Heat Index Calculator
Calculate the heat index ("feels like" temperature) from air temperature and humidity.
Try an example
113.1°F
About this calculator
What Is the Heat Index?
The heat index, sometimes called "humiture" or the "feels-like" temperature in summer, combines air temperature and relative humidity to estimate how hot it actually feels. High humidity slows the evaporation of sweat, which is the body's main way of cooling off, so the same temperature feels hotter in humid air than in dry air. For instance, an air temperature of 83°F with 70% relative humidity feels like about 88°F. This heat index calculator takes temperature and either relative humidity or dew point and returns the apparent temperature.
How the Heat Index Is Determined
The U.S. National Weather Service uses a regression equation (the Rothfusz equation) fitted to a model of how the body responds to heat and humidity. It is intended for shade conditions, with light wind, and for air temperatures of about 80°F (27°C) and above; below that, the heat index is essentially the air temperature. Standing in direct sunlight can make it feel up to 15°F hotter, and strong, hot winds can add to the effect.
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.
Heat Index Danger Levels
- 80–90°F (27–32°C): caution — fatigue is possible with prolonged exposure and activity.
- 91–103°F (33–39°C): extreme caution — heat cramps and heat exhaustion are possible.
- 103–124°F (39–51°C): danger — heat exhaustion is likely, and heat stroke is possible.
- 125°F (52°C) and above: extreme danger — heat stroke is highly likely.
Staying Safe in the Heat
- Drink water regularly, before you feel thirsty, and go easy on alcohol and caffeine.
- Limit strenuous activity to cooler hours and take frequent breaks in shade or air conditioning.
- Wear light, loose-fitting, light-colored clothing and a hat.
- Never leave people or pets in a parked car.
- Know the warning signs of heat illness: heavy sweating, dizziness, nausea, rapid pulse, confusion — and get help immediately if they appear.
Individual sensitivity varies with age, hydration, medications, fitness and acclimatization. For the cold-weather counterpart, use the Wind Chill Calculator, and for the moisture side, the Dew Point Calculator.
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.
