Heat Index Calculator
| Actual Temperature | - |
| Risk Category | - |
What Is Heat Index?
Heat index combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity reduces the body's ability to cool itself by evaporating sweat, which makes the same air temperature feel hotter.
How to Use This Heat Index Calculator
- Fill in the input fields on the left with your own numbers - every field is pre-filled with a sample value so you can see the layout working immediately.
- Click the Calculate button to instantly generate your result in the panel on the right.
- Read your result, along with the supporting breakdown table and chart, to understand not just the answer but how it was reached.
- Change any value and click Calculate again as many times as you like - there is no limit on how often you can use the heat index calculator, and nothing is saved or sent anywhere.
Frequently Asked Questions
Where does this formula come from?
This calculator uses the Rothfusz regression, the equation adopted by the US National Weather Service to compute heat index from temperature and humidity.
What heat index level is dangerous?
Values above roughly 39°C (103°F) fall into the "danger" category, where heat cramps or heat exhaustion become likely with prolonged exposure or physical activity.
Tips for Getting Accurate Results from the Heat Index Calculator
- Double-check every input before calculating - a misplaced decimal point or wrong unit is the most common cause of an unexpected result.
- Make sure the units shown next to each field (kg, cm, %, hours, and so on) match the units of the number you are typing in; convert first if they don't.
- If a result looks off, try the calculation with round, simple numbers first (like 10 or 100) to confirm the tool is behaving the way you expect, then switch back to your real figures.
- Bookmark or save this page if you use the heat index calculator regularly, since all calculations run instantly in your browser with no sign-up required.
How to Print or Save This Heat Index Calculator as a PDF
- Fill in your values and click Calculate.
- Click the Print link at the top of the page.
- In the print dialog that opens, choose "Save as PDF" as the destination (instead of a physical printer) to download a PDF copy.
- Make sure "Background graphics" is turned on in the print settings so the colors print exactly as shown on screen.
- Click Save or Print - only the heat index calculator's inputs and results are included; the site navigation, sidebar, footer, and the theory/FAQ sections below are automatically left out to keep the printout clean and focused.
More Frequently Asked Questions
Is this heat index calculator free to use?
Yes, completely free, with no registration, download, or watermark - just enter your numbers and get an instant result.
Does this calculator work on mobile phones?
Yes, the layout automatically adjusts for phones and tablets, so you can use the heat index calculator on any device with a modern web browser.
Is my data stored or sent anywhere?
No - every calculation on this page runs locally in your own browser using JavaScript; nothing you type is uploaded, logged, or stored on any server.
Can I rely on this for professional or legal decisions?
This tool is designed to give a fast, well-informed estimate for everyday use. For decisions with financial, legal, medical, or safety consequences, always confirm the figures with a qualified professional in that field.