Heat Index Calculator – Calculate the 'Feels Like' Temperature

Know how hot it really feels outside with our Heat Index Calculator. Combine air temperature and humidity to calculate the apparent temperature — helping you prepare for heat-related risks during summer and outdoor activities.

Weather Conditions

Air Temperature

Relative Humidity (%)

Heat index is most significant when temperature is above 80°F (27°C) and humidity is above 40%.

Results

Enter temperature and humidity to calculate heat index

Heat Index Safety Guide

Heat Index	Category	Health Effects
Below 80°F	Normal	No significant effects
80-90°F	Caution	Fatigue with prolonged exposure
90-103°F	Extreme Caution	Heat cramps/exhaustion possible
103-124°F	Danger	Heat stroke possible
125°F+	Extreme Danger	Heat stroke imminent

How to Calculate Heat Index

Enter Air Temperature

Input the current air temperature in Fahrenheit or Celsius.

Add Relative Humidity

Enter the relative humidity percentage from your weather source or hygrometer.

Get Heat Index Results

See the "feels like" temperature with risk level and safety precautions.

Features of This Heat Index Calculator

✓NWS Formula Accuracy

Uses the official National Weather Service Rothfusz regression equation for precise heat index calculations.

✓Risk Level Classification

Automatically categorizes heat index into Caution, Extreme Caution, Danger, or Extreme Danger levels.

✓Safety Precautions Display

Shows specific health recommendations based on your calculated heat index level.

✓Dual Temperature Units

Support for both Fahrenheit and Celsius input with automatic conversion.

✓Comprehensive Safety Guide

Reference table explains health effects at different heat index levels for quick decision-making.

Frequently Asked Questions About Heat Index

What is the difference between temperature and heat index?

Temperature measures actual air heat. Heat index combines temperature and humidity to show how hot it feels to your body. High humidity prevents sweat evaporation, making it feel hotter than the thermometer reads.

At what heat index is it dangerous?

Heat index above 103°F (39°C) is considered dangerous. Above 125°F (52°C) is extreme danger where heat stroke becomes imminent. Limit outdoor activity and stay hydrated at these levels.

Why does humidity make it feel hotter?

Your body cools itself through sweat evaporation. High humidity means air is already saturated with water vapor, slowing evaporation. This reduces your body ability to cool down, making you feel hotter.

Does heat index apply in the shade?

Yes, heat index calculations assume shady conditions. Direct sunlight can increase the perceived temperature by up to 15°F (8°C) additional heat stress.

Who is most at risk from high heat index?

Children, elderly adults, pregnant women, and people with heart or lung conditions are most vulnerable. Anyone doing strenuous outdoor work or exercise in high heat index conditions should take extra precautions.

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