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Heat Index Calculator

Calculate how hot it feels when humidity is combined with air temperature. Get heat illness risk assessments and safety recommendations based on National Weather Service formulas.

Heat Index Calculator

Air Temperature

Relative Humidity

Relative humidity (0-100%)

Conditions Summary

Temperature: 85° F

Humidity: 60%

Dew Point: 69.5° F

Heat Index Results

89.3°F

Heat Index ("Feels Like")

(31.8°C • 305K)

Feels 4.3°F hotter than air temperature
Risk Level: CAUTION

Fatigue is possible with prolonged exposure and/or physical activity

Activity Recommendation: Normal activities with precautions

Heat Index in Different Units

89.3°
Fahrenheit
31.8°
Celsius
305
Kelvin
Health & Safety Recommendations
  • 🔴Stay hydrated - drink plenty of water
  • 🔴Wear lightweight, light-colored, loose-fitting clothing
Heat Index Comparisons

Similar to a warm summer day



What is Heat Index Calculator?

The heat index measures how hot it feels when relative humidity is factored with actual air temperature. High humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This calculator uses the official formula developed by the National Weather Service.

How Heat Index Works

Your body cools itself through evaporation of sweat. When humidity is high, the evaporation rate decreases, making it harder for your body to cool down. The heat index quantifies this effect, showing what the temperature "feels like" to the human body.

NWS Heat Index Categories

Caution (80-90°F)

  • • Fatigue possible with prolonged exposure
  • • Continue outdoor activities with care
  • • Stay hydrated

Extreme Danger (130°F+)

  • • Heat stroke is imminent
  • • Avoid all outdoor activities
  • • Seek immediate air conditioning

Heat-Related Illness Prevention

Heat Exhaustion Symptoms:

  • Heavy sweating, weakness, nausea
  • Headache, dizziness, muscle cramps
  • Cool, moist, pale skin

Heat Stroke Symptoms (Emergency!):

  • High body temperature (104°F+)
  • Hot, red, dry skin
  • Rapid pulse, confusion, loss of consciousness

Call 911 immediately if heat stroke is suspected!

Practical Applications

  • Weather forecasting and public health warnings
  • Outdoor work safety and scheduling
  • Sports and recreation activity planning
  • Agricultural and construction safety
  • HVAC system design and operation
  • Emergency management and heat wave response
  • Personal health and safety planning
  • Energy consumption forecasting


FAQ - Heat Index Calculator

The heat index formula is designed for hot weather conditions where humidity significantly affects comfort and safety. Below 80°F (26.7°C), the combination of temperature and humidity doesn't pose the same health risks that the heat index is meant to quantify.