Power Calculators

🌡️ Heat Index Calculator

Calculate the heat index or feels-like temperature from air temperature and relative humidity using the National Weather Service methodology.

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Enter details

Enter the air temperature. The NWS heat index equation is based on Fahrenheit internally.
Enter relative humidity from 0 to 100 percent.
Heat Index (°F)
107.637
Heat Index (°C)
42.0207
Difference from Air Temperature (°F)
11.6373
Difference from Air Temperature (°C)
6.46516
Humidity Input Valid (1 = Yes, 0 = No)
1

Also available in our Android app, with charts and full breakdown tables.

📘 About the Heat Index

Heat index combines air temperature and relative humidity to estimate how hot conditions feel to the human body.

The National Weather Service heat index equation is based on work by Robert Steadman and the Rothfusz regression.

The calculation is intended for hot weather and is not simply a general-purpose feels-like temperature for every weather condition.

Heat index values are designed for shady conditions with light wind.

✏️ Worked examples

Example 1 - 96°F and 50% RH

You enter
Air Temperature96
Temperature UnitFahrenheit (°F)
Relative Humidity50%
You get
Heat Index (°F)107.637
Heat Index (°C)42.0207
Difference from Air Temperature (°F)11.6373
Difference from Air Temperature (°C)6.46516
Humidity Input Valid (1 = Yes, 0 = No)1

A temperature of 96°F and relative humidity of 50% produces a heat index of about 108°F using the NWS methodology.

Try this example →

Example 2 - 35°C and 60% RH

You enter
Air Temperature35
Temperature UnitCelsius (°C)
Relative Humidity60%
You get
Heat Index (°F)113.09
Heat Index (°C)45.0502
Difference from Air Temperature (°F)18.0903
Difference from Air Temperature (°C)10.0502
Humidity Input Valid (1 = Yes, 0 = No)1

The calculator converts 35°C to Fahrenheit internally and then applies the NWS calculation.

Try this example →

Example 3 - Very humid conditions

You enter
Air Temperature90
Temperature UnitFahrenheit (°F)
Relative Humidity85%
You get
Heat Index (°F)117.47
Heat Index (°C)47.4834
Difference from Air Temperature (°F)27.4701
Difference from Air Temperature (°C)15.2612
Humidity Input Valid (1 = Yes, 0 = No)1

At 90°F and 85% relative humidity, the high-humidity portion of the NWS methodology becomes important.

Try this example →

🧭 How to use this calculator

What you enter

  • Air TemperatureEnter the air temperature. The NWS heat index equation is based on Fahrenheit internally.
  • Temperature Unit
  • Relative Humidity (%)Enter relative humidity from 0 to 100 percent.

What you get

  • Heat Index (°F)
  • Heat Index (°C)
  • Difference from Air Temperature (°F)
  • Difference from Air Temperature (°C)
  • Humidity Input Valid (1 = Yes, 0 = No)

💡 Rothfusz heat index equation

  • The regression uses air temperature in Fahrenheit and relative humidity in percent.
  • The equation contains temperature, humidity and interaction terms.
  • The NWS applies additional adjustments under specific low- and high-humidity conditions.
  • The calculation is not intended for extreme conditions outside the range for which the regression was developed.

💡 Low temperature conditions

  • The full Rothfusz regression is not appropriate when the resulting heat index is below approximately 80°F.
  • The NWS uses a simpler Steadman-consistent equation in these conditions.
  • This calculator performs the same selection using the preliminary simple calculation.

💡 Sunlight and wind

  • Heat index values are intended for shady conditions.
  • Direct sunshine can increase heat stress substantially compared with a shaded heat-index value.
  • Heat index is not the same as WBGT, which includes additional environmental factors.

💡 Safety

  • High heat index values can indicate increased risk of heat illness.
  • Children, older adults, people with certain medical conditions and people performing strenuous activity can be more vulnerable to heat.
  • Use official heat warnings and local emergency guidance when conditions are dangerous.

❓ Frequently asked questions

What is heat index?

Heat index combines air temperature and relative humidity to estimate how hot conditions feel to people.

What formula does this calculator use?

It uses the National Weather Service Rothfusz regression together with the NWS preliminary equation and humidity adjustments.

Why does humidity increase heat index?

Higher humidity reduces the efficiency of evaporative cooling from the skin, making hot air feel hotter.

Does heat index include sunlight?

No. The standard NWS heat index is intended for shady conditions. Direct sunshine can increase heat exposure.

Is heat index the same as WBGT?

No. WBGT incorporates additional environmental factors and is used for heat-stress assessment in active populations.

Can I enter Celsius?

Yes. Celsius is converted to Fahrenheit internally before the NWS equation is applied.

What humidity should I enter?

Enter relative humidity as a percentage from 0 to 100.