Enter the air temperature and the relative humidity below; the heat index (how hot it actually feels) and its NWS risk category update as you type - nothing is uploaded and no calculation waits on a server round-trip.
Use the Fahrenheit / Celsius dropdown to switch units at any time; the temperature label and the result both switch with it. Humidity is entered as a plain percentage from 0 to 100.
The reference table below the result lists the four official NWS heat-index categories (Caution, Extreme Caution, Danger, Extreme Danger) and the typical effect of each, so you can see where today's reading falls.
For a full field-by-field walkthrough, see the heat index calculator step-by-step guide.
Heat Index Calculator
Enter the air temperature and the relative humidity to get the heat index - the apparent (feels-like) temperature the National Weather Service uses to gauge heat-related risk - along with its official NWS risk category, updated live as you type.
The heat index is computed with the same Rothfusz-regression formula published by the National Weather Service, not a rough estimate. It combines temperature and humidity because high humidity slows the body's ability to cool itself through sweat evaporation, so the same temperature can feel far hotter on a humid day.
The calculation runs locally in the browser - no upload and no account.
Frequently Asked Questions
What does the heat index calculator do?
It takes the air temperature and relative humidity you enter and computes the heat index (the apparent, feels-like temperature) using the National Weather Service's Rothfusz-regression formula, then shows the matching NWS risk category.
Why does humidity change how hot it feels?
High humidity slows evaporative cooling from sweat, so the body cannot shed heat as efficiently. The same air temperature can feel noticeably hotter at 70% humidity than at 20% humidity - the heat index number captures that difference.
What do the risk categories (Caution, Extreme Caution, Danger, Extreme Danger) mean?
They are the National Weather Service's official heat-index bands: Caution (80-90F), Extreme Caution (90-103F), Danger (103-124F), and Extreme Danger (125F and above), each with an increasing likelihood of heat-related illness with prolonged exposure or activity.