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Heat Index Calculator

Find the heat index, or how hot it feels, from air temperature and humidity, using the National Weather Service formula, with the risk level.

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ItemValue
The heat index is for shady, light-wind conditions. Full sun can raise it by up to about 15°F. Follow local heat warnings, drink water and rest in the shade. This is not medical advice.

Quick Answer

The heat index combines temperature and humidity to show how hot it feels. At 90°F with 60% humidity, it feels like about 100°F, which is in the Extreme Caution range. The National Weather Service uses the Rothfusz formula for heat index values of 80°F and above.

Last updated: 2 November 2026 ·How we calculate this: the National Weather Service heat index equation

Table of Contents

How This Heat Index Calculator Works

Enter the air temperature and relative humidity. The calculator follows the National Weather Service method: it uses a simple formula for mild conditions and the Rothfusz regression, with its corrections, for heat index values of 80°F and above. It then shows the NWS risk level.

Quick explanation: humid air slows sweat from drying, so your body cools less well and it feels hotter.

  • Shade: the values assume shade and light wind.
  • Sun: direct sun can add up to about 15°F.
  • Risk level: from Caution to Extreme Danger.

How to Use the Heat Index Calculator

  1. Choose Fahrenheit or Celsius.
  2. Enter the air temperature from a forecast or thermometer.
  3. Enter the relative humidity.
  4. Press Calculate.
  5. Read how hot it feels and the risk level, then plan breaks and water.

Core Formula

HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²

  • T: air temperature in °F. RH: relative humidity in %.
  • Very dry air: if RH is under 13% and T is 80 to 112°F, a small amount is subtracted.
  • Very humid air: if RH is over 85% and T is 80 to 87°F, a small amount is added.
  • Mild conditions: below about 80°F, a simpler formula is used.

Key Terms

  • Heat index: the apparent temperature felt by people when humidity is combined with air temperature.
  • Relative humidity: how much moisture the air holds compared with the most it can hold.
  • Heat exhaustion: a heat illness with heavy sweating, weakness and nausea.
  • Heat stroke: a medical emergency where the body can no longer control its temperature.

Worked Example

It is 95°F with 50% humidity.

  • The simple check gives over 80°F, so the full formula applies
  • Heat index = 105.2°F
  • Risk level: Danger

Real-Life Examples

  • An outdoor job: at 92°F with 65% humidity, it feels like about 108°F, in the Danger range, so plan shade and water breaks.
  • A sports practice: at 88°F with 70% humidity, it feels like about 100°F. Many coaches change practice times at these levels.
  • Desert heat: at 105°F with 10% humidity, it feels like about 99°F, close to the air temperature, because dry air lets sweat evaporate.
  • A humid summer day: 85°F with 90% humidity feels like about 102°F.

Heat Index vs Wind Chill

  • Heat index: how hot it feels when humidity is high, used in warm weather.
  • Wind chill: how cold it feels in wind, used at 50°F and below.
  • Both are "feels like" values: they describe the effect on people, not the thermometer reading.

Heat Risk Levels

Heat indexLevelPossible effects
80 to 90°FCautionFatigue with long exposure or activity
90 to 103°FExtreme cautionHeat cramps or heat exhaustion possible
103 to 124°FDangerHeat exhaustion likely, heat stroke possible
125°F and aboveExtreme dangerHeat stroke highly likely

Common Mistakes to Avoid

  • Ignoring direct sun. The heat index assumes shade, so full sun feels much hotter.
  • Waiting to feel thirsty. Drink regularly in the heat.
  • Wearing dark, heavy clothing. Light, loose clothes help you cool.
  • Leaving children or pets in cars. Car temperatures rise fast and can be deadly.
  • Ignoring warning signs. Confusion, hot dry skin or fainting need emergency help.

Frequently Asked Questions

It is how hot it feels when humidity is added to the air temperature, in the shade.

The National Weather Service uses the Rothfusz regression equation, with corrections for very dry or very humid air.

103°F and above is in the NWS Danger range, and 125°F and above is Extreme Danger.

In hot, humid weather, yes. Weather apps often show the heat index as the feels-like value.

The formula assumes light wind. Strong hot winds can make heat stress worse in some conditions.

Your body cools by sweat evaporating. Humid air slows evaporation, so you cool less.

Heavy sweating, cold and clammy skin, weakness, dizziness, headache and nausea. Move to a cooler place, loosen clothing and sip water.

A very high body temperature, hot red skin, confusion or fainting. Call 911 and cool the person right away.

Older adults, young children, outdoor workers, athletes and people with some health conditions or medicines.

Yes. In very dry air, sweat evaporates quickly, so it can feel slightly cooler than the thermometer shows.

Yes. This calculator converts your input to °F, applies the formula, and shows both units.

Limit hard activity, stay in shade or air conditioning, drink water often, and check on others.

References & Sources

SourceUsed ForLink
NOAA Weather Prediction Center, The heat index equation The equation and corrections used here. https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
National Weather Service, Heat forecast tools Heat risk forecasts and safety tips. https://www.weather.gov/safety/heat-tools