Calculate Heat Index (Feels Like Temperature)
Use this calculator to estimate how hot it feels when temperature and humidity are combined. This is based on the NOAA/NWS heat index method.
What is the heat index?
The heat index is often called the “feels like” temperature. It combines actual air temperature with relative humidity to estimate how hot conditions feel to the human body. When humidity is high, sweat evaporates more slowly. Since evaporation is one of your body’s primary cooling mechanisms, reduced evaporation can make warm weather feel much more dangerous.
For example, an air temperature of 95°F can feel like well above 100°F when humidity is elevated. That extra thermal stress increases dehydration risk, fatigue, and the chance of heat-related illness.
How this heat index calculator works
This page uses a standard weather-service approach based on the NOAA/NWS formula (Rothfusz regression) with common low-range handling and adjustment factors. In plain English, the calculator does the following:
- Takes your temperature and humidity inputs.
- Converts Celsius to Fahrenheit when needed.
- Applies heat index equations used in meteorology.
- Adjusts for specific humidity/temperature edge cases.
- Optionally adds a direct-sun estimate (+15°F).
- Returns a safety category and practical guidance.
Important accuracy note
The official heat index model is most reliable when air temperature is at least 80°F and relative humidity is at least 40%. Outside that range, the output is still useful for orientation, but should be treated as an approximation rather than a strict operational value.
Heat index categories and risk levels
Public weather agencies commonly use risk bands to communicate heat danger. The exact language may vary by region, but these categories are widely recognized:
- Below 80°F: Lower risk for most healthy people, but caution is still warranted during prolonged outdoor work.
- 80–90°F (Caution): Fatigue possible with prolonged exposure or activity.
- 90–103°F (Extreme Caution): Heat cramps and heat exhaustion become more likely.
- 103–124°F (Danger): Heat cramps/exhaustion likely; heat stroke becomes possible.
- 125°F and above (Extreme Danger): Heat stroke is highly likely with continued exposure.
Why humidity makes heat feel worse
Your body cools itself by sweating. Sweat has to evaporate to remove heat effectively. When the air is already moist, evaporation slows down dramatically. The result is higher skin temperature, rising internal stress, and a greater chance of overheating, especially during exercise or hard labor.
Factors not fully captured by a simple calculator
- Wind speed and airflow
- Direct solar radiation and surface reflection
- Clothing type and protective gear
- Hydration status and physical condition
- Age, medications, and chronic health conditions
Practical heat safety tips
- Drink fluids regularly; do not wait until thirsty.
- Take frequent shade or indoor cooling breaks.
- Schedule strenuous tasks for morning/evening hours.
- Wear lightweight, breathable, light-colored clothing.
- Never leave children or pets in parked vehicles.
- Use a buddy system for outdoor work in high heat.
- Watch for symptoms: dizziness, nausea, confusion, rapid pulse, headache.
When to seek urgent help
If someone has a very high body temperature, confusion, fainting, altered behavior, or stops sweating in extreme heat, call emergency services immediately. Move the person to a cooler place and begin cooling while waiting for professional care.
FAQ
Is heat index the same as “feels like” temperature in weather apps?
Not always. Some apps display a broader “apparent temperature” that may combine wind chill, humidity, and other factors. Heat index specifically refers to hot-weather temperature plus humidity effects.
Can I use Celsius?
Yes. Enter temperature in Celsius, and the calculator will convert it, compute the heat index, then show results in both °C and °F.
Why is there a direct sunlight checkbox?
Standard heat index assumes shade. In full sun, perceived heat load can be significantly higher, so this option adds a simple +15°F approximation for a more conservative outdoor estimate.
Bottom line
A heat index calculator is a quick way to turn raw weather data into practical risk awareness. If the number moves into danger zones, reduce exertion, increase cooling breaks, hydrate aggressively, and monitor for symptoms. Heat risk can escalate faster than most people expect.