What is the heat index?
The heat index estimates how hot conditions feel to the human body when humidity slows the evaporation of sweat. It combines shaded air temperature with relative humidity into one apparent-temperature value.
How the calculation works
This calculator follows the National Weather Service procedure: it first evaluates the simpler Steadman approximation, then uses the Rothfusz regression when conditions warrant a heat index near or above 80°F. The official low-humidity and high-humidity adjustments are included.
Heat index risk levels
- 80–90°F / 27–32°C: caution with prolonged activity.
- 90–103°F / 32–39°C: extreme caution.
- 103–124°F / 39–51°C: danger.
- 125°F / 52°C or higher: extreme danger.
These bands are guidance, not a diagnosis. Age, medication, hydration, clothing, activity and sun exposure can change individual risk. Follow local heat alerts and move to a cooler place if you feel unwell.
Frequently asked questions
Why is heat index different from temperature?
High humidity makes sweat evaporate less efficiently, so the body has more difficulty cooling itself and conditions can feel hotter.
Does heat index include direct sunlight?
No. The standard value assumes shade and light wind. Direct sunshine can increase apparent heat substantially.
When is the heat index formula valid?
The full Rothfusz regression is intended for hot, humid conditions. For milder conditions this calculator uses the simpler National Weather Service method.
Method and sources
Last reviewed: July 11, 2026.
