🌡️ Heat Index Calculator
This calculator estimates the temperature felt by the body as a result of air temperature and relative humidity
Use Relative Humidity
Heat Index
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Use Dew Point Temperature
Heat Index
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Heat Index Calculator
Use our advanced Heat Index Calculator to quickly and accurately determine the apparent temperature—the combination of air temperature and humidity that your body actually feels. Also known as Humiture, the Heat Index is a vital measure for assessing the risk of heat-related illnesses and planning outdoor activities safely.
What is the Heat Index (Humiture)?
The Heat Index (HI) is a measure of how hot it feels when relative humidity is factored in with the actual air temperature.2 It is often referred to as Humiture and is conceptually similar to the Wind Chill Temperature Index, which measures how cold it feels.
For example, an air temperature of 83{F} combined with 70 % relative humidity can make the outside feel like 88{F}. This difference between the actual and perceived temperature is due to a mixture of air temperature, humidity, and wind speed.
Why Does Humidity Make it Feel Hotter?
The human body naturally cools itself through the evaporation of sweat. When sweat evaporates from the skin, it removes heat, lowering your core body temperature.
- High Relative Humidity means the air already contains a large amount of moisture.
- This saturated air reduces the rate of sweat evaporation.
- When sweat can’t evaporate quickly, the body’s natural cooling mechanism is hindered, leading to a rise in perceived temperature.7
The Heat Index is an attempt to quantify this effect, reflecting the increased thermal load on the human body. While it can technically be measured indoors, it is most commonly used by weather services like the National Weather Service (NWS) to communicate outdoor risk.
How to Calculate the Heat Index: Formulas and Methodology
Calculating the Heat Index is a complex process. The official NWS Heat Index formula—developed by George Winterling in 1978 and later refined using the Rothfusz Regression Equation—is a sophisticated equation meant to provide the most accurate estimate for temperatures of 80{F} or higher and relative humidity of 40% or more.
Our Heat Index Calculator uses the widely accepted Rothfusz regression equation to provide estimates based on two input methods:
Method 1: Air Temperature and Relative Humidity
This is the standard and most direct method for calculating HI. You input the current Air Temperature (in Celsius, Fahrenheit, or Kelvin) and the percentage of Relative Humidity (0%-100%).
Method 2: Air Temperature and Dew Point Temperature
This advanced method is valuable when relative humidity data isn’t readily available. The calculator first uses the Magnus Formula to derive the relative humidity from the Dew Point Temperature (the temperature at which the air becomes saturated with moisture), and then uses that humidity to calculate the Heat Index.
Note: The Heat Index calculation makes certain assumptions about typical human factors (body mass, clothing, physical activity, wind speed). Due to these variables, the calculated heat index is an estimate and may not perfectly reflect every individual’s subjective perception of heat.
Health Risks and Effects of the Heat Index:
A high Heat Index indicates dangerous conditions where the body’s natural cooling is severely compromised.
Monitoring the Heat Index is critical to preventing heat-related illnesses such as heat cramps,
heat exhaustion, and potentially fatal heat stroke.
| Heat Index Value | Hazard Category | Potential Health Effects |
|---|---|---|
| 80°F–90°F (27°C–32°C) | Caution | Fatigue is possible with prolonged exposure and activity. Heat cramps possible with continued activity. |
| 90°F–105°F (32°C–41°C) | Extreme Caution | Heat cramps and heat exhaustion are possible. Heat stroke is possible with prolonged or continued physical activity. |
| 105°F–130°F (41°C–54°C) | Danger | Heat cramps and heat exhaustion are likely. Heat stroke is probable with continued activity. |
| Over 130°F (Over 54°C) | Extreme Danger | Heat stroke is imminent. Avoid all outdoor activities. |
Who is Most At-Risk?
Everyone is susceptible to heat stress, but certain groups are at greater risk of overheating and dehydration:
- Young Children: They have a larger skin surface area relative to their body size, may produce more heat during exercise, and are less aware of the need to rest and re-hydrate.
- Elderly Individuals: Their bodies may not adjust to heat changes as efficiently.
- Individuals with Certain Health Conditions: This includes people with obesity, diabetes, heart disease, cystic fibrosis, and other chronic conditions.
- Pregnant Women and those undergoing menopause.
Hydration is Key: By the time you feel thirst, you are already in the early stages of dehydration. It is crucial to drink plenty of fluids before, during, and after any outdoor activities, especially during high Heat Index conditions.
Related Calculators:
Air Temperature is the actual reading from a thermometer, while the Heat Index shows how hot it feels to your body. The Heat Index combines temperature and humidity to reflect the reduced cooling effect of sweat evaporation in humid conditions. When humidity is high, the Heat Index is usually higher than the real temperature.
Your body cools itself by sweating—when sweat evaporates, it removes heat. But when humidity is high, the air is already full of moisture, so sweat can’t evaporate easily. This makes it harder for your body to cool down, causing you to feel hotter than the actual temperature.
Not directly. The official Heat Index formula only uses air temperature and humidity, assuming a light wind of around 5 mph. While a gentle breeze can make you feel a bit cooler by helping sweat evaporate, its effect is minor compared to humidity.
Be cautious when the Heat Index is between 90°F and 105°F (32°C–41°C)—heat exhaustion or cramps are possible. Above 105°F (41°C), the danger rises sharply, and heat stroke becomes likely. Avoid physical activity during these conditions, especially under direct sunlight, which can make it feel up to 14°F hotter.
Children are more vulnerable because they produce more heat during activity and sweat less efficiently. Older adults and people with health issues like heart disease or diabetes may also struggle to regulate temperature or stay hydrated, making them more prone to overheating and dehydration.