Health & Wellness
Calculate the "feels like" temperature based on actual temperature and humidity. The heat index measures how hot it actually feels when accounting for humidity.
Heat Index
105.9 °F
Extreme Caution: Heat exhaustion and heat cramps are possible. Heat stroke is possible with prolonged exposure and/or physical activity.
The heat index is a measure of how hot it actually feels when humidity is factored in. Unlike simple temperature, which only measures air warmth, the heat index reflects the difficulty your body has in cooling itself through perspiration. On a humid day, sweat cannot evaporate efficiently, so your core body temperature rises faster and heat-related illness becomes more likely at lower air temperatures. The heat index is only meaningful at temperatures of 80 degrees Fahrenheit (26.7 degrees Celsius) or above, where humidity begins to significantly affect perceived temperature.
The calculator applies the Rothfusz regression formula, which blends temperature (in degrees F) and relative humidity (as a percentage) to produce a heat index value. Humidity traps body heat by blocking evaporative cooling; as humidity rises, the heat index climbs faster than the raw temperature. The formula accounts for the non-linear relationship between these two variables, ensuring accuracy across the full range of practical conditions. If temperature is below 80 degrees F, the heat index equals the air temperature because humidity has negligible effect at cooler conditions.
HI = a1 + a2*T + a3*RH + a4*T*RH + a5*T^2 + a6*RH^2 + a7*T^2*RH + a8*T*RH^2 + a9*T^2*RH^2The Rothfusz regression uses polynomial coefficients (a1 through a9) fitted to empirical heat-stress data. T is temperature in degrees F and RH is relative humidity as a percentage. This polynomial approximation captures how humidity's effect on heat stress accelerates as both temperature and humidity increase. The formula is valid for temperatures greater than or equal to 80 degrees F and is the same formula used by the National Weather Service for official heat index forecasts.
| Input | Result | Notes |
|---|---|---|
| Temperature: 95 degrees F, Humidity: 50% | Heat Index: 107 degrees F | At 95 degrees F with moderate humidity, the air feels 12 degrees F hotter than the thermometer. Outdoor activity should be limited; take frequent breaks in shade. |
| Temperature: 90 degrees F, Humidity: 80% | Heat Index: 106 degrees F | High humidity at a lower base temperature creates a high heat index. Sweat cannot evaporate efficiently; risk of heat exhaustion rises sharply. |
| Temperature: 85 degrees F, Humidity: 90% | Heat Index: 102 degrees F | Even at a moderate 85 degrees F, extreme humidity (90%) makes it feel like 102 degrees F. Outdoor exertion should be minimised; drink water frequently and monitor for symptoms. |
Humidity blocks evaporative cooling, your body's primary heat-dissipation mechanism. Sweat must evaporate to cool you; in humid air, it just drips off unused, trapping heat inside. In the desert at 95 degrees F with low humidity, sweat evaporates rapidly and cools you efficiently, so the heat feels more tolerable.
Relative humidity (RH) is the percentage of moisture in the air compared to the maximum the air can hold at that temperature. It's reported in most weather apps and forecasts. A reading of 80% RH means the air holds 80% of the moisture it could hold at that temperature.
No. Heat index accounts for how humidity reduces cooling; wind chill (used in winter) accounts for how wind accelerates cooling. They are opposite effects. Wind chill makes cold feel colder; high humidity makes heat feel hotter.
The relationship between temperature, humidity, and perceived heat is non-linear. Simple averaging or addition does not accurately predict heat stress. The Rothfusz polynomial was fitted to empirical human heat-stress data and produces accurate results across the full range of practical conditions.
Seek air conditioning or shade immediately. Limit exertion to essential tasks only. Drink water constantly, even if not thirsty. Wear loose, light-coloured clothing. Check on elderly neighbours, children, and pets. Heat stroke can develop in minutes; if someone shows confusion, fainting, or cessation of sweating, call emergency services.
Not usually. Heat index applies to outdoor or uncontrolled environments. Indoors with air conditioning, the air temperature is the relevant measure. However, in poorly ventilated or non-air-conditioned indoor spaces (warehouses, cars, tents), high outdoor heat index can lead to dangerous indoor conditions.