Quick start
- Enter air temperature in Fahrenheit.
- Enter relative humidity percent.
- Calculate to see apparent temperature in Fahrenheit and Celsius.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- 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 method to estimate how hot warm, humid air can feel to a person. It starts with the simple branch, then uses the Rothfusz regression when the preliminary value reaches about 80 F.
Match each input label on the calculator to the real measurement, amount, rate, unit, or setting for your job.
The formula in plain language
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.
The example cards on the calculator page show a complete set of inputs and the kind of answer you should expect.
How to read the answer
Read the main result first. Then check the smaller lines for the totals, units, ranges, counts, or formula steps behind it.
- The main answer is heat index.
- Celsius gives metric context.
- Humidity confirms how much moisture was used in the estimate.
- A 90 F day at 70% relative humidity is about 105.9 F heat index in this model.
Common mistakes to avoid
If the answer looks strange, the most likely cause is a small input mismatch: a mixed unit, copied value, wrong mode, missing label, or result used for the wrong job.
- Do not use heat index as the only heat-safety signal.
- Do not ignore direct sun, exertion, wind, clothing, or health conditions.
- Do not treat a heat-index chart or calculator as a replacement for local alerts.
- Follow local heat advisories and emergency guidance.
Example numbers to sanity-check
Use these as quick checks before trusting your own result. They also make the heat-index chart idea easier to understand.
- 90 F with 70% relative humidity is about 105.9 F heat index.
- 95 F with 35% relative humidity is about 96.5 F heat index.
- 100 F with 55% relative humidity is about 123.6 F heat index.
What the number cannot know
The calculator only sees temperature and humidity. It does not know if you are in full sun, working hard, wearing heavy clothing, taking medication, dehydrated, or under a local heat warning.
- Use local NWS heat alerts before outdoor plans.
- Use extra caution for children, older adults, outdoor workers, athletes, and anyone with health risks.
- If someone is confused, fainting, very hot, or showing heat-stroke warning signs, do not wait for a calculator result.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Heat Index Calculator
About 105.9 F heat index
About 96.5 F heat index
About 123.6 F heat index
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 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.
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.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Everyday Tools Browse the full category for related tools that help with the same job.
- All free tools Search the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guides Find more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resources Start here when you are not sure which calculator page fits.
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 save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.