Heat Index Calculator

Q: What is the difference between heat index and wet bulb temperature?

The heat index tells you what the temperature feels like to your body. The wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air. 7 It is read from a thermometer whose bulb is kept wet, so that evaporation cools it below the dry thermometer's reading. 8 Both use humidity, but they measure different things. Scientists have argued that a wet bulb temperature above 35°C for extended periods would cause hyperthermia, because the body could no longer get rid of its metabolic heat. 10 That is 95°F. The NWS heat index classes start at 80°F. 2

Q: Can I use dew point instead of relative humidity?

Yes. Click the "Temp + Dew Point" tab at the top of the calculator. Enter your air temperature and dew point temperature, and the calculator will convert the dew point to relative humidity automatically using the Magnus formula. 4 Then it calculates the heat index from there.

Q: Does the heat index work below 80°F?

It works below 80°F too, with a different formula. The full NWS regression is not appropriate when the heat index would be below about 80°F. 1 For those conditions the NWS uses a simpler formula that gives values consistent with Steadman's original results. 1 The calculator follows the same rule, so it still shows a heat index below 80°F (about 27°C).

Q: What is the dew point and how does it relate to heat index?

The dew point is the temperature to which air must be cooled to reach saturation. 6 A higher dew point means more moisture is present in the air. 6 More moisture means your sweat evaporates slower, which makes it feel hotter. The calculator can use the dew point to figure out relative humidity, which it then uses to find the heat index.

Q: What does absolute humidity mean in the results?

Absolute humidity is the actual amount of water vapor in the air, measured in grams per cubic meter (g/m³). Unlike relative humidity, which changes with temperature, absolute humidity tells you exactly how much moisture is present. Higher values mean more water vapor, which makes it harder for your body to cool down.

Q: What is vapor pressure shown in the results?

Vapor pressure is the partial pressure of water vapor in the air. 9 The calculator shows it in hectopascals (hPa), next to the saturation vapor pressure . The saturation vapor pressure is the vapor pressure at which water vapor is in equilibrium with a flat surface of liquid water at the same temperature. 9 Relative humidity is the actual vapor pressure divided by the saturation vapor pressure, times 100. 4

Q: Is the heat index accurate at night?

Yes, the heat index works at any time of day. It only needs the air temperature and humidity. However, heat index values were devised for shady, light-wind conditions, and full sunshine can raise them by up to 15°F . 3 At night there is no sun exposure, so the calculated heat index is usually a closer match to how it actually feels.

Q: How does humidity make it feel hotter?

Your body cools itself by sweating. When sweat evaporates from your skin, it carries heat away. When the air already has a lot of moisture (high humidity), your sweat cannot evaporate as fast. This means your body holds onto more heat, and the temperature feels higher than it actually is.

Q: What is the highest possible heat index?

The formula itself has no fixed upper limit. The NWS classes a heat index of 125°F or higher as Extreme Danger, where heat stroke is highly likely. 2 That is about 52°C. The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman. 1 Conditions like these are rare but extremely dangerous.

Q: Who is most at risk from a high heat index?

The people most at risk include older adults , young children , people with chronic medical conditions, outdoor workers, and athletes. 11 Anyone who is not used to hot weather or who does not drink enough water is also at higher risk. Certain medications can also make it harder for your body to handle heat.

Heat Index Calculator

Introduction

The Heat Index Calculator helps you find out what the temperature actually feels like outside. When the air is hot and humid, your body has a harder time cooling itself through sweat. This makes it feel much hotter than the thermometer shows. The heat index combines the air temperature and the relative humidity to give you a single number: the "feels like" temperature. This number is important because high heat index values can be dangerous and lead to heat exhaustion or heat stroke. Simply enter the current temperature and humidity level, and the calculator will show you the heat index right away. Use this tool to stay safe and plan ahead on hot, sticky days.

How to Use Our Heat Index Calculator

Enter the air temperature and relative humidity below to find out what the temperature actually feels like outside. The calculator will give you the heat index, also known as the "feels like" temperature.

