The temperature that constitutes a fever varies by age and the method of taking it. Typically, any temperature over 100o F (37.7o C) is considered a fever.

A fever involves a higher than normal body temperature.

Most commonly, they develop due to the body trying to fight an infection or illness.

The range that healthcare professionals classify as a fever varies slightly depending on a person’s age.

This article explores what constitutes a fever in various age groups, possible causes of fever, and at what point a person should seek medical attention for them.

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Typically, an average body temperature is about 98.6°F (37°C). However, normal body temperatures may also vary slightly throughout the day.

For example, body temperatures tend to be lowest in the morning.

Individuals also may have their own normal temperature, which may be slightly lower or higher than the “norm.”

Normal body temperature also varies depending on the method that people use to take the temperature. For example, temperatures taken orally are usually lower than a rectal temperature.

Overall, a normal body temperature can vary by 0.9–1.8°F (0.5°F to 1°F) depending on the anatomic site.

Below are ranges for normal body temperatures according to age. All measurements are oral, except in babies, where oral temperatures are difficult to obtain.

Normal temperatures include:

  • 0–2 years: A normal rectal temperature range for babies is 97.9–100.3°F (36.637.9°C).
  • 3–10 years: When taken orally, a normal temperature for this age group is 95.9–99.5°F (35.537.5°C).
  • 11–65 years: A normal oral temperature is 97.6–99.6°F (36.437.5°C).
  • Over 65 years: Some older adults may have a lower baseline temperature than younger people. A normal oral temperature for this age group is 96.4–98.5°F (35.736.9°C).

What counts as a fever varies slightly by age and the method of taking the temperature. These different techniques include:

  • rectally
  • orally
  • ear
  • under the arm

A rectal temperature is the most accurate measure of core body temperature. However, other methods are less invasive and more convenient. In addition, other approaches can be more accurate to determine if someone has a fever.

Note that an oral temperature is usually slightly lower than a rectal temperature.

The chart below lists what constitutes a fever by age group.

Age Type of reading Fever
02 yearsRectal100.4°F (38°C)
310 years Oral100.4°F (38°C)
1165 yearsOral100.4°F (38°C)
Over 65 yearsOral100°F (37.7°C)

Experts categorize fevers as either low grade or high grade. Although the classification can vary slightly, a fever under 102.2°F (39°C) as a rectal temperature reading is a low-grade fever.

How to take a temperature correctly depends on the body site.

One of the most common ways is with an oral thermometer. In addition, most doctors do not recommend using a glass or mercury thermometer. Instead, healthcare providers usually suggest using a digital thermometer.

To take an oral temperature using a digital thermometer, follow the steps below:

  1. Make sure the thermometer is clean.
  2. Read the directions that came with the thermometer.
  3. Turn the device on.
  4. Place the tip of the thermometer under the tongue and keep the mouth closed.
  5. Wait until the device beeps, which signals a reading.
  6. Check the reading.

It is best to avoid eating or drinking anything for about 10 minutes before taking an oral temperature. The temperature of the food and drink can alter the results and lead to an inaccurate reading.

Another option to check for fever is an ear thermometer. To get the most accurate results:

  1. Place the tip into the ear according to the directions.
  2. Wait for the beep.
  3. Remove the device.
  4. Check the reading.

Several conditions, including diseases and infections, can lead to a fever.

Additional symptoms often paint a clearer picture of what is causing a fever. Possible causes may include the following:


A fever is one of the symptoms of COVID-19. Additional indicators may include:

  • coughing
  • fatigue
  • shortness of breath
  • loss of taste and smell
  • muscle aches

Learn more about the symptoms, treatment, and prevention of COVID-19 here.

Common infections

Various other types of viral and bacterial infections can also trigger a fever, which include:

  • seasonal flu
  • strep throat
  • urinary tract infections
  • gastroenteritis


A fever can indicate certain types of blood cancers, including lymphoma.

Health experts believe that cancer cells produce chemical signals that increase body temperature, leading to a fever.


Heat-related illnesses, such as heatstroke, can increase temperature and trigger a fever.

Traumatic injuries and pain

A fever can also occur after trauma to the body that causes pain.

Healthcare professionals state that trauma can trigger the release of prostaglandins and cytokines by the white blood cells, which may cause an increase in body temperature.

Fever may also develop after cardiac arrest.

Drug use

Withdrawal from alcohol and amphetamine use can increase body temperature and lead to a fever.

Certain types of prescription medications, such as drugs to treat seizures, may also cause a fever as a side effect.

It is not always necessary to seek medical care for a fever, especially if it is low grade or only lasts a short time.

However, there are some circumstances where a person should seek medical help, such as the following:

  • a fever in a baby younger than 3 months
  • a baby becomes lethargic
  • a fever higher than 103°F (39.4°C)
  • a fever that comes and goes for a week or more
  • a fever that lasts for more than 2 days
  • if additional symptoms occur, such as:

The temperature that constitutes a fever varies slightly by age and where on the body someone takes the temperature.

Usually, rectal temperatures are the most accurate at measuring core temperature.

Although it varies slightly by age, an oral temperature over 100.4°F (38°C) counts as a fever.

In many cases, fever as a sole symptom is not dangerous. But it could be the sign of an infection, including COVID-19.

If a person is unsure of the cause of a fever, or if a fever occurs in a baby less than 3 months old, they should consult with a healthcare provider.