Enter temperature and humidity to get the heat index — how hot it actually feels.
How this calculator arrives at its answer
High humidity makes hot weather feel hotter because sweat evaporates more slowly. This calculator uses the US National Weather Service formula to combine temperature and humidity into a feels-like heat index.
It’s most accurate at 80°F or above with humidity of 40% or more.
Step by step
- Enter the air temperature in Fahrenheit.
- Enter the relative humidity.
- Read the heat index and caution level.
Formula reference
NWS Rothfusz regression of temperature and humidity
Worked example
On the values this calculator opens with, the feels like is 105.9°F. Underneath, In Celsius comes out at 41.1°C and Caution level at Danger. Change any field and every figure updates as you type.
Common questions
What is the heat index?
The temperature it feels like when humidity is combined with the actual air temperature — always at or above the real temperature in humid heat.
Why does humidity make heat worse?
Humid air slows sweat evaporation, so your body cools less effectively and feels hotter.
When is the heat dangerous?
Heat index above about 103°F warrants real caution; above 125°F is extreme danger. Stay hydrated and cool.