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
Enter air temperature and relative humidity to estimate heat index, then check the Fahrenheit and Celsius apparent-temperature result.
Inputs explained Result checks Example values Runs in your browser
Formula steps
- Compute the simple NWS heat index branch first.
- Use the Rothfusz regression and humidity adjustments when the preliminary value reaches about 80 F.
- 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.
What people use it for
Estimate how hot humid weather feels.
Compare air temperature with heat index.
Convert apparent temperature to Celsius.
Understand why humidity changes heat stress.
Quick examples
Humid heat
90 F and 70% RHAbout 105.9 F heat index
Dryer heat
95 F and 35% RHAbout 96.5 F heat index
Danger check
100 F and 55% RHAbout 123.6 F heat index
Need the guide or a nearby tool?
Need a slower walkthrough, a related calculator, or the full library? These links keep you close to the task you started.
- How to use the Heat Index Calculator See slower examples, input meanings, formula notes, result limits, and mistakes to avoid.
- Everyday Tools Browse related calculators and utilities in the same category.
- Calculator and utility guides Find more guides that use real tool inputs and example numbers.
- All free tools Search every live tool, converter, calculator, AI helper, and browser utility.
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 measurements, amounts, units, or options the page asks for.
What is the Heat Index Calculator doing with my inputs?
In plain language: The calculator uses the National Weather Service heat index method: a simple branch first, then the Rothfusz regression and standard humidity adjustments when the preliminary value reaches about 80 F. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Heat Index Calculator inputs mean?
Temperature F: the air temperature in degrees Fahrenheit. Relative humidity %: how much water vapor is in the air compared with the most it could hold at that temperature. Heat index: an apparent-temperature estimate for hot, humid conditions.
How should I read the Heat Index Calculator answer?
Read the headline answer, then check the smaller lines beside it. For everyday tools, those lines usually show the distance, time, cost, units, or setting that made the answer change.
What should I double-check before trusting the answer?
Heat risk depends on sun, exertion, wind, hydration, clothing, health, and local warnings. Treat this as a weather-math estimate, not a safety clearance. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
Why can direct sun make heat feel worse?
Heat index is usually based on air temperature and humidity in shade-like conditions. Direct sun, hard activity, heavy clothing, and low wind can raise real heat stress.
Is heat index the same as the real air temperature?
No. Air temperature is the thermometer reading. Heat index is a feels-like estimate for people in hot, humid weather.
Why does humidity raise the heat index?
High humidity makes sweat evaporate more slowly. That can make the same air temperature feel hotter to your body.
Can I use Celsius with this calculator?
Enter Fahrenheit on this page. The result also shows Celsius, so you can read the apparent temperature in both units after calculating.
Does wind speed belong in heat index?
Not in this calculator. Heat index uses temperature and humidity. Wind, sun, work level, clothing, and local alerts can still change real heat risk.
When should I ignore the calculator and use official help?
Use local heat advisories first. If someone is confused, fainting, very hot, or showing heat-stroke warning signs, treat it as urgent and follow emergency guidance.
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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