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Heat Index Calculator

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Heat index is 99.7 °F.

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How to use this calculator

  1. Enter Air temperature
    Enter the air temperature in degrees Fahrenheit.
  2. Enter Relative humidity
    Enter relative humidity as a whole-number percentage; this calculator works with values from 0% to 100%.
  3. Read Heat index

Heat index (°F) vs Air temperature (°F)

134.181.8180101.3

Heat index (°F) Air temperature (°F)

What this calculates

This calculator turns two everyday weather readings, air temperature and relative humidity, into one number: the heat index. That number describes what the temperature feels like to your body. 3

The formula behind it refines a multiple regression analysis. That analysis was carried out by Lans P. Rothfusz. It was described in a 1990 National Weather Service technical paper. 1

What Is the Heat Index?

The heat index measures how hot the air feels to your body. It factors in relative humidity along with the actual air temperature. A thermometer alone only shows air temperature. The heat index adds humidity to describe the real feel. 5

No single true equation defines the heat index. Instead, it comes from a model. That model is built from a collection of equations. Together, those equations produce one heat index value. 7

How Heat Index Is Calculated

The calculator starts with a simple formula. That formula averages air temperature with a Steadman-based estimate. If the result reaches 80°F or higher, the calculator switches formulas. It then applies a fuller regression equation instead. 1

Two adjustments can change that regression result. When relative humidity is less than 13%, an amount is subtracted. This applies when air temperature is between 80°F and 112°F. When relative humidity is above 85% and temperature is 80°F to 87°F, an amount is added instead. 1

Using Relative Humidity or Dew Point

Heat index is computed using air temperature and dew point temperature. 2

Relative humidity in the formula is a percentage value. It is entered as a whole integer, not a decimal fraction. A reading of 50% humidity is simply written as 50. This keeps the formula's inputs consistent. 7

Reading the Heat Index Chart

A published heat index chart lists air temperature down one side. Relative humidity runs across the top of the chart. Each cell inside shows a resulting heat index value. The chart's rows span air temperatures from 80°F to 110°F. 4

To find a heat index, you can look at a chart like that one. You can also check a calculator such as this one. As an example, air temperature of 96°F and relative humidity of 65% gives a heat index of 121°F. This example comes directly from the National Weather Service safety page. 5

What Heat Index Values Mean for Your Health

Heat index values carry real health meaning, not just comfort. Once the heat index reaches 103°F or higher, risk increases. Prolonged exposure at that level can lead to heat disorders. Physical activity in that heat can also lead to heat disorders. 6

A heat index of 80°F to 90°F is labeled caution. At that level, fatigue is possible with prolonged exposure or activity. From 90°F to 103°F is extreme caution, where heat stroke, heat cramps, or heat exhaustion become possible. 6

From 103°F to 124°F is danger. Heat cramps or heat exhaustion are likely there. Heat stroke becomes possible in that range too. 125°F or higher is extreme danger, where heat stroke is highly likely. 6

Staying Safe When Heat Index Is High

Drink plenty of water when the heat index is high. Also drink non-alcoholic, decaffeinated fluids. Do this even before you feel thirsty. 9

Slow down during high heat index conditions. Reduce, eliminate, or reschedule strenuous activity. Save it for the coolest part of the day. Wear lightweight, loose-fitting, light-colored clothing instead, since it reflects heat and sunlight. Sunburn reduces your body's ability to dissipate heat. 9

Your body's ability to cool itself can be challenged then. Heat-related illness becomes more likely as a result. A fan can help when the heat index stays below the high 90s. At higher heat index values, a fan can make you hotter instead. 8

How Accurate Is the Heat Index?

The heat index formula comes from multiple regression analysis. Because of that method, its values carry an error. That error is about plus or minus 1.3°F. Temperature and relative humidity are the only two variables used. Other body and clothing assumptions are still built into the numbers. 7

The model assumes a person 5 feet 7 inches tall. It also assumes a weight of 147 pounds. Clothing is assumed to be long trousers and a short-sleeved shirt. That clothing covers 84% of the skin. The model assumes walking outdoors at 3.1 miles per hour, with a wind of 5 knots. 7

Key facts

  • The heat index, also called apparent temperature, comes from extensive biometeorological studies of the body. 7 7
  • Printed heat index chart values reach as high as 137°F for the hottest, most humid conditions. 4 4
  • A heat wave is abnormally hot weather generally lasting more than two days, with or without high humidity. 9 9
  • During a heat wave, check on older, sick, or frail people who may need help. 9 9
  • Newborns, infants, and young children are particularly vulnerable to heat-related illness and death. 9 9

How this is calculated

Combine air temperature and humidity using the published heat-index method. A simple estimate selects the appropriate calculation; humidity adjustments apply where specified. These equations use °F.

Heat index

If S > 79: R + A Otherwise: S
  • S = Simple estimate
  • R = Rothfusz regression
  • A = Humidity adjustment
Understand each part

These names are shorthand for the exact calculations below, not different formulas.

  1. Simple estimate (S)

    The preliminary estimate from air temperature and relative humidity.

