🌡️ Heat Index Calculator
Calculate how hot it really feels based on temperature and humidity
What is Heat Index?
The heat index, often called the “feels like” temperature, combines air temperature and relative humidity to determine how hot it actually feels to the human body. While your thermometer might read 90°F, the heat index could be well over 100°F when humidity is high.
Understanding heat index is crucial for staying safe during hot weather. The human body cools itself through perspiration, but when humidity is high, sweat doesn’t evaporate as easily. This makes it harder for your body to cool down, increasing the risk of heat-related illnesses.
Why Heat Index Matters
Every summer, thousands of Americans are affected by extreme heat. According to the CDC, heat-related illnesses send more than 65,000 people to emergency rooms annually. The heat index helps you understand the real danger level and take appropriate precautions.
The National Weather Service issues heat advisories and warnings based on heat index values. When the heat index reaches dangerous levels, outdoor activities become risky, and vulnerable populations like the elderly and young children are especially at risk.
Heat Index Risk Levels
Caution (80-90°F)
At this level, fatigue is possible with prolonged exposure and physical activity. Stay hydrated and take breaks in the shade when working or exercising outdoors.
Extreme Caution (90-105°F)
Heat cramps and heat exhaustion are possible with prolonged exposure or strenuous activity. Limit outdoor activities to morning or evening hours, and take frequent breaks in air-conditioned spaces.
Danger (105-130°F)
Heat cramps and heat exhaustion are likely, and heat stroke is possible. Outdoor physical activity should be rescheduled or canceled. If you must be outside, stay in the shade and drink plenty of water.
Extreme Danger (130°F+)
Heat stroke is imminent at these levels. Seek air-conditioned shelter immediately. Outdoor activities should be completely avoided, and vulnerable individuals should be checked on frequently.
Protecting Yourself from Heat-Related Illness
When the heat index is high, taking preventive measures can save your life. Here’s what you need to do:
- Stay Hydrated: Drink plenty of water throughout the day, even if you don’t feel thirsty. Avoid alcohol and caffeinated beverages, which can dehydrate you.
- Dress Appropriately: Wear lightweight, light-colored, loose-fitting clothing. A wide-brimmed hat and sunglasses provide additional protection.
- Time Your Activities: Schedule outdoor activities during cooler parts of the day—early morning or evening. Avoid the peak heat hours between 10 AM and 4 PM.
- Take Breaks: Find air-conditioned spaces to cool down regularly. Shopping malls, libraries, and community centers often serve as cooling stations during heat waves.
- Never Leave Kids or Pets in Cars: Even with windows cracked, car interiors can reach lethal temperatures in minutes.
Using This Calculator
Our heat index calculator provides instant results based on current temperature and humidity levels. It’s designed to be simple and accurate, using the same formula employed by the National Weather Service.
How to Calculate
- Select your preferred temperature unit (Fahrenheit or Celsius)
- Enter the current temperature
- Enter the current relative humidity percentage
- Click “Calculate Heat Index”
You’ll instantly see the heat index value and the associated risk level, along with recommended safety precautions for the current conditions.
Frequently Asked Questions
What’s the difference between temperature and heat index?
Temperature measures the actual air temperature, while heat index accounts for humidity to show how hot it really feels. On a humid day, 90°F can feel like 105°F or higher.
How is heat index calculated?
This calculator uses the Rothfusz regression equation adopted by the National Weather Service. It factors in the complex relationship between temperature and humidity to provide scientifically accurate results.
When should I check the heat index?
Check it before heading outdoors, planning activities, exercising, or working outside. It’s especially important during summer months and heat waves.
Can I trust this calculator?
Yes! Our calculator uses the official formula from the National Weather Service, ensuring accurate and reliable results you can depend on for safety decisions.
Stay Safe This Summer
Heat-related illnesses are preventable. By staying informed about the heat index and taking appropriate precautions, you can enjoy outdoor activities safely throughout the summer months.
Use this calculator regularly to stay ahead of dangerous heat conditions. Your health and safety are worth those few seconds it takes to check the heat index before heading out.