Heat Index Calculator

Calculate the heat index (feels-like temperature) based on air temperature and relative humidity. See heat danger levels and safety precautions.

Enter conditions

Heat Index Reference Chart
Temp \ RH

Values shown are the heat index (feels-like temperature). Colors indicate NWS danger levels.

How the Heat Index is Calculated

The heat index is calculated using the Rothfusz regression equation, the same formula used by the National Weather Service (NWS).

The formula combines air temperature and relative humidity to estimate the human-perceived equivalent temperature. At high humidity, the body's ability to cool itself through sweat evaporation is reduced, making conditions feel much hotter.

When is it valid? The heat index formula applies when the air temperature is at least 80°F (27°C) and relative humidity is at least 40%. Below these thresholds, the actual air temperature is the best measure of comfort.

Note: The heat index does not account for direct sun exposure. If you are in direct sunlight, the feels-like temperature may be up to 15°F higher than the calculated heat index.

Frequently Asked Questions

What is the heat index and how is it calculated?▾
The heat index, also called the "feels-like" temperature, combines air temperature and relative humidity to estimate how hot it actually feels. This calculator uses the official NWS Rothfusz regression equation.
Why does humidity make it feel hotter?▾
High humidity slows sweat evaporation, which is your body's primary cooling mechanism. When sweat cannot evaporate efficiently, your body retains more heat, making the air feel significantly hotter than the actual temperature.
What are the heat index danger levels?▾
The NWS defines four levels: Caution (80-90°F), Extreme Caution (91-103°F), Danger (104-125°F), and Extreme Danger (126°F+). Each level indicates increasing risk of heat-related illness.
When is the heat index valid?▾
The heat index formula is designed for air temperatures at or above 80°F (27°C) with relative humidity of 40% or higher. Below these thresholds, the air temperature itself is the best indicator of how hot it feels.
Is this heat index calculator free and accurate?▾
Yes. This calculator uses the same Rothfusz regression equation used by the National Weather Service. It runs entirely in your browser with no signup or data collection required.