Air Temperature: Type in the current air temperature. This is the temperature you see on a thermometer. You can enter it in degrees Fahrenheit or Celsius, depending on what you prefer.

Relative Humidity: Enter the relative humidity as a percentage. This tells the calculator how much moisture is in the air. You can find this number from your local weather report or a home weather station. Humidity must be between 0% and 100%.

If you know the dew point temperature instead of the relative humidity, switch to the "Temp + Dew Point" tab. The calculator will convert the dew point to relative humidity automatically.

What Is the Heat Index?

The heat index is what the temperature actually feels like to your body when you combine the air temperature with the humidity.² You might hear people call it the "apparent temperature" or "feels-like temperature." On a hot, humid day, your body has a harder time cooling itself through sweat because the moisture in the air slows down evaporation.² This makes it feel much hotter than the thermometer says.

The heat index is the warm-weather counterpart to wind chill, which measures how cold it feels when wind strips heat from your body during cold weather. Together, these two metrics give you a complete picture of how weather conditions actually affect your body year-round.

Why Does the Heat Index Matter?

The heat index is important because it tells you how dangerous the heat really is. When the heat index is high, your body can overheat quickly. This can lead to serious health problems like heat exhaustion or heat stroke, which can be life-threatening. Children, older adults, and people who work or exercise outdoors are especially at risk.¹¹

Knowing the heat index is especially valuable for athletes and anyone who exercises outdoors. High heat index values mean you should slow down, hydrate more, and consider shortening your workout.

Heat Index Danger Levels

Caution: at a heat index of 80–90°F, fatigue is possible with prolonged exposure and/or physical activity.² That is about 27–32°C.
Extreme Caution: at 90–103°F, heat stroke, heat cramps, or heat exhaustion are possible with prolonged exposure and/or physical activity.² That is about 32–39°C.
Danger: at 103–124°F, heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged exposure and/or physical activity.² That is about 39–51°C.
Extreme Danger: at 125°F or higher, heat stroke is highly likely.² That is about 52°C and above.

How Is the Heat Index Calculated?

The National Weather Service (NWS) calculates the heat index with a regression equation worked out by Lans P. Rothfusz and described in a 1990 NWS technical attachment.¹ It takes the air temperature in degrees Fahrenheit and the relative humidity in percent. The regression is not used when the heat index would be below about 80°F.¹ In that case a simpler formula is used: it is worked out first and averaged with the temperature, and the full regression is used only if that average is 80°F or higher.¹ Because the regression is a statistical fit, it has an error of about ±1.3°F.¹³

The NWS method adds two small adjustments to the Rothfusz regression. One is subtracted when the relative humidity is below 13% and the temperature is between 80°F and 112°F, and one is added when the relative humidity is above 85% and the temperature is between 80°F and 87°F.¹ The wet bulb temperature comes from Stull's 2011 equation, which is valid for relative humidities between 5% and 99% at sea-level pressure.⁵ The calculator also computes derived meteorological values such as the dew point, wet bulb temperature, vapor pressure, and absolute humidity, all of which help paint a fuller picture of atmospheric moisture conditions.

Tips for Staying Safe in High Heat

When the heat index is high, drink plenty of water, stay in the shade or air conditioning when possible, wear light and loose clothing, and avoid heavy outdoor activity during the hottest parts of the day. Always check the heat index before spending time outside. The actual temperature alone does not tell the full story.

If you're planning outdoor work or construction projects during hot weather, the heat index should guide your work-rest schedule. High heat index conditions reduce worker productivity and increase the risk of heat-related illness.

Proper nutrition and monitoring your health metrics also play a role in heat safety. Tracking your heart rate zones during outdoor exercise helps you recognize when your body is overheating before symptoms become serious. On extreme heat days, consider moving workouts indoors.

