The heat index calculator computes the apparent "feels like" temperature using the official NOAA/NWS Rothfusz regression equation. Enter air temperature and relative humidity to get the true heat stress level and danger classification.
Temperature & Humidity
Enter temperature and humidity to calculate the heat index.
Heat Index (Feels Like)
Safety Recommendations
NWS Heat Index Reference
Feels-like temperature at common temperature/humidity combinations (°F)
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How to Use the Heat Index Calculator
The heat index calculator uses the official NOAA National Weather Service Rothfusz regression equation — the same formula used in weather forecasts and emergency alert systems. Understanding the heat index helps you make safer outdoor activity decisions when temperature and humidity combine dangerously.
The Rothfusz Formula
The main equation uses a polynomial regression with 9 coefficients to model the body's physiological response to heat and humidity. Two adjustments apply: (1) when relative humidity is below 13% and temperature is between 80-112°F, the heat index is adjusted downward slightly (dry air aids evaporation); (2) when humidity exceeds 85% and temperature is 80-87°F, the heat index is adjusted upward. Below 80°F, a simplified Steadman equation applies.
Heat Index vs. Air Temperature
The NWS issues heat advisories when the heat index exceeds 100°F for at least 2 hours. Heat warnings occur when heat index reaches 105°F or higher for 2+ hours. In Phoenix on a typical summer day (108°F, 15% humidity), the heat index is actually lower than the air temperature (~104°F) because dry air allows efficient evaporation. In Houston (95°F, 75% humidity), the heat index is 123°F — deadly conditions despite a seemingly moderate air temperature.
Outdoor Safety at High Heat Index
At heat index 90-103°F (Extreme Caution): schedule outdoor work in early morning before 9 AM; take 10-minute breaks every hour; drink 8 oz of water every 20 minutes. At 103-124°F (Danger): move outdoor activities to shaded areas only; limit exertion to 20 minutes maximum; never work alone. At 125°F+ (Extreme Danger): suspend all outdoor activity; heat stroke can develop in 10-15 minutes for anyone.
FAQ
What is the heat index?
The heat index (also called 'feels like' or 'apparent temperature') combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity prevents sweat from evaporating, which impairs the body's cooling mechanism. The NWS considers heat index above 103°F (39°C) dangerous and above 125°F (52°C) extremely dangerous.
What formula does this calculator use?
This calculator uses the official NOAA/NWS Rothfusz regression equation, which is the industry standard for heat index calculation. It applies the main polynomial regression with two adjustments: one for low humidity (below 13%) in the 80-112°F range, and one for high humidity (above 85%) in the 80-87°F range. This matches the formula used in official NWS weather products.
At what heat index should I be concerned?
NWS guidelines: Caution (80-90°F / 27-32°C) — fatigue possible with prolonged exposure; Extreme Caution (90-103°F / 32-39°C) — sunstroke, heat cramps, heat exhaustion possible; Danger (103-124°F / 39-51°C) — heat cramps/exhaustion likely, heatstroke possible; Extreme Danger (125°F+ / 52°C+) — heatstroke highly likely with continued exposure.
Why does humidity make it feel hotter?
Your body cools itself primarily through sweat evaporation. When humidity is high (above 70%), the air is already saturated with moisture, so sweat can't evaporate as efficiently. This means your body retains more heat. At 95°F with 90% humidity, the heat index reaches 130°F — that's the same thermal stress as 130°F dry air on your skin.
Is this heat index calculator free?
Yes, completely free. All calculations run locally in your browser using the official NWS Rothfusz formula. No signup required.