Heat Index Calculator

What Is Heat Index?

Heat index (HI) is sometimes referred to as the "apparent temperature". The HI, given in degrees F, is a measure of how hot it feels when relative humidity (RH) is factored in with the actual air temperature.

The human body cools itself primarily by evaporating sweat. When the air is humid, evaporation slows and the body retains more heat, so the same air temperature feels hotter than it would on a dry day. Heat index quantifies this effect: for example, an air temperature of 90°F at 70% relative humidity feels like roughly 105°F. Because sustained heat index values above 90°F increase the risk of heat cramps, heat exhaustion, and heat stroke, HI is widely used to set work/rest schedules, plan outdoor activities, and issue heat advisories.

The following equation approximates the heat index. There are many assumptions made to produce this, far too many to list here. The equation was obtained by multiple regression analysis and has an error of ±1.3°F. It is a curve fit to the results of a full thermodynamic model of the human body, which is why the coefficients look arbitrary—each term captures part of the combined effect of temperature and humidity on perceived heat.

For continuous on-site measurement of heat stress conditions, see our selection of heat index monitors.

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783×10^-3T² - 5.481717×10^-2R²
+ 1.22874×10^-3T²R + 8.5282×10^-4TR² - 1.99×10^-6T²R²
where
T = ambient dry bulb temperature in °F
R = relative humidity (%)