Heat Index Calculator

Gauge the real-feel temperature on sweltering days. The heat index merges air temperature and humidity to estimate how hot conditions feel to the human body, helping you plan hydration, rest breaks, and safety precautions.

Enter degrees Fahrenheit. Convert from °C by multiplying °C × 1.8 + 32.
Measure ambient humidity as a percentage between 0 and 100.

Values are estimates and do not replace on-site safety monitoring. Watch for heat illness symptoms and follow local guidelines.

Examples

  • 90°F with 70% humidity ⇒ Feels like 106°F, indicating dangerous heat stress potential
  • 84°F with 40% humidity ⇒ Feels like 84°F, so sweat can still evaporate efficiently
  • 98°F with 55% humidity ⇒ Feels like 120°F, a level requiring frequent rest and cooling

FAQ

How should I interpret different heat index values?

80–90°F signals caution, 90–103°F is extreme caution, 103–124°F is danger, and above 125°F is extreme danger for heat-related illness.

Is the calculation valid for every climate?

It works best for warm-weather conditions with temperatures above 80°F and humidity above 40%. Outside those ranges, results become less reliable.

Does wind or shade change the perceived temperature?

Yes. The heat index assumes shady, light-wind conditions. Direct sun or still air can make it feel far hotter than the calculated value.

Additional Information

  • The National Weather Service formula is most accurate for temperatures above 80°F and relative humidity over 40%.
  • Direct sunlight can add up to 15°F to the perceived temperature compared with the shaded conditions assumed here.
  • High humidity limits sweat evaporation, which is why the heat index often exceeds the actual air temperature.

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