90 F, 70% RH
- Celsius
- 41.0677892222 C
- Air temperature
- 90 F
- Humidity
- 70%
Heat illness risk depends on sun, exertion, hydration, wind, and health. Follow local heat advisories.
Heat Index Calculator
Use this free heat index calculator to estimate apparent temperature from air temperature and humidity using the NWS regression.
Research-backed assumptions Formula steps Examples included Private in-browser use
Formula steps
- Use the NWS Rothfusz regression for warm, humid conditions.
- Apply humidity adjustment when the formula range calls for it.
- Report the apparent temperature in Fahrenheit and Celsius.
How to use the heat index calculator
- Enter air temperature in Fahrenheit and relative humidity percent.
- Press Calculate heat index to estimate apparent temperature.
- Use the result with caution in sun, exercise, or poor ventilation.
- Follow local heat advisories and safety guidance.
Common uses
Estimate how hot humid weather feels.
Compare air temperature with heat index.
Convert apparent temperature to Celsius.
Understand why humidity changes heat stress.
Examples
Higher apparent temperature
Adjusted heat index
High heat index estimate
Frequently asked questions
Plain-language answers about when to use the tool, what it does with your inputs, what to double-check, and how privacy works.
When should I use the Heat Index Calculator?
Use it when your task matches one of these common needs: Estimate how hot humid weather feels. Compare air temperature with heat index. It works best when you already know the values, dates, units, or settings the page asks for.
What is the Heat Index Calculator doing with my inputs?
In plain language: The calculator uses the National Weather Service Rothfusz regression for heat index and applies the standard humidity adjustments. The examples on the page are there so you can compare your inputs with a filled-out calculation before copying the answer.
What should I double-check before trusting the answer?
Heat risk depends on sun, exertion, wind, hydration, clothing, health, and local warnings. Do not rely on a calculator alone. Also check that you used the right unit, date, scale, or mode because small input changes can change the result.
Does the site save what I enter?
No. The calculator runs in your browser tab. Your recent answers stay only on the page while you use it, and they are not sent to a server.
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