Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and relative humidity.
A heat index calculator estimates the perceived temperature based on air temperature and relative humidity, indicating heat stress risk.
Examples
Hot humid day
Moderate heat
Frequently Asked Questions
Why does humidity make heat feel worse?
What is the difference between heat index and wet-bulb temperature?
Quick Tips
- •The heat index is calculated for shady conditions. Direct sun can add up to 15°F.
- •Stay hydrated and take breaks when the heat index exceeds 91°F.
- •Danger level heat index (103°F+) means heat stroke is possible — limit outdoor activity.
- •Humidity matters because high moisture prevents sweat from evaporating efficiently.
A heat index calculator estimates the perceived temperature based on air temperature and relative humidity, indicating heat stress risk.
How to Use This Calculator
Select your temperature unit, enter the air temperature and relative humidity. The calculator uses the NWS Rothfusz regression equation to compute the heat index and danger level.
Understanding the Formula
The NWS heat index uses a multiple regression equation: HI = -42.379 + 2.049T + 10.143RH - 0.225TRH - 0.00684T² - 0.0548RH² + 0.00123T²RH + 0.000853TRH² - 0.00000199T²RH², with adjustments for extreme conditions.
Examples
Hot humid day
Temperature: 95°F, Humidity: 70%. Heat Index = 124°F. Danger level: Danger. Heat cramps and exhaustion likely.
Moderate heat
Temperature: 85°F, Humidity: 50%. Heat Index = 86°F. Caution level. Stay hydrated during prolonged activity.
Frequently Asked Questions
Why does humidity make heat feel worse?
Your body cools itself by evaporating sweat. When humidity is high, sweat evaporates more slowly, reducing your body's cooling efficiency. This makes the effective temperature feel higher than the actual air temperature.
What is the difference between heat index and wet-bulb temperature?
Heat index estimates how hot it "feels" to people and is derived from temperature and humidity. Wet-bulb temperature is a physical measurement using a thermometer wrapped in a wet cloth. Both indicate heat stress, but wet-bulb is used more in industrial and military settings.
Assumptions & Limitations
- Uses the Rothfusz regression equation, valid for temperatures above 80°F (27°C) and humidity above 40%.
- Assumes shaded conditions — direct sunlight can add up to 15°F to the heat index.
- Does not account for wind speed, which can reduce perceived heat.