Heat Index Calculator

The heat index was developed in 1978 by George Winterling and refined by the U.S. National Weather Service. It estimates how hot the human body feels by combining air temperature and relative humidity. The formula assumes a person is in shade with a light wind — full sun can increase the feels-like temperature by up to 15°F (8°C).

High humidity is dangerous because it reduces the rate at which sweat evaporates from your skin. Evaporation is your body's primary cooling mechanism; when it is impaired, your core temperature rises. At a heat index of 103°F (39°C), the body struggles to maintain a safe temperature, and heat-related illness becomes likely with prolonged exposure.

The heat index is most relevant in humid climates. In dry climates like the Southwest United States, actual temperatures may exceed 110°F (43°C) but feel closer to the actual temperature because low humidity allows sweat to evaporate efficiently. Use our Dew Point Calculator to understand how moisture content affects comfort at any temperature.

• Stay hydrated: Drink water before you feel thirsty. Avoid alcohol and excessive caffeine, which increase dehydration.
• Limit outdoor activity: Schedule exercise and strenuous work for early morning or evening when temperatures are lower.
• Never leave anyone in a parked car: Interior temperatures can rise 20°F (11°C) in 10 minutes, even with windows cracked.
• Know the warning signs: Heavy sweating, weakness, dizziness, nausea, and muscle cramps signal heat exhaustion. Confusion, fainting, and hot, dry skin signal heat stroke — call 911 immediately.
• Check on vulnerable people: Infants, elderly adults, outdoor workers, and people with chronic conditions are at highest risk.

The OSHA Heat Illness Prevention program requires employers to provide water, rest, and shade for outdoor workers when the heat index reaches specific thresholds. Knowing your local heat index helps you plan safe outdoor activities and recognize when conditions become dangerous. When temperatures drop, check our Wind Chill Calculator for cold-weather safety.