Heat Index Calculator

What Is the Heat Index?

The heat index (also called the apparent temperature or "feels like" temperature) combines air temperature and relative humidity to describe how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, reducing the body's ability to cool itself. A temperature of 32 C (90 F) at 70% humidity produces a heat index of 41 C (106 F), meaning the heat stress on your body is equivalent to 41 C in dry air. Enter temperature and humidity in the calculator above to find the heat index instantly.

How Is the Heat Index Calculated?

The National Weather Service uses the Rothfusz regression equation, a polynomial formula with nine terms involving temperature and relative humidity. The simplified version: HI = -42.379 + 2.049T + 10.143R - 0.225TR - 0.007T squared - 0.055R squared + 0.001T squared R + 0.001TR squared - 0.000002T squared R squared, where T is temperature in Fahrenheit and R is relative humidity percentage. Adjustments apply for very low humidity (below 13%) or very high humidity (above 85%) at certain temperature ranges. The calculator implements the full NWS algorithm with all corrections for accurate results.

Heat Index Danger Levels

Caution (27-32 C / 80-90 F): Fatigue possible with prolonged exposure and physical activity. Extreme Caution (32-39 C / 90-103 F): Sunstroke, muscle cramps, and heat exhaustion possible. Danger (39-51 C / 103-124 F): Sunstroke, cramps, and heat exhaustion likely. Heat stroke possible with continued activity. Extreme Danger (above 51 C / 125 F): Heat stroke highly likely. The NWS issues heat advisories when the heat index is expected to reach 40 C (105 F) for two or more hours, and excessive heat warnings for heat indices reaching 41 C (106 F) or higher.

How Does Humidity Make Heat Dangerous?

The human body cools itself primarily through sweat evaporation. In dry air, sweat evaporates quickly and efficiently removes heat from the skin. In humid air, the moisture gradient between your skin and the surrounding air is smaller, slowing evaporation dramatically. At 100% humidity, sweat cannot evaporate at all, eliminating the body's primary cooling mechanism. This is why 35 C at 80% humidity (heat index 46 C) is far more dangerous than 40 C at 20% humidity (heat index 36 C). The dry heat allows your body to cool effectively, while the humid heat traps thermal energy against your skin.

Heat Index vs Wind Chill

Heat index measures how hot it feels by adding the effect of humidity to temperature. Wind chill measures how cold it feels by adding the effect of wind to temperature. Both describe the apparent temperature experienced by the human body rather than the actual air temperature measured by a thermometer. Heat index applies when temperature is above 27 C (80 F) and humidity is above 40%. Wind chill applies when temperature is below 10 C (50 F) and wind speed is above 5 km/h. Between these ranges, the actual air temperature closely matches how it feels to exposed skin.

Protecting Yourself in High Heat Index Conditions

When the heat index exceeds 32 C (90 F): drink water before feeling thirsty (at least 250 ml every 20 minutes during activity). Wear lightweight, light-colored, loose-fitting clothing. Take frequent breaks in shade or air conditioning. Avoid peak sun hours (10 AM to 4 PM). Never leave children or pets in parked cars where interior temperature can reach 70 C (160 F) in minutes. Monitor for heat illness symptoms: heavy sweating suddenly stopping, confusion, rapid pulse, nausea, and loss of consciousness. Heat stroke is a medical emergency requiring immediate cooling and emergency services.

Heat Index for Athletes and Workers

OSHA recommends modified work schedules when the heat index exceeds 32 C: more frequent rest breaks, access to water and shade, and a buddy system to watch for heat illness. Youth sports organizations cancel or modify practices above certain heat index thresholds (typically 35-40 C depending on the sport). Marathon organizers monitor heat index for runner safety, with many events canceled or converted to unofficial timing above 28 C wet bulb globe temperature. Military training uses heat categories (green, yellow, red, black flag conditions) based on wet bulb globe temperature, which is related to but more comprehensive than the standard heat index.

Frequently asked questions