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Heat Index Calculator

Calculate the apparent "feels-like" temperature felt by the body when combining relative humidity with air temperature.

Calculate Heat Index

Generally defines felt heat starting at 80°F (26.7°C).
Percent between 0 and 100.

Result

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Enter temperature and relative humidity, then click Calculate.

Understanding the Heat Index

The heat index (apparent temperature) is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. High relative humidity prevents the body's natural cooling mechanism (evaporation of sweat) from working efficiently, making the air feel hotter than it actually is.

Rothfusz Regression Equation

The National Weather Service (NWS) calculates the heat index using a multi-parameter regression equation representing human physiological heat exchange:

HI = −42.379 + 2.04901523×T + 10.14333127×R − 0.22475541×T×R − 0.00683783×T2 − 0.05481717×R2 + 0.00122874×T2×R + 0.00085282×T×R2 − 0.00000199×T2×R2

Where T is air temperature in °F, and R is relative humidity in %.

NWS Advisory Classifications

  • Caution (80°F to 90°F / 27°C to 32°C): Fatigue is possible with prolonged exposure and activity.
  • Extreme Caution (90°F to 103°F / 32°C to 39°C): Heat stroke, heat cramps, and heat exhaustion are possible.
  • Danger (103°F to 124°F / 39°C to 51°C): Heat cramps or heat exhaustion are likely; heat stroke is possible.
  • Extreme Danger (125°F / 52°C and above): Heat stroke is highly imminent.

FAQ

Why does humidity make the air feel warmer?

Your body cools itself by evaporating moisture from the skin. When the air is already saturated with water vapor (high humidity), sweat cannot evaporate as quickly. As a result, your body retains more heat.

Does wind reduce the heat index?

Yes, wind speed can improve sweat evaporation rates, but the standard NWS Heat Index assumes a light wind speed (approx 5.7 mph or 9.2 km/h). Under extreme heat and calm winds, apparent temperature can be even higher.

Disclaimer. RapidRatio is informational only. Apparent temperature indices are estimates based on standard shade environments. Direct exposure to full sunlight can increase heat index values by up to 15°F (8°C). Consult local weather advisories during heat waves.