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Calculator Answers

Heat Index Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

The Heat Index (HI) or apparent temperature is a measure of how hot it feels to the human body when relative humidity is combined with the air temperature. High humidity slows the evaporation of sweat, making it harder for the body to cool itself. The Heat Index is calculated using the NOAA/NWS Rothfusz regression model and Steadman's thermoregulation equations. Weather services and safety organizations use the heat index to issue heat advisories and prevent heat-related illnesses like heat exhaustion or stroke.

Calculate apparent temperature ('feels like' temperature) from ambient air temperature and relative humidity using the standard NOAA/NWS Rothfusz regression.

See also: Temperature Converter, Water Intake Calculator, and Calories Burned Calculator.

Quick Answer

Calculate the heat index ('feels like' temperature) from ambient air temperature and relative humidity. View safety classifications and risk warnings based on National Weather Service thresholds.

Environmental inputs

Scale

%

NWS Heat Index Thresholds

  • 80°F to 90°F: Caution. Fatigue is possible.
  • 91°F to 103°F: Extreme Caution. Heat stroke or cramps possible.
  • 104°F to 129°F: Danger. Heat cramps/exhaustion likely.
  • 130°F or above: Extreme Danger. Heat stroke highly likely.
Apparent temperature

Heat Index (Feels Like)

99.7°F

≈ 37.6°C

Air Temperature90°F (32.2°C)
Relative Humidity60%
Apparent Temp (°F)99.7°F
Apparent Temp (°C)37.6°C

NOAA Risk Category: Extreme Caution

Heat stroke, heat cramps, or heat exhaustion are possible with prolonged exposure and/or physical activity. Limit strenuous activity.

The band names follow the National Weather Service heat index chart. We could not obtain the exact band boundaries as published text: they appear only inside the chart image on the NWS page, so the cutoffs used here are read from that chart rather than quoted.

* Apparent temperature estimates are for shady conditions with a light breeze. Full exposure to direct sunshine can increase heat index values by up to 15°F (8°C).

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Examples

Temp = 90 °F, Humidity = 60%

Heat Index ≈ 100 °F (Extreme Caution)

Temp = 95 °F, Humidity = 70%

Heat Index ≈ 124 °F (Danger)

Temp = 75 °F, Humidity = 80%

Apparent Temp ≈ 77 °F (Normal)

How it works

The heat index calculator uses the dry-bulb air temperature and relative humidity to estimate the thermal load on the human body. It runs a simple Steadman check first, and if the average temperature is ≥ 80°F, it applies the full 9-parameter Rothfusz regression with low/high humidity corrections.

Rothfusz Regression · HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²

Where T is temperature in Fahrenheit, R is relative humidity (%), and c₁–c₉ are constant coefficients.

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Frequently asked questions

The heat index, also known as the apparent temperature, is a measure of how hot it feels to the human body when relative humidity is combined with the air temperature. High humidity slows the body's natural cooling mechanism (evaporation of sweat), making it feel hotter than the actual temperature.

The NWS uses the Rothfusz regression equation — a multi-term mathematical model based on work by R.G. Steadman. The calculator applies the full 9-coefficient regression along with NWS adjustments for low and high humidity at specific temperatures.

The NWS heat index is designed to apply when air temperature is 80 °F (27 °C) or higher and relative humidity is 40% or higher. Outside this range, the calculator uses the simple Steadman formula to estimate apparent temperature.

The NWS classifies heat index values into four warning categories: Caution (80–90 °F), Extreme Caution (91–103 °F), Danger (104–129 °F), and Extreme Danger (130 °F or higher). Proactively review hydration and outdoor exposure guidelines for each level.

No. Heat index values are calculated for shady, light-wind conditions. Exposure to full, direct sunshine can increase the heat index values by up to 15 °F (8 °C).