Heat Index Calculator

The Heat Index — sometimes called 'apparent temperature' or 'humiture' — measures how hot it actually feels to the human body when both air temperature and relative humidity are combined. High humidity prevents sweat from evaporating efficiently, reducing your body's ability to cool itself and making the air feel hotter than the thermometer reads.

This calculator uses the Rothfusz regression equation developed by the U.S. National Weather Service. It applies a multi-variable polynomial formula combining temperature (°F) and relative humidity to estimate perceived temperature. For lower temperatures (below 80°F / 27°C), a simpler Steadman formula is used instead, as the full regression is less accurate at those ranges.

Your body cools itself primarily through sweating. When the air is already saturated with moisture (high relative humidity), sweat cannot evaporate as quickly, so your body retains more heat. At low humidity, sweat evaporates fast and cooling is efficient — even in high heat. At high humidity, that cooling mechanism is impaired, causing the temperature to feel significantly hotter.

Heat stress is typically classified into five levels: Safe (below 80°F / 27°C feels-like), Caution (80–90°F / 27–32°C), Extreme Caution (90–103°F / 32–39°C), Danger (103–124°F / 39–51°C), and Extreme Danger (above 124°F / 51°C). At Extreme Danger levels, heat stroke is highly likely without immediate action.

A Heat Index above 103°F (39°C) is classified as Dangerous, and above 124°F (51°C) is Extreme Danger. At these levels, prolonged exposure or physical activity can quickly lead to heat cramps, heat exhaustion, or life-threatening heat stroke. Vulnerable populations — the elderly, young children, and those with chronic illnesses — are at risk at lower thresholds.

No — they are related but different. The Heat Index uses relative humidity (expressed as a percentage), while dew point is an absolute measure of moisture in the air (in degrees). Dew point is often considered a more accurate indicator of human discomfort, but the Heat Index formula converts relative humidity into a practical 'feels-like' temperature that most people find intuitive.

Wind is not factored into the Heat Index calculation. However, wind can make the apparent temperature feel cooler by increasing sweat evaporation — this effect is captured by the separate Wind Chill index used in cold weather. In hot conditions, wind only helps if the air temperature is below body temperature; very hot winds can actually increase heat stress.

The Rothfusz Heat Index formula is most accurate when the air temperature is at or above 80°F (27°C) and relative humidity is 40% or higher. For cooler temperatures or lower humidity, the calculator automatically applies the Steadman approximation. Results outside this range may be less precise, and the NOAA Heat Index chart is the authoritative reference for edge cases.