Find how hot it really feels. This free heat index calculator combines air temperature and humidity (°F or °C) using the NWS formula, showing the feels-like temperature and heat-risk level.
The heat index is how hot it feels when humidity is combined with air temperature. High heat index raises the risk of heat exhaustion and heat stroke, especially during exertion — stay hydrated, take breaks in the shade or air conditioning, and check on vulnerable people. Seek medical help for symptoms like confusion, fainting or a very high body temperature.
Conditions
°F
%
Enter the air temperature and the relative humidity (from a weather app or thermometer). The heat index is most meaningful in warm, humid conditions, roughly above 80 °F (27 °C).
Feels like
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Enter temperature and humidity
Heat index
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Difference
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In °F
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In °C
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Based on the US National Weather Service (Rothfusz) heat index. High humidity slows sweat evaporation, so the body cools less effectively and the air feels hotter than the thermometer reading.
Use this free heat index calculator to find how hot it really feels. Enter the air temperature and relative humidity to get the heat index, or “feels-like” temperature, with a heat-risk note.
What this heat index calculator does
A heat index calculator combines temperature and humidity into a feels-like figure. When humidity is high, sweat evaporates more slowly, so the body cools less and the air feels hotter than the thermometer shows.
How the heat index works
Heat index uses the NWS (Rothfusz) regression on
air temperature (°F) and relative humidity (%).
It applies mainly above about 80 °F (27 °C).
At 90 °F and 70% humidity it feels like about 106 °F; at 100 °F and 40%, around 109 °F.
Heat index risk levels
Heat index
Risk
80–90 °F
Caution — fatigue with prolonged exposure
90–103 °F
Extreme caution — cramps and exhaustion possible
103–124 °F
Danger — heat exhaustion or stroke likely
125 °F+
Extreme danger — heat stroke highly likely
Staying safe in the heat
When the heat index is high, drink water, limit exertion, take breaks in shade or air conditioning, and watch for heat illness. Confusion, fainting, or a very high body temperature are medical emergencies — get help immediately.
How to use the heat index calculator
Choose °F or °C.
Enter the air temperature and humidity.
Read the feels-like temperature and risk note.
Heat Index FAQ
How is the heat index calculated?
It uses the NWS Rothfusz formula on air temperature and relative humidity. For example, 90 °F at 70% humidity feels like about 106 °F.
What's the difference between heat index and temperature?
Temperature is what the thermometer reads; heat index adds the effect of humidity to show how hot it feels to the body.
Why does humidity make it feel hotter?
Your body cools by evaporating sweat. High humidity slows that evaporation, so you cool less effectively and the air feels hotter.
When is the heat index dangerous?
Above about 103 °F, heat exhaustion and heat stroke become likely with activity or prolonged exposure; 125 °F+ is extreme danger.
Does sunshine change the heat index?
Yes. The formula assumes shade; direct sun can add roughly 15 °F to the feels-like temperature.
Is the heat index calculator free?
Yes, this heat index calculator is completely free, needs no sign-up, and gives instant results in your browser.