Formulas used

Heat Index – Simple Formula (HI avg < 80°F) ¹

HI_{simple} = 0.5\left(T + 61.0 + 1.2\,(T - 68.0) + 0.094\,RH\right)

Heat Index – Rothfusz Regression (HI avg ≥ 80°F) ¹³

HI = -42.379 + 2.04901523\,T + 10.14333127\,RH - 0.22475541\,T\cdot RH - 6.83783 \times 10^{-3}\,T^{2} - 5.481717 \times 10^{-2}\,RH^{2} + 1.22874 \times 10^{-3}\,T^{2}\,RH + 8.5282 \times 10^{-4}\,T\,RH^{2} - 1.99 \times 10^{-6}\,T^{2}\,RH^{2}

Dew Point from Relative Humidity (Magnus Formula) ⁴

T_d = \frac{b\,\gamma}{a - \gamma}, \quad \gamma = \ln\!\left(\frac{RH}{100}\right) + \frac{a\,T_c}{b + T_c}, \quad a = 17.625,\; b = 243.04

Relative Humidity from Dew Point ⁴

RH = 100 \times \exp\!\left(\frac{a\,T_d}{b + T_d} - \frac{a\,T_c}{b + T_c}\right)

Wet Bulb Temperature (Stull 2011)

T_w = T_c\,\arctan\!\left(0.151977\sqrt{RH + 8.313659}\right) + \arctan(T_c + RH) - \arctan(RH - 1.676331) + 0.00391838\,RH^{1.5}\,\arctan(0.023101\,RH) - 4.686035

Saturation & Actual Vapor Pressure

e_s = 6.112 \times \exp\!\left(\frac{17.67\,T_c}{T_c + 243.5}\right), \quad e_a = e_s \times \frac{RH}{100}

Absolute Humidity

AH = \frac{100\,e_a}{461.5\,(T_c + 273.15)} \times 1000 \quad \text{(g/m}^3\text{, with } e_a \text{ in hPa)}

Frequently asked questions

What is the difference between heat index and wet bulb temperature?

The heat index tells you what the temperature feels like to your body. The wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air.⁷ It is read from a thermometer whose bulb is kept wet, so that evaporation cools it below the dry thermometer's reading.⁸ Both use humidity, but they measure different things. Scientists have argued that a wet bulb temperature above 35°C for extended periods would cause hyperthermia, because the body could no longer get rid of its metabolic heat.¹⁰ That is 95°F. The NWS heat index classes start at 80°F.²

Can I use dew point instead of relative humidity?

Yes. Click the "Temp + Dew Point" tab at the top of the calculator. Enter your air temperature and dew point temperature, and the calculator will convert the dew point to relative humidity automatically using the Magnus formula.⁴ Then it calculates the heat index from there.

Does the heat index work below 80°F?

It works below 80°F too, with a different formula. The full NWS regression is not appropriate when the heat index would be below about 80°F.¹ For those conditions the NWS uses a simpler formula that gives values consistent with Steadman's original results.¹ The calculator follows the same rule, so it still shows a heat index below 80°F (about 27°C).

What is the dew point and how does it relate to heat index?

The dew point is the temperature to which air must be cooled to reach saturation.⁶ A higher dew point means more moisture is present in the air.⁶ More moisture means your sweat evaporates slower, which makes it feel hotter. The calculator can use the dew point to figure out relative humidity, which it then uses to find the heat index.

What does absolute humidity mean in the results?

Absolute humidity is the actual amount of water vapor in the air, measured in grams per cubic meter (g/m³). Unlike relative humidity, which changes with temperature, absolute humidity tells you exactly how much moisture is present. Higher values mean more water vapor, which makes it harder for your body to cool down.

What is vapor pressure shown in the results?

Vapor pressure is the partial pressure of water vapor in the air.⁹ The calculator shows it in hectopascals (hPa), next to the saturation vapor pressure. The saturation vapor pressure is the vapor pressure at which water vapor is in equilibrium with a flat surface of liquid water at the same temperature.⁹ Relative humidity is the actual vapor pressure divided by the saturation vapor pressure, times 100.⁴

Is the heat index accurate at night?

