Heat Index Calculator

Enter the air temperature and relative humidity, then click Calculate

Heat index: 93°F

Air temperature: 85.0 °F / 29.4 °C
Relative humidity: 70.0%
Heat index: 92.7 °F
Heat index (Celsius): 33.7 °C
Risk level: Extreme caution
Within NWS range: Yes

The NWS heat index assumes a shady, light-wind location. Direct sun can raise the apparent temperature by up to 15 °F. Heat index is a screening estimate, not a medical diagnosis. People with heat sensitivity, older adults, and children may be at higher risk.

What this calculator does

The Heat Index Calculator estimates how hot the air feels when temperature and relative humidity are combined. It uses the National Weather Service (NWS) heat index method, including the Rothfusz regression and the standard low-humidity and high-humidity adjustments.

Heat index is also called apparent temperature. It is not a measured temperature: the air temperature stays the same, but high humidity slows the evaporation of sweat, so the body feels hotter and has more difficulty cooling itself. The result includes a risk category from the NWS scale.

How to use this calculator

Enter the air temperature.
Choose Fahrenheit, Celsius, or Kelvin.
Enter the relative humidity as a percentage from 1 to 100.
Click Calculate.

The default example uses 85 °F and 70% relative humidity. The NWS regression returns about 92.7 °F, which rounds to 93 °F and falls in the "Extreme caution" category. The result card also shows the Celsius equivalent and whether the temperature is inside the range where the heat index is most reliable.

Heat index formula

For T ≥ 80 °F:
HI = −42.379
     + 2.04901523T
     + 10.14333127R
     − 0.22475541TR
     − 0.00683783T²
     − 0.05481717R²
     + 0.00122874T²R
     + 0.00085282TR²
     − 0.00000199T²R²

T = air temperature in °F
R = relative humidity in percent

The calculator applies two NWS corrections. When humidity is below 13% and the temperature is between 80 and 112 °F, it subtracts a dryness adjustment. When humidity is above 85% and the temperature is between 80 and 87 °F, it adds a small adjustment. Below 80 °F, it uses the NWS simple formula, which is less precise and meant only as a rough estimate.

What the results mean

The risk category follows the NWS scale. Below 80 °F there is no significant heat risk. From 80 to 90 °F the level is "Caution," from 90 to 103 °F it is "Extreme caution," from 103 to 125 °F it is "Danger," and at 125 °F or above it is "Extreme danger." These categories describe the potential for heat-related illness with prolonged exposure or activity.

The heat index assumes a shady, light-wind location. Direct sunlight can raise the apparent temperature by up to about 15 °F, and a hot wind can make conditions feel worse.

Assumptions and limitations

The NWS heat index is defined for shady conditions with light wind, not direct sun or a enclosed car.
Individual risk depends on age, health, medication, hydration, clothing, and activity level.
Relative humidity must be greater than 0% and at most 100%.
Below 80 °F the result is a rough estimate; the NWS recommends caution when using heat index outside its validated range.
The calculator does not model wet-bulb temperature, solar radiation, or acclimatization.
This is a screening tool and not medical advice. Heat exhaustion and heat stroke are medical emergencies.

Frequently asked questions

What is the difference between heat index and dew point?

Heat index combines temperature and humidity into one "feels like" number. Dew point measures the amount of moisture in the air and is most useful for judging comfort and condensation. A high dew point usually means a high heat index when the temperature is also high.

Why does humidity make it feel hotter?

Sweat cools the body by evaporating. When the air is already humid, evaporation is slower, so the body sheds heat less efficiently. The result is a higher apparent temperature and a greater risk of heat stress.

What should I do at the danger level?

The NWS "Danger" category means heat cramps and heat exhaustion are likely and heat stroke is possible with continued activity. Reduce strenuous activity, drink water, move to a cool place, and check on people at higher risk. Seek emergency help for confusion, fainting, or a high body temperature.

Reference tables

Temperature unit conversions

Air Temperature	Fahrenheit °F Celsius °C Kelvin
80 °F	80
90 °F	90
100 °F	100
110 °F	110

Heat index notes

Celsius	Fahrenheit	Notes
27–32	80–90	Caution: fatigue possible with prolonged exposure
32–41	90–105	Extreme caution: heat cramps and exhaustion possible
41–54	105–130	Danger: heat cramps or heat exhaustion likely
Above 54	Above 130	Extreme danger: heat stroke likely

The heat index assumes a shady, light-wind location. Direct sun can raise the apparent temperature by up to 15 °F.

Source

Reference: Calculator.net — Heat Index Calculator. Formula source: National Weather Service heat index documentation.

Disclaimer

This calculator provides an estimate for planning and is not medical advice. Heat index values are guidance, not a diagnosis. Follow local heat warnings and seek medical help if you suspect heat illness.

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