Dew Point Calculator
How muggy the air feels.
Open →
Weather
Heat Index Calculator
Enter temperature and humidity to get the feels-like heat and NWS risk level.
Feels like
—
The apparent temperature.
| Heat index (°C) | — |
|---|---|
| Risk level | — |
| Temperature | — |
| Humidity | — |
Show our math
Enter your figures above to see the step-by-step working.
Everything computed client-sideEverything computed client-side — nothing leaves this page
How the Heat Index Calculator Works
The heat index — the “feels-like” temperature in hot weather — combines air temperature and relative humidity to estimate how hot it actually feels to the body. When humidity is high, sweat evaporates slowly, so you feel hotter than the thermometer reads.
This calculator uses the National Weather Service Rothfusz regression, the same equation behind official heat advisories, and flags the NWS risk category from Caution up to Extreme Danger.
heat index = Rothfusz regression of temperature (°F) and relative humidity (%)
Frequently Asked Questions
What is the heat index?
It’s the apparent temperature — how hot it feels when relative humidity is factored in with the actual air temperature.
When is the heat index dangerous?
The NWS flags Extreme Caution at 90–103°F, Danger at 103–125°F, and Extreme Danger above 125°F, when heat stroke becomes likely.
Why does humidity make it feel hotter?
Sweat cools you by evaporating. High humidity slows evaporation, so your body can’t shed heat as easily and it feels hotter.
More From The Desk
Same plain method, different figuresWind Chill Calculator
Feels-like cold.
Open →Feels-Like Temperature Calculator
True apparent temperature.
Open →Relative Humidity Calculator
Humidity from dew point.
Open →