Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and relative humidity.
Result
Fill in the fields above, then press Calculate.
A heat index calculator estimates the perceived temperature based on air temperature and relative humidity, indicating heat stress risk.
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.
Result
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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.