About this Heat Index
The Heat Index (HI), sometimes called the 'apparent temperature', measures how hot it really feels when relative humidity is factored in with the actual air temperature. High humidity prevents sweat from evaporating, reducing the body's ability to cool itself.
Who this calculator is for
- People working or exercising outdoors in hot weather.
- Parents, coaches, and event planners watching for heat stress risk.
- Anyone comparing how much worse humidity makes a hot day feel.
When to use it
- Check the apparent temperature before outdoor activity.
- Estimate heat stress when humidity is high.
- Decide whether additional cooling breaks or shade are needed.
How to use this calculator
- Enter the Air Temperature.
- Enter the Relative Humidity percentage.
- The result shows the Heat Index and the associated Danger Level.
Key assumptions
- The formula is most reliable at 80°F or higher and humidity of 40% or more.
- Shade is assumed; direct sunlight can make conditions feel hotter.
- This is a weather safety estimate, not a medical diagnosis.
Accuracy and testing
Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.
How this calculator works
The calculator uses the Rothfusz regression equation used by the National Weather Service (NWS). This complex formula approximates the heat index for temperatures of 80°F or higher and humidity of 40% or more.
Inputs explained
Air Temperature
Must be 80°F or higher for the Heat Index formula to apply reliably.
Relative Humidity
The amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature.
Understanding your results
The result is categorized by risk:
* Caution (80-90°F): Fatigue possible.
* Extreme Caution (90-103°F): Heat cramps/exhaustion possible.
* Danger (103-124°F): Heat cramps/exhaustion likely. Heat stroke possible.
* Extreme Danger (125°F+): Heat stroke highly likely.
Example scenarios
Dry Heat
90°F, 20% Humidity. Heat Index: 86°F. (Feels cooler because sweat evaporates fast).
Swampy
90°F, 80% Humidity. Heat Index: 113°F. (Dangerous conditions).
Key Features
- NWS Rothfusz Formula
- Safety Warnings
- Humidity Adjustment
Tips & Best Practices
- Heat Index values assume shady conditions. Direct sunlight can increase the values by up to 15°F.
- Drink plenty of water and stay in air-conditioned areas when the index hits the 'Danger' zone.
Glossary of Terms
- Relative Humidity
- A percentage indicating how saturated the air is with water vapor.
- Heat Stroke
- A life-threatening condition where the body's temperature rises above 104°F.