Heat Index Calculator

The heat index tells you how hot it really feels when humidity is combined with air temperature. High humidity slows the evaporation of sweat, so your body cools less effectively.

How to use the Heat Index Calculator

Enter the air temperature and choose °F or °C.
Enter the relative humidity as a percentage.
Read the heat index and its risk category.

How it works

This calculator uses the Rothfusz regression used by the U.S. National Weather Service, including its adjustments for very low and very high humidity. Below about 80 °F (27 °C) a simpler formula is used, as the NWS recommends.

Heat index values assume shade and a light wind. Direct sunshine can raise the heat index by up to 15 °F (8 °C).

Heat index (°F) by temperature and humidity

Temp \ RH	40%	50%	60%	70%	80%
80 °F	80	81	82	83	84
85 °F	84	86	89	93	97
90 °F	91	95	100	106	113
95 °F	99	105	113	123	134
100 °F	109	118	129	143	158
105 °F	121	134	149	166	187

Frequently asked questions

What heat index is dangerous?

The NWS classes a heat index of 103–124 °F (39–51 °C) as “danger” and 125 °F (52 °C) or higher as “extreme danger”.

What is the heat index at 90 °F and 70% humidity?

About 106 °F — well into the “extreme caution” range.

Is the heat index the same as the “feels like” temperature?

In hot weather, yes. Weather apps usually show the heat index as “feels like” above about 80 °F and wind chill in the cold.