidity (%)
%
Formula Notes
The Rothfusz regression equation is valid when temperature ≥ 80°F (27°C) and relative humidity ≥ 40%. Below these thresholds, humidity has less impact on perceived temperature.
Heat Index
Feels Like (°F)
—
Feels Like (°C)
—
Danger Level
—
What is the Heat Index?
The heat index (also called apparent temperature or "feels like") combines air temperature and relative humidity to measure perceived temperature. High humidity slows the evaporation of sweat from skin, reducing the body's ability to cool itself — making it feel hotter than the actual air temperature.
The Rothfusz equation used here is the formula adopted by the US National Weather Service. It's accurate for temperatures ≥ 80°F (27°C) and humidity ≥ 40%. A heat index above 40°C (104°F) is potentially dangerous; above 54°C (130°F) can cause heat stroke in minutes.
lightbulb Example
35°C (95°F), 80% humidity:
1Using Rothfusz equation
2HI ≈ 124°F (51°C)
✓ Extreme Danger — heat stroke risk