Heat Index Calculator
See how hot it really feels by combining the air temperature with relative humidity, using the National Weather Service formula.
This uses the National Weather Service Rothfusz regression, the same formula behind official heat advisories. Values assume shade and a light breeze; direct sun can add up to 15°F.
How it works
When it's humid, your sweat can't evaporate as easily, so your body struggles to cool itself and the air feels hotter than the thermometer says. The heat index rolls that effect into a single 'feels like' temperature.
This calculator uses the Rothfusz regression — the same equation the National Weather Service uses to issue heat advisories. Enter the air temperature and the relative humidity, and it returns the apparent temperature. It's most accurate above about 80°F (27°C).
One thing to remember: the official heat index assumes you're in the shade with a light breeze. Standing in direct sunlight can add as much as 15°F to what you actually feel, so treat the number as a floor on a sunny day.
Frequently asked questions
Why does 90°F with high humidity feel like over 100°F?
At 90°F and 70% humidity the heat index is about 106°F. Muggy air keeps your sweat from evaporating, which is your main way of shedding heat, so your body feels much hotter than the reading.
Is the heat index accurate in dry heat?
Below roughly 40% humidity the heat index can actually be lower than the air temperature, and below 80°F the formula isn't really meant to apply. It's built for hot, humid conditions where the risk is highest.
When does the heat index become dangerous?
The NWS flags caution above about 90°F, extreme caution above 103°F, and danger above 124°F. At those levels heat cramps, exhaustion, and stroke get much more likely, especially with exertion.