What is the difference between the heat index and the air temperature?

Air temperature is what a thermometer in the shade reads. The heat index is how hot it actually feels once humidity is factored in, because humidity changes how well your sweat can evaporate. At 90°F the air temperature is 90 — but at 70% humidity the heat index is about 106°F, because the muggy air slows evaporative cooling and your body cannot shed heat as fast. The heat index is the number that predicts heat illness, not the raw thermometer reading.

Why does humidity make it feel hotter?

Your main cooling system is sweat evaporating off your skin, which carries heat away. Evaporation only works if the surrounding air can absorb more water vapour. When humidity is high the air is already near saturation, so sweat sits on your skin instead of evaporating — you keep generating heat but cannot dump it. That is why a 95°F day at 40% humidity (heat index ~95°F) feels far more bearable than 90°F at 80% (heat index ~113°F): the cooler day with dry air actually moves less load onto your body.

What heat index is dangerous?

The NWS uses four bands. Caution starts at a heat index of 80°F (27°C): fatigue is possible with prolonged activity. Extreme Caution begins at 90°F (32°C): heat cramps and exhaustion are possible. Danger starts at 103°F (39°C): heat exhaustion is likely and heat stroke is possible with continued exposure. Extreme Danger is 124°F (51°C) and up: heat stroke is highly likely. This tool labels every reading with its band and the matching warning, so you can decide whether outdoor exertion is safe.

How does the heat index relate to wind chill?

They are mirror images. Heat index applies in hot weather and tells you how much hotter it feels because humidity blocks your sweat from cooling you. Wind chill applies in cold weather and tells you how much colder it feels because wind strips warm air off your skin faster. Both are "apparent temperature" measures — what your body feels rather than what the thermometer reads — but they use entirely different inputs (humidity vs. wind speed) and only one applies at a time. This calculator covers the hot side.

Is the °C result the same as the official °F number?

Yes. The Rothfusz regression is defined in Fahrenheit, so when you enter Celsius the tool converts your temperature to °F, runs the exact same equation and adjustments, then converts the heat index back to °C. You get the official NWS value, just displayed in the unit you chose — there is no separate, less-accurate "metric formula" involved.

Decide if a midday run is safe

It is 88°F and the phone says 75% humidity before your lunchtime run. Enter 88 and 75 and the heat index comes back around 103°F — the bottom of the Danger band, where heat exhaustion is likely with sustained exertion. That is the signal to move the run to early morning or cut it short, rather than trusting the "only 88°F" reading that hides how much the humidity is loading your body.

Call a heat day on a construction site

A site supervisor checks conditions before the afternoon shift: 95°F, 60% humidity. The heat index lands near 114°F — solidly in Danger. Knowing the apparent temperature, not just the air temperature, is what justifies scheduling extra water breaks, rotating crews out of direct sun, and watching for heat-illness symptoms before someone goes down.

Judge whether it is too hot to walk the dog

Dogs cannot sweat to cool down, so the heat index matters even more for them. At 90°F and 70% humidity the feels-like is about 106°F. Pavement will be far hotter, and a thick-coated dog has almost no way to shed that load — the Danger reading tells you to skip the midday walk and go at dawn instead.

Compare two cities for a summer trip

One destination is 100°F at 20% humidity (heat index ~103°F), the other 88°F at 85% (heat index ~113°F). The "hotter" city by the thermometer actually feels cooler, because dry desert air lets sweat evaporate freely. Running both through the calculator flips the intuition and helps you pack and plan for the one that will genuinely be harder on your body.

Set an attic or greenhouse work cutoff

A poorly ventilated attic reads 96°F with 55% humidity from a datalogger. The heat index is about 112°F — well into Danger. Rather than guessing how long you can safely work up there, the apparent temperature gives a concrete threshold for scheduling the job for a cooler hour or adding forced ventilation first.

Heat Index Calculator — Real "Feels-Like" Temperature from Heat and Humidity

Air temperature + humidity to the real "feels-like" temperature, with NWS risk level — exact Rothfusz regression, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Air temperature (°F)

Relative humidity (%)

Feels like112.2°FActual air temperature: 92°F

Risk levelDanger

Heat cramps and heat exhaustion likely; heat stroke possible with prolonged exposure. Avoid exertion outdoors.

