Heat Index Calculator
Apparent temperature from heat and humidity.
Enter your values
Enter the air temperature in °F.
Enter the relative humidity in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Feels like
- 108.3 °F
- In Celsius
- 42.4 °C
- Added by humidity
- 16.3 °F
- Risk level
- Danger
- Guidance
- Limit outdoor exertion; heat illness likely
Quick answer
The heat index combines temperature and humidity using the NWS Rothfusz regression; it applies above about 80°F.
At a glance
| What it does | Apparent temperature from heat and humidity. |
|---|---|
| Category | Weather |
| Inputs needed | Air temperature, Relative humidity |
| Main output | Feels like |
| Cost | Free — no sign-up, no download |
How to use the Heat Index Calculator
- 1Enter the air temperature in Fahrenheit.
- 2Enter the relative humidity.
- 3Read the feels-like temperature and risk band.
Inputs explained
- Air temperature(°F)
- Enter the air temperature in °F.
- Relative humidity(%)
- Enter the relative humidity in %.
Worked example
Using the values the calculator loads with:
Inputs
- Air temperature92 °F
- Relative humidity65 %
Results
- Feels like108.3 °F
- In Celsius42.4 °C
- Added by humidity16.3 °F
- Risk levelDanger
- GuidanceLimit outdoor exertion; heat illness likely
Frequently asked questions
Why does humidity make heat worse?
Sweat cools you by evaporating. In humid air it evaporates slowly, so your body cannot shed heat.
Is heat index measured in shade?
Yes. Full sun can add up to 15°F to the values shown.
What do I need to enter into the Heat Index Calculator?
Just 2 values: air temperature, relative humidity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Heat Index Calculator work out the answer?
The heat index combines temperature and humidity using the NWS Rothfusz regression; it applies above about 80°F. The working is shown step by step so you can verify the result by hand.
Is the Heat Index Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.