Why does humidity make it feel hotter?

Your body cools itself by evaporating sweat. When the air is already moist, evaporation slows, so less heat leaves your skin and the air feels hotter than the actual temperature.

Why does the calculator require at least 80°F?

The Rothfusz regression was fitted for hot conditions and is only valid at roughly 80°F and above. Below that, heat index is not meaningful, and cold-weather "feels like" effects are handled by a wind chill calculator instead.

What do the danger levels mean?

Below 80°F is None; 80–89°F is Caution; 90–104°F is Extreme Caution; 105–129°F is Danger; and 130°F or above is Extreme Danger. Higher bands signal a rising risk of heat cramps, exhaustion, and heat stroke.

Does the heat index account for sun and wind?

No. The standard heat index assumes shade and light wind. Direct sunlight can add up to roughly 15°F to how hot it feels, so outdoor exposure in the sun is more dangerous than the number alone suggests.

Heat Index Calculator

Temperature

°F

Humidity50%

Heat Index105.2°F

Danger LevelDanger

This heat index calculator estimates the "feels-like" temperature by combining air temperature with relative humidity using the National Weather Service Rothfusz regression. When humidity is high, sweat evaporates slowly and the body cannot cool itself as effectively, so the air feels hotter than the thermometer reads. The result includes a danger level so you can judge the risk of heat cramps, exhaustion, or heat stroke during hot, muggy conditions.

Formula

HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R − 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R + 0.00085282·T·R² − 0.00000199·T²·R²

T: Air temperature in degrees Fahrenheit (must be 80°F or above)
R: Relative humidity as a percentage (0 to 100)
HI: Heat index (apparent temperature) in degrees Fahrenheit

How it works

Enter the air temperature in degrees Fahrenheit (80°F or above, the range where the regression applies) and the relative humidity as a percentage.
The calculator evaluates the multi-term Rothfusz regression and applies low-humidity and high-humidity corrections where appropriate.
It reports the apparent (feels-like) temperature and maps it to a danger level from None through Caution, Extreme Caution, Danger, and Extreme Danger.

Worked example

An afternoon at 90°F with 70% relative humidity.

Substitute T = 90 and R = 70 into the Rothfusz regression.
The humidity is between 13% and 85%, so no low- or high-humidity correction is applied.
The regression evaluates to about 105.9°F.

It feels like about 105.9°F, which falls in the Danger band (105–129°F).

Frequently asked questions

Why does humidity make it feel hotter?: Your body cools itself by evaporating sweat. When the air is already moist, evaporation slows, so less heat leaves your skin and the air feels hotter than the actual temperature.
Why does the calculator require at least 80°F?: The Rothfusz regression was fitted for hot conditions and is only valid at roughly 80°F and above. Below that, heat index is not meaningful, and cold-weather "feels like" effects are handled by a wind chill calculator instead.
What do the danger levels mean?: Below 80°F is None; 80–89°F is Caution; 90–104°F is Extreme Caution; 105–129°F is Danger; and 130°F or above is Extreme Danger. Higher bands signal a rising risk of heat cramps, exhaustion, and heat stroke.
Does the heat index account for sun and wind?: No. The standard heat index assumes shade and light wind. Direct sunlight can add up to roughly 15°F to how hot it feels, so outdoor exposure in the sun is more dangerous than the number alone suggests.

Related calculators

Dew Point Calculator Wind Chill Calculator