Heat Index Calculator
Calculate the heat index ("feels like" temperature) from air temperature and humidity.
Try an example
113.1°F
About this calculator
How This Heat Index Calculator Works
Enter the air temperature and relative humidity, and the calculator returns the heat index — how hot it actually feels once humidity is factored in.
Worked Example
At 95°F with 60% relative humidity, the heat index is about 113°F — falling into the National Weather Service's "danger" category for heat-related illness with prolonged exposure.
The Formula
This calculator uses the National Weather Service's Rothfusz regression, a polynomial formula fit to a more complex heat-stress model, applied once the average of temperature and a simpler estimate reaches 80°F or higher. Below that threshold, a simpler linear approximation is used instead, since the full regression isn't reliable at lower temperature/humidity combinations.
Why Humidity Makes Heat Feel Worse
The body's main defense against overheating is evaporating sweat, which pulls heat away from the skin. High humidity means the surrounding air is already close to saturated with moisture, so sweat evaporates more slowly — the body can't shed heat as effectively, making the same air temperature feel considerably hotter and increasing real physiological heat stress, not just perceived discomfort.
Heat Index Risk Categories
- 80-90°F — caution: fatigue possible with prolonged exposure or activity
- 90-103°F — extreme caution: heat cramps or heat exhaustion possible
- 103-124°F — danger: heat exhaustion likely, heat stroke possible
- 125°F+ — extreme danger: heat stroke highly likely
Worked example
At 95°F with 60% relative humidity, the heat index is about 113°F — in the National Weather Service's "danger" category for heat-related illness.
Frequently asked questions
Why does humidity make heat feel worse?
The body cools itself primarily through sweat evaporation. High humidity slows that evaporation, so the body can't shed heat as effectively — making the same air temperature feel hotter and increasing the risk of heat-related illness.
Is heat index the same as 'real feel' or 'feels like' temperature shown in weather apps?
It's closely related — many weather apps' 'feels like' temperature is heat index in hot conditions and wind chill in cold conditions, sometimes with additional adjustments for sun exposure.