Yes, the heat index works at any time of day. It only needs the air temperature and humidity. However, heat index values were devised for shady, light-wind conditions, and full sunshine can raise them by up to 15°F.³ At night there is no sun exposure, so the calculated heat index is usually a closer match to how it actually feels.

How does humidity make it feel hotter?

Your body cools itself by sweating. When sweat evaporates from your skin, it carries heat away. When the air already has a lot of moisture (high humidity), your sweat cannot evaporate as fast. This means your body holds onto more heat, and the temperature feels higher than it actually is.

What is the highest possible heat index?

The formula itself has no fixed upper limit. The NWS classes a heat index of 125°F or higher as Extreme Danger, where heat stroke is highly likely.² That is about 52°C. The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman.¹ Conditions like these are rare but extremely dangerous.

Who is most at risk from a high heat index?

The people most at risk include older adults, young children, people with chronic medical conditions, outdoor workers, and athletes.¹¹ Anyone who is not used to hot weather or who does not drink enough water is also at higher risk. Certain medications can also make it harder for your body to handle heat.

What should I do if the heat index is over 105°F?

When the heat index is above 105°F, you should limit outdoor activity to only what is essential. Stay in air conditioning as much as possible. For moderate activity in the heat that lasts less than 2 hours, drink 1 cup (8 oz.) of water every 15 to 20 minutes.¹² Wear lightweight, light-colored clothing. Never leave children or pets in a parked car. Watch for signs of heat stroke like confusion, hot dry skin, or fainting.

What is the difference between heat index and feels-like temperature?

They are essentially the same thing in hot weather. "Feels-like temperature" is a general term that weather apps use. In warm conditions, the feels-like temperature is the heat index. In cold and windy conditions, the feels-like temperature is the wind chill. The heat index specifically measures the combined effect of heat and humidity.

Can the heat index be lower than the actual temperature?

Yes. When the relative humidity is low, the heat index can be lower than the air temperature.² For example, at 100°F and 15% relative humidity the heat index is 96°F.² In dry air, sweat evaporates faster and the body actually feels cooler.²

How often should I check the heat index?

On hot days, check the heat index before going outside and again during the hottest part of the day, usually between noon and 4 PM. If you are working or exercising outdoors, check it every hour or two. Conditions can change quickly, especially if humidity rises after a rain shower.

Sources

The Heat Index Equation. NOAA National Weather Service, Weather Prediction Center. Accessed September 29, 2026.

What is the heat index? NOAA National Weather Service, Amarillo, TX Weather Forecast Office. Classification table. Accessed September 29, 2026.

Heat Forecast Tools. NOAA National Weather Service. Heat Index. Accessed September 29, 2026.

Lawrence MG. The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications. Bulletin of the American Meteorological Society. 2005;86(2):225-233. doi:10.1175/BAMS-86-2-225. Accessed September 29, 2026.

Stull R. Wet-Bulb Temperature from Relative Humidity and Air Temperature. Journal of Applied Meteorology and Climatology. 2011;50(11):2267-2269. doi:10.1175/JAMC-D-11-0143.1. Accessed September 29, 2026.

Glossary: Dew Point. NOAA National Weather Service. Accessed September 29, 2026.

Glossary: Wet-Bulb Temperature. NOAA National Weather Service. Accessed September 29, 2026.

Glossary: Psychrometer. NOAA National Weather Service. Accessed September 29, 2026.

Glossary: Vapor Pressure. NOAA National Weather Service. Accessed September 29, 2026.

Sherwood SC, Huber M. An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences. 2010;107(21):9552-9555. doi:10.1073/pnas.0913352107. Accessed September 29, 2026.

Heat and Your Health: Risk Factors. Centers for Disease Control and Prevention. Accessed September 29, 2026.

Heat Stress: Workplace Recommendations. National Institute for Occupational Safety and Health. Hydration. Accessed September 29, 2026.

Rothfusz LP. The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index). Technical Attachment SR 90-23. NOAA National Weather Service, Southern Region Headquarters. 1990;p. 2. Accessed September 29, 2026.

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