Heat Index Calculator - Calculate Apparent Temperature

Heat Index Calculator

The Heat Index Calculator determines how hot it feels outside based on air temperature and relative humidity. The heat index represents the perceived temperature that the human body feels when heat and humidity are combined.

Weather Conditions

Temperature Unit

Fahrenheit (°F)Celsius (°C)

Air Temperature (°F)

Humidity Input Method

Relative Humidity (%)Dew Point

Relative Humidity (%)

0%50%100%

Quick Examples

Heat Index Results

Heat Index Temperature

89.3°F

Feels Like Difference

4.3° hotter

Relative Humidity

60.0%

Comfort Level

Slightly Uncomfortable

ADVISORY

Fatigue possible with prolonged exposure and physical activity.

Heat Index Chart (°F)

Temp	40%	50%	60%	70%	80%	90%
80°F	80	81	82	83	84	86
85°F	84	86	89	93	97	102
90°F	91	95	100	106	113	122
95°F	99	105	113	123	134	147
100°F	109	118	129	143	158	176
105°F	121	134	149	166	187	209
110°F	136	152	171	194	219	247

Values show heat index in °F for different temperature and humidity combinations

How to Use the Heat Index Calculator

Step-by-Step Instructions

1. Select your preferred temperature unit (Fahrenheit or Celsius)
2. Enter the actual air temperature
3. Choose humidity input method (relative humidity or dew point)
4. Enter the humidity value or dew point temperature
5. View the calculated heat index and safety recommendations

Quick Tips

• Heat index is most accurate for temperatures above 80°F (27°C)
• Higher humidity makes temperatures feel hotter
• Use safety warnings to plan outdoor activities
• The chart shows heat index values for common conditions

Understanding Heat Index

What is Heat Index?

Heat index is the perceived temperature that the human body feels when air temperature is combined with relative humidity. It represents how hot it actually feels outside when humidity is factored in.

How It Works

High humidity reduces the body's ability to cool itself through sweating. When sweat doesn't evaporate efficiently, the body retains more heat, making the air feel hotter than the actual temperature.

Health Implications

Extreme heat index values can lead to heat exhaustion, heat stroke, and other heat-related illnesses. It's important to take precautions when the heat index is high.

Heat Index Formula

National Weather Service Formula

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783×10⁻³T² - 5.481717×10⁻²R² + 1.22874×10⁻³T²R + 8.5282×10⁻⁴TR² - 1.99×10⁻⁶T²R²

Where: HI = Heat Index (°F), T = Air Temperature (°F), R = Relative Humidity (%)

Simplified Formula (for quick estimates)

HI = 0.5 × (T + 61.0 + ((T-68.0) × 1.2) + (R × 0.094))

This simplified version is used when the full formula result is close to the air temperature.

Applications

Weather Forecasting

Meteorologists use heat index to provide more accurate "feels like" temperatures in summer weather reports.

Sports and Recreation

Athletes and outdoor enthusiasts use heat index to plan training schedules and hydration strategies.

Workplace Safety

Construction and outdoor workers use heat index to determine safe working conditions and break schedules.

Public Health

Health officials use heat index data to issue heat warnings and plan cooling center operations.

Example Calculations

Example 1: Hot Summer Day

Air Temperature: 90°F (32.2°C)

Relative Humidity: 60%

Heat Index: 100°F (37.8°C)

Feels Like: 10° hotter than actual temperature

Example 2: Extreme Heat

Air Temperature: 95°F (35°C)

Relative Humidity: 80%

Heat Index: 118°F (47.8°C)

Feels Like: 23° hotter than actual temperature

Frequently Asked Questions

At what temperature is heat index most accurate?

Heat index is most accurate for air temperatures above 80°F (27°C) and relative humidity above 40%. Below these values, the heat index may be less than the actual temperature.

How does wind affect heat index?

The standard heat index calculation assumes light wind conditions (less than 10 mph). Strong winds can help cool the body and reduce the perceived temperature, while very light winds may make it feel even hotter.

What heat index is dangerous?

Heat index values above 105°F (40.6°C) are considered dangerous and can lead to heat exhaustion. Values above 130°F (54.4°C) are extremely dangerous and can cause heat stroke.