Last updated: 2026-06-16

The heat index is the temperature the human body actually perceives when the measured air temperature is combined with the relative humidity. It is often called the "feels like" temperature. Because humid air slows the evaporation of sweat — the body's main cooling mechanism — a humid 32 °C day feels considerably hotter than a dry one at the same reading. This calculator converts a temperature and humidity pair into an apparent temperature and a corresponding heat-risk category.

How the heat index is calculated

Most weather services use the Rothfusz regression, a polynomial fitted to a detailed model of human heat balance published by the U.S. National Weather Service. It takes the air temperature $T$ (in degrees Fahrenheit) and the relative humidity $R$ (in percentage points) and returns the apparent temperature:

The calculator accepts Celsius, Fahrenheit, or Kelvin and handles the conversions for you, so the formula above runs on the Fahrenheit values internally.

Worked example

Consider an afternoon at $32\,^\circ\mathrm{C}$, about $89.6\,^\circ\mathrm{F}$, with relative humidity of sixty percent. Substituting the Fahrenheit temperature and humidity into the regression gives:

So the air feels roughly $5\,^\circ\mathrm{C}$ hotter than the thermometer shows, which places the conditions in the "extreme caution" band where heat exhaustion becomes possible with prolonged activity.

Risk categories and limits

The National Weather Service groups apparent temperatures into caution, extreme caution, danger, and extreme danger bands, reflecting the rising likelihood of heat cramps, exhaustion, and heatstroke. The regression is defined for shade temperatures of about 27 °C (80 °F) and above and assumes a light wind; standing in direct sun can add several degrees. It is least accurate at very low or very high humidity. For a related measure of how humid the air is, see the Dew Point, Absolute Humidity & THI Calculator calculator, and in cold weather the Wind Chill Calculator calculator gives the analogous "feels like" figure.