Heat Index Calculator
How hot does it really feel? Heat index from temperature and humidity.
The Heat Index Calculator computes the 'feels like' temperature from air temperature and relative humidity using the NOAA Rothfusz regression — the standard behind every heat advisory. A 35 °C day at 60% humidity feels like 43 °C, because humidity blocks the sweat evaporation your body relies on to cool.
The output includes the National Weather Service caution scale, from 'caution' up to 'extreme danger', so the feels-like number comes with practical guidance about heat exhaustion risk. Hydration, shade and rest matter increasingly above the caution line.
The index is defined for temperatures of 27 °C (80 °F) or higher, and the tool explains when your input falls outside that range instead of returning a misleading number. Inputs accept °C or °F.
Heat index (feels like)
37.1 °C
≈ 98.7 °F
NWS caution level
Extreme caution
sunstroke possible with prolonged exposure
NOAA heat index (Rothfusz regression). It models how humidity blocks sweat evaporation — the reason 35 °C at 80% humidity feels far worse than 35 °C dry. Stay hydrated and shade-cooled above the 'caution' threshold.
How to use the Heat Index Calculator
- Enter the air temperature.
- Enter the relative humidity in percent.
- Choose °C or °F.
- Read the feels-like temperature.
- Check the NWS caution level and take heat seriously above it.
Frequently asked questions
Why does humidity make heat feel worse?
Your body cools by evaporating sweat. Humid air is already saturated, so evaporation slows and the body cannot shed heat — the heat index models exactly this effect, which is why 35 °C at 80% humidity can feel like 50 °C.
What are the NWS heat levels?
Feels-like 27–32 °C is caution, 33–40 °C extreme caution (sunstroke possible), 41–52 °C danger (heat exhaustion likely), and above 52 °C extreme danger with heat stroke risk. This tool maps your result onto that scale.
Is the heat index valid in the shade?
It assumes shaded conditions. Direct sun adds roughly 8–15 °C of effective heat on top of the index, which is why guidance says to add that margin on sunny days.
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