Calculator
Heat Index Calculator
Find out how hot it actually feels, using the U.S. National Weather Service formula.
What is heat index?
Heat index is how hot it actually feels to the body when humidity is factored in with air temperature — high humidity slows the evaporation of sweat, which is the body’s main cooling mechanism, so the same temperature feels hotter as humidity rises.
Frequently asked questions
How is heat index calculated?
This calculator uses the U.S. National Weather Service formula, a regression equation built from air temperature and relative humidity that estimates the equivalent "feels like" temperature. It's more complex than a simple average because the humidity effect isn't linear — it accelerates as both temperature and humidity climb.
Why does the note mention 80°F as a threshold?
Below about 80°F, sweat evaporates efficiently enough that humidity has little practical effect on how hot the air feels, so the formula isn't meaningfully different from air temperature. The heat index effect becomes significant once temperature and humidity are both high enough to impair the body's cooling.
What heat index levels are considered dangerous?
The NWS uses tiered warnings: roughly 90-103°F (32-39°C) is a "caution" range where heat exhaustion is possible with prolonged exposure, 103-124°F (39-51°C) is "extreme caution" to "danger," and above 125°F (52°C) is classified as "extreme danger" with heat stroke highly likely. These thresholds assume some physical activity and direct sun exposure.
Does heat index account for sun exposure or wind?
No — the standard heat index formula assumes shade and light wind. Direct sunlight can add the equivalent of up to 15°F to the perceived heat index, so actual conditions in full sun can feel meaningfully hotter than the calculated number.
The NWS formula is most accurate for temperatures at or above 80°F (27°C). Below that, humidity has little effect on perceived temperature.