    See the definition of S0.5 * (air_temperature + 61 + (air_temperature - 68) * 1.2 + relative_humidity * 0.094)
  2. Rothfusz regression (R)

    The published polynomial combines temperature and humidity. Its full coefficients are available below.

    See the definition of R-42.379 + 2.04901523 * air_temperature + 10.14333127 * relative_humidity - 0.22475541 * air_temperature * relative_humidity - 0.00683783 * air_temperature * air_temperature - 0.05481717 * relative_humidity * relative_humidity + 0.00122874 * air_temperature * air_temperature * relative_humidity + 0.00085282 * air_temperature * relative_humidity * relative_humidity - 0.00000199 * air_temperature * air_temperature * relative_humidity * relative_humidity
  3. Humidity adjustment (A)

    Apply the low- or high-humidity adjustment only when its conditions hold; otherwise this term is zero.

    See the definition of A(relative_humidity <= 13 ? 0 - (relative_humidity <= 13 ? (13 - relative_humidity) / 4 * ((17 - abs(air_temperature - 95)) / 17) ^ 0.5 : ((relative_humidity > 85 ? (air_temperature <= 87 ? 1 : 0) : 0) == 1 ? (relative_humidity - 85) / 10 * (87 - air_temperature) / 5 : 0)) : (relative_humidity <= 13 ? (13 - relative_humidity) / 4 * ((17 - abs(air_temperature - 95)) / 17) ^ 0.5 : ((relative_humidity > 85 ? (air_temperature <= 87 ? 1 : 0) : 0) == 1 ? (relative_humidity - 85) / 10 * (87 - air_temperature) / 5 : 0)))
Follow the calculation with your values
  1. Heat index: 99.7 °F
    Show arithmetic
    -42.379 + 2.04902 * 90 + 10.1433 * 60 - 0.224755 * 90 * 60 - 0.00683783 * 90 * 90 - 0.0548172 * 60 * 60 + 0.00122874 * 90 * 90 * 60 + 0.00085282 * 90 * 60 * 60 - 0.00000199 * 90 * 90 * 60 * 60 + 0
    Full branch logic0.5 * (90 + 61 + (90 - 68) * 1.2 + 60 * 0.094) > 79 ? -42.379 + 2.04901523 * 90 + 10.14333127 * 60 - 0.22475541 * 90 * 60 - 0.00683783 * 90 * 90 - 0.05481717 * 60 * 60 + 0.00122874 * 90 * 90 * 60 + 0.00085282 * 90 * 60 * 60 - 0.00000199 * 90 * 90 * 60 * 60 + (60 <= 13 ? 0 - (60 <= 13 ? (13 - 60) / 4 * ((17 - abs(90 - 95)) / 17) ^ 0.5 : ((60 > 85 ? (90 <= 87 ? 1 : 0) : 0) == 1 ? (60 - 85) / 10 * (87 - 90) / 5 : 0)) : (60 <= 13 ? (13 - 60) / 4 * ((17 - abs(90 - 95)) / 17) ^ 0.5 : ((60 > 85 ? (90 <= 87 ? 1 : 0) : 0) == 1 ? (60 - 85) / 10 * (87 - 90) / 5 : 0))) : 0.5 * (90 + 61 + (90 - 68) * 1.2 + 60 * 0.094)
Full formulas, symbols and sources

heat_index = 0.5 * (air_temperature + 61 + (air_temperature - 68) * 1.2 + relative_humidity * 0.094) > 79 ? -42.379 + 2.04901523 * air_temperature + 10.14333127 * relative_humidity - 0.22475541 * air_temperature * relative_humidity - 0.00683783 * air_temperature * air_temperature - 0.05481717 * relative_humidity * relative_humidity + 0.00122874 * air_temperature * air_temperature * relative_humidity + 0.00085282 * air_temperature * relative_humidity * relative_humidity - 0.00000199 * air_temperature * air_temperature * relative_humidity * relative_humidity + (relative_humidity <= 13 ? 0 - (relative_humidity <= 13 ? (13 - relative_humidity) / 4 * ((17 - abs(air_temperature - 95)) / 17) ^ 0.5 : ((relative_humidity > 85 ? (air_temperature <= 87 ? 1 : 0) : 0) == 1 ? (relative_humidity - 85) / 10 * (87 - air_temperature) / 5 : 0)) : (relative_humidity <= 13 ? (13 - relative_humidity) / 4 * ((17 - abs(air_temperature - 95)) / 17) ^ 0.5 : ((relative_humidity > 85 ? (air_temperature <= 87 ? 1 : 0) : 0) == 1 ? (relative_humidity - 85) / 10 * (87 - air_temperature) / 5 : 0))) : 0.5 * (air_temperature + 61 + (air_temperature - 68) * 1.2 + relative_humidity * 0.094)

air_temperature
Air temperature (°F)
relative_humidity
Relative humidity (%)
heat_index
Heat index (°F)
abs
absolute value

