The heat index, often called the “feels-like” temperature or apparent temperature, is a crucial weather measurement that combines air temperature and relative humidity to determine how hot it really feels to the human body. Our bodies cool down by sweating, which evaporates and lowers our skin temperature. However, when the humidity and temperature are high, the evaporation process slows down, making it feel significantly hotter than the actual air temperature. This calculator helps you understand that real-feel temperature for better weather safety.

How to Calculate the Heat Index

The heat index formula used by this calculator is the U.S. National Weather Service’s standard, the Rothfusz regression equation. This complex, multi-variable equation provides an accurate real-feel temperature by mapping the combined effects of heat and humidity.

This calculator uses the official NWS methodology, which first calculates a simple heat index. If that value is 80°F or higher, it proceeds with the more complex regression formula to provide the most accurate reading for high-heat conditions.

Heat Safety Tips

Knowing the heat index is the first step. The next is taking action to prevent heat-related illnesses like heat exhaustion and heatstroke. Here are some key heat safety tips:

Stay Hydrated: Drink plenty of water throughout the day, even before you feel thirsty. Avoid sugary drinks and alcohol.
Limit Outdoor Activity: Reduce strenuous activities, especially during the hottest part of the day (usually 10 a.m. to 4 p.m.).
Wear Appropriate Clothing: Choose lightweight, light-colored, and loose-fitting clothing.
Stay in Cool Places: Spend as much time as possible in air-conditioned environments.
Never Leave People or Pets in Cars: Temperatures inside a parked car can become lethal in minutes.

Heat Index Chart

The chart below shows the heat index (apparent temperature) for a given air temperature and relative humidity. Values are in Fahrenheit (°F).

Temp (°F) → RH (%) ↓	80	84	88	92	96	100
40	80	84	89	95	103	112
50	81	86	93	101	110	123
60	82	88	97	107	119	132
70	84	92	102	114	129	144

Caution: 80°F to 90°F

Extreme Caution: 90°F to 103°F

Danger: 103°F to 124°F

Extreme Danger: 125°F or higher

Frequently Asked Questions

High humidity slows down the evaporation of sweat from your skin. Since evaporation is the body’s primary way of cooling down, this process becomes less effective in humid conditions. As a result, you feel hotter than the actual air temperature because your body retains more heat.

The heat index describes how hot it feels during warm weather by combining temperature and humidity. Conversely, wind chill describes how cold it feels during cold weather by combining temperature and wind speed. Wind chill quantifies the increased rate of heat loss from the body due to wind.

Symptoms of heat exhaustion include heavy sweating, cold or clammy skin, a fast but weak pulse, nausea, muscle cramps, tiredness, dizziness, and headache. If you experience these, move to a cool place, loosen your clothes, and sip water. Seek medical help if symptoms worsen or don’t improve within an hour.

Temp (°F) → RH (%) ↓	80	84	88	92	96	100
40	80	84	89	95	103	112
50	81	86	93	101	110	123
60	82	88	97	107	119	132
70	84	92	102	114	129	144

Temp (°F) → RH (%) ↓	80	84	88	92	96	100
40	80	84	89	95	103	112
50	81	86	93	101	110	123
60	82	88	97	107	119	132
70	84	92	102	114	129	144

Temp (°F) → RH (%) ↓	80	84	88	92	96	100
40	80	84	89	95	103	112
50	81	86	93	101	110	123
60	82	88	97	107	119	132
70	84	92	102	114	129	144