Weather / 01
Heat Index Calculator
Work out how hot it feels from the temperature and humidity, with the US National Weather Service formula and its caution and danger levels, in °F or °C.
Heat Index Calculator: The calculator uses the National Weather Service's heat index algorithm: the Rothfusz regression, with the NWS adjustments for very dry and very humid air, and a simpler formula when the result is below 80 °F. The result is placed in the NWS bands: caution from 80 °F (27 °C), extreme caution from 90 °F (32 °C), danger from 103 °F (39 °C), and extreme danger from 125 °F (52 °C). Runs 100% locally in your browser with zero server file uploads.
- Category
- Everyday tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
Why humid heat is worse
The body sheds heat mainly by sweating, and sweat only cools as it evaporates. In humid air evaporation slows, so the same temperature puts more strain on the body. The heat index expresses that strain as the temperature that would feel the same at a moderate humidity.
Older people, babies and young children, people with heart or lung conditions, and anyone working or exercising hard are most at risk. Drink water regularly, rest in shade or air conditioning, and use the water reminder if you tend to forget.
Reading a forecast
Forecasts give temperature and relative humidity, and many give a feels-like figure that uses the heat index in hot weather. To switch between scales, use the Celsius to Fahrenheit converter. In winter, the wind chill calculator does the same job for cold and wind.
How to use it
- Choose °F or °C.
- Enter the air temperature and relative humidity from a forecast or weather station.
- Read the heat index and what the risk level means.
Privacy & limitations
Everything is calculated in your browser; only your choice of °F or °C is remembered.
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Frequently asked questions
What is the heat index?
How hot it feels when humidity is taken into account. Sweat cools the body by evaporating, and humid air slows evaporation, so 90 °F at 70% humidity feels like 106 °F.
Is the heat index measured in the shade?
Yes. The heat index assumes shade and a light wind. In full sunshine it can feel up to 15 °F (8 °C) hotter, and hard work or heavy clothing adds more.
At what heat index should I stop working outside?
There is no single cut-off, but risk rises through the NWS bands. In extreme caution and above, take regular breaks in shade, drink water often, and watch for heat exhaustion: heavy sweating, weakness, dizziness, nausea. Confusion or hot dry skin can mean heat stroke, an emergency: call for help and cool the person straight away.
How is the heat index different from the wet-bulb globe temperature?
The wet-bulb globe temperature (WBGT) also counts sunshine and wind and is used by armies, sports bodies, and workplace rules. It needs special instruments; the heat index needs only the temperature and humidity in a forecast.
Free tool · runs in your browser · no account required