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 temperature and humidity you type stay in the browser - the heat index is computed on this device, with no upload and no account.
Same 95F air temperature, four different heat-index risk levels
Heat Index Calculator shows why an air-temperature reading alone cannot tell you the real heat risk. Hold the air temperature fixed at 95F and change only the relative humidity, and that one unchanged reading crosses four different NWS risk categories - from Extreme Caution in dry air, through Danger at a common summer-afternoon 60%, up to Extreme Danger at 80%. That is exactly why the calculator asks for humidity and not just temperature.
| Relative humidity | Heat index | NWS category |
|---|---|---|
| 20% | 91F | Extreme Caution |
| 40% | 99F | Extreme Caution |
| 60% | 113F | Danger |
| 80% | 134F | Extreme Danger |
Why the same formula does not apply at every extreme
At or above 80F apparent temperature, Heat Index Calculator applies two correction terms published alongside the NWS Rothfusz regression. In very dry air (relative humidity under 13%, air temperature 80-112F) the base regression slightly overstates the risk, so a dry-air correction is subtracted - at 100F and 10% humidity the uncorrected regression works out to about 94.8F, and the correction brings the displayed value down to 94.1F. In very humid air (above 85% humidity, air temperature 80-87F) the regression slightly understates the risk instead, so a humid-air correction is added - at 85F and 90% humidity the uncorrected value of about 101.6F becomes 101.8F once the correction is applied.
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.
What does the calculator show below 80 degrees apparent temperature?
Below 80F, the NWS regression formula the calculator otherwise uses is not defined, so Heat Index Calculator falls back to the simpler temperature-humidity average the National Weather Service itself uses at lower readings. The result switches to the full regression, with its correction terms, only once the apparent temperature reaches 80F.
Does the heat index shown here account for direct sunlight?
No. Heat Index Calculator reports the standard shade-value heat index published by the NWS. Full sun exposure can add up to 15F to the effective heat index per NWS guidance, so conditions in direct sun can feel meaningfully hotter than the number shown here.
Should I rely on this instead of an official NWS heat advisory?
No. This calculator is a general estimate for understanding how temperature and humidity combine, not a substitute for an official National Weather Service heat advisory or warning - always follow local NWS alerts for your area.