Converter tool

Heat Index Calculator

This heat index calculator shows how hot it really feels once humidity is in the mix. Enter the air temperature and the humidity, and you'll get the feels-like temperature in Fahrenheit and Celsius, plus the National Weather Service caution level. It updates as you type, so it's quick to check before you head out, and it's free with no sign-up.

Feels-like temp
NWS caution bands
F and C
Humidity effect
Heat index chart

Last updated July 23, 2026 NWS formula By Muhammad Younus

Feels like

106 °F

41°C

Celsius

Danger

Caution level

+16°

Above actual

Estimates only, not medical advice. The heat index assumes shade and light wind. Direct sun can add up to 15°F, so hydrate, take breaks, and watch for heat illness.

Air temperature (°F) Relative humidity (%)

The heat index matters most above about 80°F; below that, temperature and humidity barely change how it feels. Relative humidity is what a weather app or hygrometer shows.

The basics

How does the heat index work?

The heat index answers a simple question: how hot does it actually feel? Your body cools itself by sweating, and sweat works by evaporating off your skin. When the air is dry, that evaporation is fast and efficient, so you stay comfortable. When the air is humid, it's already loaded with moisture, so the sweat lingers and your cooling stalls, which makes the same temperature feel far worse. The NWS captured this in the Rothfusz regression, a formula that takes the air temperature and the relative humidity and folds them into one feels-like number. At 90 degrees with 70 percent humidity it lands near 106, well into the danger zone, while that same 90 in dry desert air feels close to the real reading. The effect grows as it gets hotter, so humidity barely matters at 80 but dominates at 100. This tool runs the official formula, converts between Fahrenheit and Celsius, and tells you which caution band you're in, so you'll know at a glance whether it's safe to push hard outside.

heat index = NWS Rothfusz regression of temp and humidity

Step by step

Checking the heat, step by step

Here's the quick routine before you head outside, and it's just three steps:

1
Enter the temperatureType the air temperature, in Fahrenheit or Celsius.
2
Enter the humidityAdd the relative humidity from your weather app.
3
Read the feels-likeSee the heat index and which caution band it's in.

Quick reference

Heat index chart

Here's the feels-like temperature in Fahrenheit at a few air temperatures and humidity levels. You'll see how fast it climbs as the air gets muggy.

Air temp	40% RH	60% RH	80% RH
80°F	80	82	84
85°F	84	90	96
90°F	91	100	113
95°F	98	114	133
100°F	109	129	150

Methodology

Where this number comes from

Every feels-like reading here runs the NWS Rothfusz regression, the same equation behind official heat advisories, so the results track the NWS heat index chart rather than a made-up formula. That equation assumes you're in the shade with a light wind, which is why direct sun can add up to 15°F on top of what you see, and why it only kicks in above about 80°F, where humidity starts to bite. Because the whole thing runs in your browser from that one published equation, nothing you type is sent anywhere, and you can sanity-check any value against the chart above. If your weather app shows only the dew point instead of humidity, convert it with our dew point calculator, then enter that percent to get the heat index.

FAQ

Frequently asked questions

You enter the air temperature and the humidity, and it works out the heat index, which is how hot it actually feels. It uses the National Weather Service formula, the same one behind the heat advisories you hear about, then flags which caution band you're in. The feels-like number updates as you type, and it all happens on your device.

The heat index is the feels-like temperature, combining the actual air temperature with the humidity. When the air is humid, sweat evaporates slower, so your body cools less well and the heat feels worse than the thermometer says. At 90 degrees with 70 percent humidity, for example, it feels more like 106. That's why a humid 90 can be dangerous while a dry 90 is merely hot, and it's the number forecasters use to warn people.

Because your body cools itself by sweating, and sweat only cools you as it evaporates. When the air is already full of moisture, evaporation slows down, so your built-in cooling system can't keep up and your core temperature creeps up. Dry air lets sweat evaporate fast, which is why a desert 95 can feel easier than a muggy 88. The heat index captures that effect, turning temperature and humidity into one feels-like figure.

The NWS uses four bands. From about 80 to 90 it's caution, where fatigue is possible. From 90 to 103 is extreme caution, with cramps and heat exhaustion possible. From 103 to 124 is danger, where heat exhaustion is likely and heat stroke possible. At 125 and above it's extreme danger, with heat stroke highly likely. These rise faster in direct sun, so add up to 15 degrees if you're standing in it, and take the warnings seriously.

It comes from the Rothfusz regression, a formula the NWS fitted to how heat and humidity affect the body. It blends the temperature, the humidity, and several combined terms into one number, with small tweaks at very low or very high humidity. You don't have to run it by hand, since this tool does it for you, but the short version is that both heat and moisture push the result up, and humidity matters more as the temperature climbs.

At 90 degrees Fahrenheit with 80 percent humidity, the heat index is about 113, so it feels a lot closer to 113 than to 90. That sits in the danger band, where heat exhaustion is likely and heat stroke is possible, so it's a good moment to ease off, drink water, and get to shade or air conditioning. Enter any temperature and humidity above and you'll see the exact feels-like number and its caution level.

The heat index works the same in Celsius, it just uses metric numbers. Switch the unit toggle to C, type the air temperature in Celsius, and you'll get the feels-like temperature in Celsius along with the caution band. As a quick guide, 32 C at 70 percent humidity feels like roughly 41 C, which is firmly in the extreme caution to danger range.

Yes, with one extra step. The heat index formula uses relative humidity rather than the dew point, so if your forecast only lists the dew point, convert it to relative humidity with our dew point calculator first, then type that percent above. A higher dew point simply means more moisture in the air, and that extra moisture is exactly what pushes the feels-like temperature up.

Yes, it's completely free, with no sign-up. Type the temperature and humidity, switch between Fahrenheit and Celsius, and read the feels-like number and caution level in a tap. Bookmark it for planning a workout, a game, or yard work on a hot day, and use it as a safety guide, since it can't account for your own health or the sun overhead.

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