    Worked examples

    Example using the starting values: Heat index 99.7 °F

    See inputs and calculation

    Starting values

    Air temperature
    90 °F
    Relative humidity
    60 %
      1. Heat index: 99.7 °F
        Show arithmetic
        -42.379 + 2.04902 * 90 + 10.1433 * 60 - 0.224755 * 90 * 60 - 0.00683783 * 90 * 90 - 0.0548172 * 60 * 60 + 0.00122874 * 90 * 90 * 60 + 0.00085282 * 90 * 60 * 60 - 0.00000199 * 90 * 90 * 60 * 60 + 0
        Full branch logic0.5 * (90 + 61 + (90 - 68) * 1.2 + 60 * 0.094) > 79 ? -42.379 + 2.04901523 * 90 + 10.14333127 * 60 - 0.22475541 * 90 * 60 - 0.00683783 * 90 * 90 - 0.05481717 * 60 * 60 + 0.00122874 * 90 * 90 * 60 + 0.00085282 * 90 * 60 * 60 - 0.00000199 * 90 * 90 * 60 * 60 + (60 <= 13 ? 0 - (60 <= 13 ? (13 - 60) / 4 * ((17 - abs(90 - 95)) / 17) ^ 0.5 : ((60 > 85 ? (90 <= 87 ? 1 : 0) : 0) == 1 ? (60 - 85) / 10 * (87 - 90) / 5 : 0)) : (60 <= 13 ? (13 - 60) / 4 * ((17 - abs(90 - 95)) / 17) ^ 0.5 : ((60 > 85 ? (90 <= 87 ? 1 : 0) : 0) == 1 ? (60 - 85) / 10 * (87 - 90) / 5 : 0))) : 0.5 * (90 + 61 + (90 - 68) * 1.2 + 60 * 0.094)

      Frequently asked questions

      What is the heat index?
      The heat index is expressed as an apparent temperature. It states that value in degrees Fahrenheit. This gives a single number for how conditions feel. 1
      How to Calculate Heat Index?
      The full regression formula multiplies air temperature and relative humidity together. It also squares each of those values separately. Nine fixed coefficients combine these terms, such as -42.379 and 2.04901523. The result is the heat index in degrees Fahrenheit. 1
      Is the heat index accurate?
      The regression equation is not valid for extreme conditions. This applies to temperature and humidity combinations beyond a certain range. That range is the one Steadman originally studied. Outside it, the equation's accuracy is not established. 1
      What are the factors affecting heat index?
      Heat index rises when either air temperature or relative humidity increases. It falls when either one decreases instead. Apparent temperature can even come out lower than air temperature. That happens under the right combination of conditions. 7
      How do I estimate the heat index?
      To estimate the heat index, you need two readings. The first is air temperature, in degrees Fahrenheit. The second is relative humidity, as a percentage. Both values feed directly into the formula. 6
      How do you interpret the heat index of 80 °F?
      At 80°F air temperature and 50% relative humidity, the heat index is 80.8°F, only slightly above the air temperature itself. At the same 80°F with 100% relative humidity instead, the heat index rises to about 87°F. Near 80°F, the heat index stays fairly close to actual temperature. 24
      Is the heat index of 105 °F hot?
      Yes, depending on humidity. At 105°F air temperature and 0% relative humidity, the heat index is 95.3°F. At the same 105°F with 50% relative humidity instead, it rises to 133.9°F. Higher humidity at that temperature makes it feel much hotter. 2
      What is Wet Bulb Globe Temperature (WBGT)?
      WBGT is a different heat-stress measurement. It uses temperature, humidity, wind, and solar radiation together. Other weather parameters can factor in too. It is a particularly effective indicator for outdoor workers and athletes. 5
      How does WBGT differ from Heat Index?
      WBGT readings serve a different purpose than heat index. They can inform decisions to modify activity. This applies during exercise or outdoor work. 5

      Limitations & common mistakes

      • This calculator works with relative humidity of 0% to 100%, and entries outside that range cannot return a heat index value.
      • The published chart's red area has no numbers; it only indicates extreme danger.
      • The regression equation carries an error of about plus or minus 1.3°F and is not valid for extreme temperature and humidity combinations beyond the range Steadman studied.
      • The formula assumes a specific body size, clothing, activity level, and wind speed, so actual conditions for any one person may differ.

      By Editorial Team, Assembled from the cited primary sources; not yet expert-reviewed

      Last updated

      Sources current as of

      How we review

      Formula, sources, and review

      This calculation follows the formula published on this page and rests on 9 cited sources, listed under Sources below. Last checked .

      How we review

      Sources & methodology

      ConstantValueUnitSource
      heat_index_c10.51
      heat_index_c2611
      heat_index_c3681
      heat_index_c41.21
      heat_index_c50.0941
      heat_index_c6792
      heat_index_c7-42.3791
      heat_index_c82.049015231
      heat_index_c910.143331271
      heat_index_c100.224755411
      heat_index_c110.006837831
      heat_index_c120.054817171
      heat_index_c130.001228741
      heat_index_c140.000852821
      heat_index_c150.000001991
      heat_index_c16131
      heat_index_c1741
      heat_index_c18171
      heat_index_c19951
      heat_index_c20851
      heat_index_c21871
      heat_index_c22101
      heat_index_c2351

      Sources