Heat Index Calculator Online

The actual temperature is what a thermometer measures in the air. The heat index (also called “apparent temperature”) is what it feels like to the human body when relative humidity is combined with the air temperature. High humidity makes it feel hotter because it reduces the body’s ability to cool itself through sweating.

Our calculator uses the standard Rothfusz regression formula developed by the National Weather Service, which is accurate to within ±1.3°F (±0.7°C) when compared to the full heat index equation. It’s the same calculation used by professional meteorologists.

Heat index values are categorized into four danger levels:

• Caution (80-90°F/27-32°C): Fatigue possible with prolonged exposure
• Extreme Caution (90-103°F/32-39°C): Heat stroke, heat cramps, or heat exhaustion possible
• Danger (103-124°F/39-51°C): Heat cramps or heat exhaustion likely, heat stroke possible
• Extreme Danger (125°F/52°C or higher): Heat stroke highly likely

High humidity slows the evaporation of sweat from your skin, which is your body’s primary way of cooling itself. When sweat can’t evaporate efficiently, your body can’t regulate its temperature as effectively, making it feel hotter than the actual air temperature.

Yes, wind can affect how the heat index feels. The standard heat index calculation assumes light wind conditions (about 6 mph or 9.7 km/h). Stronger winds can make it feel cooler than the calculated heat index, while very light or no wind can make it feel hotter.

When heat index values are high:

• Stay hydrated by drinking plenty of water
• Wear lightweight, loose-fitting, light-colored clothing
• Take frequent breaks in shaded or air-conditioned areas
• Avoid strenuous activities during the hottest parts of the day
• Never leave children or pets in parked vehicles
• Check on elderly neighbors and relatives

For cold weather, wind chill is the equivalent measurement that combines temperature and wind speed to determine how cold it feels to exposed skin. The heat index is only applicable when temperatures are above 80°F (27°C).

No, the heat index is always equal to or higher than the actual air temperature. At very low humidity levels (below about 40%), the heat index will be very close to the actual temperature because the body can cool itself efficiently through sweat evaporation.

High heat index values significantly increase the risk of heat-related illnesses during physical activity. Athletes and outdoor workers should:

• Schedule strenuous activities for cooler parts of the day
• Increase rest periods in shaded or cool areas
• Wear appropriate clothing
• Monitor for symptoms of heat illness (dizziness, nausea, headache, muscle cramps)
• Have an emergency plan in place

The standard heat index equation is only valid for temperatures above 80°F (27°C) because below this threshold, humidity has minimal effect on perceived temperature. At extremely high combinations (above about 130°F/54°C), the equation becomes less accurate because such conditions are rare and less studied, but the danger is extreme regardless of the precise value.