What this tool does

A heat index calculator that turns a thermometer reading plus relative humidity into the temperature your body actually experiences. It runs the full National Weather Service Rothfusz regression — the same equation the NWS publishes — including both official adjustments: the subtraction for hot, very dry air (relative humidity under 13%) and the addition for warm, saturated air (humidity over 85%). When the air is cool enough that the regression is out of range, it falls back to the NWS simple-average formula, so the answer is correct across the whole temperature span rather than only in the danger zone. Enter the temperature in °F or °C — the metric path converts to Fahrenheit internally, computes, and converts the result back, so the °C heat index matches the official °F figure exactly. Every reading is mapped to the NWS risk category — Caution, Extreme Caution, Danger, or Extreme Danger — with the specific heat-disorder warning for that band, so you know whether it is safe to run, work the job site, or leave a pet outside. Inputs sync to the URL so a "share this reading" link reopens the same number, your preferred unit is remembered between visits, and nothing ever leaves your browser.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 10 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Heat Index Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Decide if a midday run is safe
It is 88°F and the phone says 75% humidity before your lunchtime run. Enter 88 and 75 and the heat index comes back around 103°F — the bottom of the Danger band, where heat exhaustion is likely with sustained exertion. That is the signal to move the run to early morning or cut it short, rather than trusting the "only 88°F" reading that hides how much the humidity is loading your body.
Call a heat day on a construction site
A site supervisor checks conditions before the afternoon shift: 95°F, 60% humidity. The heat index lands near 114°F — solidly in Danger. Knowing the apparent temperature, not just the air temperature, is what justifies scheduling extra water breaks, rotating crews out of direct sun, and watching for heat-illness symptoms before someone goes down.
Judge whether it is too hot to walk the dog
Dogs cannot sweat to cool down, so the heat index matters even more for them. At 90°F and 70% humidity the feels-like is about 106°F. Pavement will be far hotter, and a thick-coated dog has almost no way to shed that load — the Danger reading tells you to skip the midday walk and go at dawn instead.
Compare two cities for a summer trip
One destination is 100°F at 20% humidity (heat index ~103°F), the other 88°F at 85% (heat index ~113°F). The "hotter" city by the thermometer actually feels cooler, because dry desert air lets sweat evaporate freely. Running both through the calculator flips the intuition and helps you pack and plan for the one that will genuinely be harder on your body.
Set an attic or greenhouse work cutoff
A poorly ventilated attic reads 96°F with 55% humidity from a datalogger. The heat index is about 112°F — well into Danger. Rather than guessing how long you can safely work up there, the apparent temperature gives a concrete threshold for scheduling the job for a cooler hour or adding forced ventilation first.

Common pitfalls

Trusting the air temperature alone on a humid day. 88°F sounds mild, but at 75% humidity the heat index is about 103°F — the Danger band. Always read the feels-like number, not the thermometer, when deciding whether outdoor activity is safe.
Assuming the hottest air temperature is the most dangerous. Dry heat lets sweat evaporate; humid heat does not. 100°F at 20% humidity (heat index ~103°F) is easier on your body than 88°F at 85% (heat index ~113°F). Compare the heat index, not the raw temperature.
Confusing heat index with wind chill. Heat index applies in hot, humid weather and uses humidity; wind chill applies in cold weather and uses wind speed. Plugging a winter wind-speed mindset into a summer decision gets the risk exactly backwards.

Privacy

Every calculation runs in your browser. The Rothfusz regression, the unit conversions, and the risk classification are plain JavaScript on this page — the temperature and humidity you enter are never sent to a server, and there is no logging of what you looked up. The one thing to know: the shareable URL encodes your inputs in the query string (e.g. ?temp=90&humidity=70&unit=f), so a "share link" pasted into chat records those numbers in the destination's access log. They are weather readings, not sensitive data, but if you would rather not, copy the result text instead of sharing the URL. Your preferred unit is stored only in this browser's localStorage.

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Heat Index Calculator — Real "Feels-Like" Temperature from Heat and Humidity

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Heat Index Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Decide if a midday run is safe

Call a heat day on a construction site

Judge whether it is too hot to walk the dog

Compare two cities for a summer trip

Set an attic or greenhouse work cutoff

Common pitfalls

Privacy

FAQ

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