Heat Index Calculator

Calculate apparent temperature (feels-like) based on air temperature and relative humidity. Essential for outdoor safety and activity planning.

Important: Standard heat index formula is valid only for temperatures ≥ 80°F (27°C) and humidity ≥ 40%. Outside this range, results are estimates.

°F
Enter air temperature (-100 to 200°)
%
Enter relative humidity (0-100%)
Hot & Humid (85°F, 75%)
Hot & Dry (95°F, 40%)
Warm & Humid (78°F, 90%)
Extreme Heat (100°F, 55%)
Comfortable (70°F, 60%)
Extreme Case (66°C, 50%)
Calculating Heat Index...

Understanding Heat Index

The Heat Index (HI) is a measure of how hot it feels when relative humidity is factored with the actual air temperature. This is important because high humidity reduces the body's ability to cool itself through perspiration.

How it works: When the body gets too hot, it begins to perspire or sweat to cool itself. If the perspiration is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body's temperature. When the atmospheric moisture content (relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions.

Heat Index Categories & Danger Levels

Heat Index Category Health Effects Precautions
80°F - 90°F
(27°C - 32°C)		 Caution Fatigue possible with prolonged exposure and physical activity Limit strenuous outdoor activity, stay hydrated
91°F - 103°F
(33°C - 39°C)		 Extreme Caution Sunstroke, heat cramps, and heat exhaustion possible Avoid strenuous activity, take frequent breaks in shade
104°F - 124°F
(40°C - 51°C)		 Danger Sunstroke, heat cramps, and heat exhaustion likely; heat stroke possible Reschedule strenuous activity, stay in air conditioning if possible
125°F+
(52°C+)		 Extreme Danger Heat stroke highly likely with continued exposure Cancel all outdoor activity, remain in air-conditioned spaces

Heat Index Formula

The Heat Index formula used by the U.S. National Weather Service is based on work by Robert G. Steadman (1979):

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²

Where: T = Temperature in °F, R = Relative Humidity percentage

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Limitations: The Heat Index equation works only when the temperature is 80°F (27°C) or higher and humidity is 40% or more. The index is calculated for shady locations with light wind conditions. Exposure to full sunlight can increase heat index values by up to 15°F (8°C).

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Wind Chill vs. Heat Index: While wind chill measures how cold it feels when wind is factored with cold temperatures, heat index measures how hot it feels when humidity is factored with hot temperatures.

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Real Feel Temperature: The Heat Index is sometimes called the "apparent temperature" or "feels-like temperature" because it represents what the temperature feels like to the human body rather than the actual air temperature.

Groups at Higher Risk

  • Infants and young children: Their bodies are less efficient at regulating temperature
  • Elderly adults: May have reduced ability to perspire and poor blood circulation
  • People with chronic medical conditions: Especially heart disease, high blood pressure, respiratory problems
  • Outdoor workers: Construction workers, farmers, landscapers, etc.
  • Athletes: Especially those not acclimated to hot conditions
  • People on certain medications: Diuretics, antihistamines, beta-blockers, etc.

Heat-Related Illness Warning Signs:

  • Heat Cramps: Painful muscle cramps, usually in legs or abdomen
  • Heat Exhaustion: Heavy sweating, weakness, cold/pale/clammy skin, fast/weak pulse, nausea/vomiting, fainting
  • Heat Stroke: High body temperature (103°F+), hot/red/dry skin, rapid/strong pulse, possible unconsciousness

Heat stroke is a medical emergency. Call 911 immediately if you suspect someone has heat stroke.

Frequently Asked Questions

Humidity makes it feel hotter because it reduces the effectiveness of sweating. Sweat cools the body through evaporation. When the air is already saturated with moisture (high humidity), sweat evaporates more slowly, reducing the body's ability to cool itself. This makes you feel hotter than the actual air temperature.

The Heat Index specifically accounts for humidity's effect on perceived temperature. "Real Feel" or "Feels Like" temperature may incorporate additional factors like wind speed, sun intensity, and even precipitation. Some weather services use proprietary formulas that include more variables than just humidity.

Typically, the Heat Index is higher than the actual air temperature when humidity is above 40% and temperature is above 80°F (27°C). In very dry conditions with low humidity, the perceived temperature might actually feel cooler than the thermometer reads because sweat evaporates quickly, cooling the body effectively.

The standard Heat Index calculation assumes shady conditions with light wind. In direct sunlight, the Heat Index can increase by up to 15°F (8°C). This is because solar radiation directly heats the body in addition to the air temperature and humidity effects.

The Heat Index formula is an approximation based on physiological studies. Individual factors like clothing, physical activity, wind speed, and direct sunlight can affect how hot a person actually feels. The formula is most accurate for average adults in light clothing, at rest, in shaded conditions with light wind.