Heat Index Calculator

You enter the air temperature and the relative 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. It then flags which caution band you're in. Everything runs in your browser, so you'll see the feels-like number update as you type, and nothing you enter leaves 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 National Weather Service 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 National Weather Service 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.

Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. 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.