Heat Index Calculator
National Weather Service Rothfusz regression with the published humidity adjustments. Type air temperature in Fahrenheit and relative humidity percent. Below the 80 °F simple-average gate the page uses the Steadham-style average. Not a medical warning product.
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How to compute NWS heat index
- Enter air temperature in degrees Fahrenheit. The default is 90 °F.
- Enter relative humidity from 0 to 100 percent. The default is 70%.
- Read heat index in °F and the Celsius conversion of that heat-index value.
- Treat the number as a shade, still-air index. Direct sun and a medical emergency are outside this page.
NWS Rothfusz heat index
Apparent temperature from T and relative humidity
| Simple gate | 0.5 (T + 61 + (T - 68) × 1.2 + RH × 0.094) |
|---|---|
| Rothfusz | Multiple regression used when the simple value is at least 80 °F |
| Dry adjustment | RH under 13% and T from 80 to 112 °F |
| Humid adjustment | RH over 85% and T from 80 to 87 °F |
| Assumption | Shade, still air |
| Advice | Not a medical warning product. |
What heat index measures
Heat index is an apparent temperature that combines air temperature and relative humidity. The NWS implementation uses Lans P. Rothfusz's multiple regression, with later adjustments for very dry or very humid air in stated temperature bands. It assumes shade and light wind. Wind chill is the cold-weather counterpart on the wind chill page.
How this page computes heat index
First the page computes a simple average 0.5 (T + 61 + (T - 68) × 1.2 + RH × 0.094). If that value is under 80 °F, that simple value is the result. Otherwise the Rothfusz polynomial in T and RH runs, then the published dry or humid adjustments apply in their bands. Relative humidity must sit from 0 to 100. Celsius is a conversion of the heat-index Fahrenheit, not a second regression.
Rothfusz, Steadham, and related heat products
The simple gate follows the Steadham-style average NWS documents as a check before the full regression. Wet-bulb globe temperature includes sun and globe temperature and is not this page. Heat outlook maps from a weather service include more than two typed numbers. Convert Celsius air temperature on the temperature converter before you use this form.
Use cases
A homework check at 90 °F and 70% RH, or turning a backyard thermometer and a cheap hygrometer into the NWS-style number. Not a workplace heat-stress program and not medical advice.
Limits
Shade and still air only. No sun factor. No live METAR. Humidity outside 0-100 fails. CZNull does not receive the numbers.