Quick start
- Enter air temperature in Fahrenheit.
- Enter relative humidity percent.
- Calculate to see apparent temperature in Fahrenheit and Celsius.
Best uses
These are the situations this tool is meant for. If your task is close to one of these, the examples and notes below can help you choose the right inputs.
- Estimate how hot humid weather feels.
- Compare air temperature with heat index.
- Convert apparent temperature to Celsius.
- Understand why humidity changes heat stress.
What this calculator is solving
The Heat Index Calculator uses the NWS heat index regression to estimate apparent temperature in warm, humid conditions.
You do not need to memorize the formula first. Start by matching each input label on the calculator to the number, date, unit, or setting you actually have.
The formula in plain language
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.
If that sounds abstract, use the example cards on the calculator page. They show a complete set of inputs and the kind of answer you should expect.
How to read the answer
Read the headline result first. Then look at the smaller supporting lines because they explain the parts behind the answer, such as totals, units, ranges, or formula steps.
- The main answer is heat index.
- Celsius gives metric context.
- Humidity confirms how much moisture was used in the estimate.
Common mistakes to avoid
If the answer looks strange, the most likely cause is a small input mismatch: the wrong unit, date, weight, scale, mode, or policy assumption.
- Do not use heat index as the only heat-safety signal.
- Do not ignore direct sun, exertion, wind, clothing, or health conditions.
- Follow local heat advisories and emergency guidance.
Research and references
These references shaped the calculator assumptions, unit choices, or safety notes.
Examples from the calculator
Higher apparent temperature
Adjusted heat index
High heat index estimate
FAQ in plain language
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.
Related tools
- Wind Chill Calculator Calculate wind chill from Fahrenheit temperature and wind speed using the NWS formula.
- Dew Point Calculator Estimate dew point from Fahrenheit temperature and relative humidity.
- BTU Calculator Estimate room air conditioner BTU capacity from room size and simple adjustments.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to paste the expression and result into notes, homework, a message, or another document. Check the units and assumptions before copying.