What is the Heat Index Calculator?
Calculate the heat index (“feels like” temperature) from air temperature and humidity. The heat index estimates how hot it feels when humidity slows the cooling effect of sweat, which matters for health and outdoor work.
How to use the Heat Index Calculator
- Enter your air temperature, your relative humidity.
- Results update automatically as you type; you can also press Calculate.
- Read the output: heat index, in celsius.
- Use the 📊 chart to see the result visually, then copy or share it.
Formula and method
Worked example
With air temperature = 90 °F, relative humidity = 60 %, the Heat Index Calculator returns heat index: 99.7 °F; in celsius: 37.6 °C.
Where the Heat Index Calculator helps in real life
The thermometer reads 32 °C (90 °F) with 70% humidity. The calculator shows a heat index well above 40 °C (105 °F), a level where heat illness becomes a real risk.
A sports coach plans an afternoon practice. Checking the feels-like temperature helps decide whether to shorten the session or add water breaks.
Common mistakes to avoid
- Using a temperature measured in direct sunlight.
- Ignoring humidity on hot days.
- Applying the heat index to cool weather, where it is not defined.
- Assuming shade conditions match full sun.
💡 Practical tip: Direct sunlight can make conditions feel several degrees hotter than the heat index, which assumes shade.
Tips for better results
- Schedule strenuous activity for cooler parts of the day.
- Drink water regularly before you feel thirsty.
- Check official heat warnings for your area.
Frequently asked questions
What does the Heat Index Calculator do?
Calculate the heat index (“feels like” temperature) from air temperature and humidity.
How is it calculated?
The Heat Index Calculator uses this method: NWS Rothfusz regression (with low/high humidity adjustments).
Can you show an example?
With air temperature = 90 °F, relative humidity = 60 %, the Heat Index Calculator returns heat index: 99.7 °F; in celsius: 37.6 °C.
Is the Heat Index Calculator accurate?
Yes. It uses standard formulas and exact constants, and the results are verified by our automated test suite. Values are displayed to up to 12 significant digits.
Is this tool free and private?
Yes — the Heat Index Calculator is completely free with no sign-up, and it runs entirely in your browser, so the values you enter are never uploaded to our servers.
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