Heat Index Calculator

Temperature and humidity — what the heat actually feels like.

°F
%

Heat index is most meaningful above about 80°F — below that, humidity has little perceived effect on comfort.

Feels like
105.9°F
Feels hotter than actual
+15.9°F

The NWS issues heat advisories when heat index exceeds 100°F and warnings above 105°F — dangerous territory for heat-related illness.

A worked example

At 90°F with 70% relative humidity, the heat index is about 106°F — significantly hotter-feeling than the actual air temperature.

Frequently asked questions

Why does humidity make it feel hotter?

Your body cools itself by sweating — but sweat only works when it evaporates, and humid air is already nearly saturated, so evaporation slows or stops. Less evaporative cooling means your body retains more heat, making high humidity feel significantly hotter than dry heat at the same temperature.

At what heat index level does it become dangerous?

The NWS issues a Heat Advisory when the heat index is expected to reach 100-105°F, an Excessive Heat Watch when 105°F or higher is possible over two or more days, and an Excessive Heat Warning when 105°F or above is imminent.

This uses the NWS Rothfusz regression, the same formula used by the National Weather Service — results are most accurate when temperature is above 80°F and relative humidity above 40%.