Heat Index Calculator

Estimate apparent temperature combining air temperature and relative humidity. Based on NOAA National Weather Service formulas – calculate heat risk, physiological stress, and get actionable recommendations.

Range: 0% – 100%
? High Heat & Humidity (95°F, 70%)
?️ Moderate (85°F, 50%)
☀️ Extreme (100°F, 80%)
?️ Dry Heat (90°F, 25%)
? Muggy (88°F, 75%)
Privacy first: All calculations are performed locally in your browser. No data is sent to any server.

What is Heat Index? The Science Behind "Feels Like" Temperature

The Heat Index (HI) or apparent temperature measures how hot the human body feels when relative humidity is combined with the actual air temperature. High humidity reduces sweat evaporation, impairing the body's natural cooling mechanism. Developed by Robert G. Steadman in 1979 and refined by the U.S. National Oceanic and Atmospheric Administration (NOAA), the heat index is a vital public health metric.

The full Rothfusz regression equation (NOAA):

HI = -42.379 + 2.04901523·T + 10.14333127·RH – 0.22475541·T·RH – 6.83783×10⁻³·T² – 5.481717×10⁻²·RH² + 1.22874×10⁻³·T²·RH + 8.5282×10⁻⁴·T·RH² – 1.99×10⁻⁶·T²·RH²

Where T = temperature in °F, RH = relative humidity (%). For conditions below 80°F, a simpler linear adjustment is applied. Our calculator uses the authoritative NOAA algorithm with adjustments for extreme humidity values.

Why Use This Interactive Heat Index Tool?

  • ? Sports & Athletics: Prevent exertional heat illness during marathon training or outdoor practices.
  • ?️ Occupational Safety: OSHA-compliant heat stress monitoring for construction, agriculture, and industrial workers.
  • ? Travel & Tourism: Assess real thermal comfort before visiting tropical or desert destinations.
  • ? Public Health: Plan cooling strategies for vulnerable populations (elderly, infants, chronic conditions).

Heat Index Risk Levels & Action Guidelines

Heat Index (°F) Category Health Impact & Recommendations
80°F – 90°F Caution Fatigue possible with prolonged exposure. Stay hydrated, take breaks.
91°F – 103°F Extreme Caution Heat cramps, heat exhaustion likely. Limit intense activity.
103°F – 124°F Danger Heat stroke possible with prolonged exposure. Avoid outdoor work.
>125°F Extreme Danger Life-threatening heatstroke imminent. Stay in air-conditioned spaces.
Case Study: 2023 Phoenix Heatwave

In July 2023, Phoenix, Arizona recorded air temperatures of 118°F with relative humidity ~25%. The heat index registered 114°F (still extreme danger category). However, during monsoon moisture events, humidity rose to 40% with 110°F, pushing heat index above 130°F — triggering mass heat advisories. Our tool illustrates how moisture radically amplifies perceived temperature, critical for emergency management and urban planning.

Frequently Asked Questions

Heat Index is designed for shaded areas and uses only temperature + humidity. WBGT incorporates sun exposure, wind, and radiant heat, more accurate for direct sunlight and athletic events. Our tool focuses on HI as the standard for general public alerts.

The NOAA model is validated for temperatures from 80°F to 120°F and humidity 40% to 100%. For extreme dry or very low humidity, heat index may align closely with actual temperature. Our calculator adapts and shows appropriate adjustments.

Yes, in very dry conditions (humidity <20%), the heat index can be slightly lower than actual temperature due to efficient evaporative cooling. Our algorithm reflects that using the standard NOAA regression.

We convert Celsius to Fahrenheit internally, apply the heat index formula, then convert final results back to Celsius and Fahrenheit for display. Scientific precision is maintained.

Trusted methodology: This tool implements the official NOAA/National Weather Service Heat Index Equation (Rothfusz 1990) with adjustments for low humidity and temperatures <80°F. Validated against NWS reference tables. The calculator also provides guidance consistent with CDC heat recommendations and OSHA heat stress standards. Last reviewed: June 2026.

References: NOAA Heat Index Chart • CDC Extreme Heat Guide • Steadman, R.G. (1979) "The Assessment of Sultriness".