Heat Index Calculator
Calculate how hot it really feels based on temperature and humidity
Enter the air temperature and relative humidity percentage.
How to Use This Heat Index Calculator
- Choose your input method: Relative Humidity percentage or Dew Point temperature
- Enter the current air temperature
- Select your temperature unit (Fahrenheit or Celsius)
- For humidity method: enter relative humidity as a percentage (0-100%)
- For dew point method: enter the dew point in the same unit as temperature
- Click 'Calculate Heat Index' to see the feels-like temperature and safety level
Example: At 90°F with 70% relative humidity, the heat index is 106°F - that's 16 degrees hotter than the actual temperature. This falls in the 'Danger' category where heat exhaustion is likely with prolonged exposure.
Tip: The heat index formula is most accurate when temperature is above 80°F and humidity is above 40%.
Why Use a Heat Index Calculator?
The heat index tells you how hot it actually feels to your body, helping you make informed decisions about outdoor activities and safety precautions.
- Planning outdoor exercise or sports activities
- Determining safe working conditions for outdoor laborers
- Deciding whether to take children or elderly outdoors
- Scheduling yard work and gardening tasks
- Planning outdoor events and festivals
- Monitoring conditions for pets and livestock
Understanding Your Results
The National Weather Service uses these heat index categories to communicate danger levels to the public.
| Result | Meaning | Action |
|---|---|---|
| 80-90°F Heat Index | Caution | Fatigue possible with prolonged exposure. Stay hydrated during outdoor activity. |
| 90-103°F Heat Index | Extreme Caution | Heat cramps and exhaustion possible. Take frequent breaks in shade or AC. |
| 103-124°F Heat Index | Danger | Heat cramps/exhaustion likely, heat stroke possible. Limit outdoor exposure. |
| 125°F+ Heat Index | Extreme Danger | Heat stroke highly likely. Avoid outdoor activity. Seek air conditioning. |
Meaning: Caution
Action: Fatigue possible with prolonged exposure. Stay hydrated during outdoor activity.
Meaning: Extreme Caution
Action: Heat cramps and exhaustion possible. Take frequent breaks in shade or AC.
Meaning: Danger
Action: Heat cramps/exhaustion likely, heat stroke possible. Limit outdoor exposure.
Meaning: Extreme Danger
Action: Heat stroke highly likely. Avoid outdoor activity. Seek air conditioning.
Note: Direct sunlight can increase the heat index by up to 15°F. These values assume shaded conditions.
About Heat Index Calculator
Formula
HI = -42.379 + 2.049T + 10.143RH - 0.225TxRH - 0.00684T² - 0.0548RH² + 0.00123T²xRH + 0.000853TxRH² - 0.00000199T²xRH² T is temperature in Fahrenheit, RH is relative humidity percentage. This regression equation approximates the physiological effect of humidity on perceived temperature.
Current Standards: The heat index formula was developed by Robert Steadman in 1979 and adopted by the National Weather Service. It assumes a person of average height and weight, walking at 3.1 mph in shade with light clothing.
Frequently Asked Questions
Why does humidity make heat feel worse?
Your body cools itself primarily through sweating - as sweat evaporates, it carries heat away from your skin. When humidity is high, the air already contains a lot of moisture, so your sweat can't evaporate as quickly. At 100% humidity, sweat doesn't evaporate at all, eliminating your body's most effective cooling mechanism.
What temperature does the heat index start to matter?
The heat index formula is designed for temperatures at or above 80°F (27°C) with relative humidity of at least 40%. Below these thresholds, the heat index approximately equals the actual air temperature. In hot, dry conditions (low humidity), the heat index can actually be lower than the air temperature.
How accurate is the heat index for outdoor work?
The official heat index assumes you're in the shade. Direct sunlight can make it feel 10-15°F hotter. For outdoor workers, OSHA recommends using the wet bulb globe temperature (WBGT) which accounts for sun exposure, wind, and radiant heat from hot surfaces.
Who is most at risk during high heat index conditions?
Elderly adults (reduced sweating ability), infants and young children (higher metabolism, can't regulate temperature well), people with chronic conditions (heart disease, obesity, diabetes), outdoor workers and athletes, and those taking certain medications (diuretics, beta-blockers, antihistamines) face elevated risk.
What's the difference between heat index and dew point?
Dew point measures the actual moisture content in the air - it's the temperature at which air becomes saturated and dew forms. Heat index combines temperature and moisture to show how hot it feels. A dew point above 70°F feels oppressively humid to most people, regardless of temperature.