Heat index calculator
Enter the air temperature and humidity to see how hot it feels in the shade, which matters most in humid summers.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter air temperature, temperature unit and relative humidity.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
First the simple form 0.5 × (T + 61 + (T − 68) × 1.2 + 0.094 RH) in °F; if its average with T reaches 80°F the Rothfusz regression is used with the NWS low and high humidity adjustments.
Example
Inputs: Air temperature = 35, Temperature unit = Celsius (°C), Relative humidity = 60 %. Result: Heat index: 45.1 °C; In the other unit: 113.1 °F; NWS risk level: Danger.
Limits and caveats
- Values assume shade and light wind; full sun can add up to about 8°C (15°F).
- A regression fit with about ±1.3°F error, less reliable outside the NWS table range.
Questions about this tool
How is the heat index different from temperature?
It adds the effect of humidity: sweat evaporates slowly in humid air, so the body feels hotter.
When does the heat index become dangerous?
From 103°F (39.4°C) the NWS rates it Danger, with heat stroke likely on long exposure.
Why do coastal cities feel hotter?
High humidity there pushes the heat index up even when the air is cooler than inland.