Heat Index Calculator
Calculate the heat index from temperature and humidity — how hot it really feels.
Why humid heat is dangerous
The heat index is how hot the air feels once humidity is factored in, and on a humid day it can be far above the thermometer reading. This calculates it from the temperature and relative humidity, so you know the true heat stress — important because humid heat is not just uncomfortable, it can be genuinely hazardous.
Enter the temperature and humidity, and see the heat index.
Sweat that cannot evaporate
The reason humidity makes heat dangerous is that your body cools itself by sweating, and sweat only cools you when it evaporates. When the air is already saturated with moisture, sweat cannot evaporate efficiently, so your cooling system fails and your core temperature can climb. That is why high heat-index days carry warnings about heat exhaustion and heatstroke, and why a moderately hot but very humid day can be more dangerous than a hotter but dry one. The heat index is a real health signal, not just a comfort number.
Questions & answers
Why does humidity make heat feel worse?
Your body cools by sweating, and sweat only cools you as it evaporates. High humidity stops it evaporating, so cooling fails and it feels hotter.
Can a humid day be more dangerous than a hotter dry one?
Yes — because the humidity blocks your body's cooling, so the heat stress can be higher despite a lower temperature.
Is anything uploaded?
No. The calculation happens in your browser.