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What temperature is considered a fever?

Medically reviewed by Sally Chao, MD. Last updated on Jan 20, 2025.

Official Answer by Drugs.com

A fever is defined as a body temperature of 38°C (100.4°F) or higher. Normal body temperature is usually 37°C (98.6°F), although it can be about a half degree Celsius higher or lower for some people and that’s normal for them.

A fever occurs when your brain sets your body temperature higher than normal. Normal body temperature can change during the day based on activity level, hormone levels and whether you are sleeping or awake.

Your normal body temperature is regulated or set by an area of your brain called your hypothalamus. Fever occurs when your body’s defense system — your immune system — reacts to an infection or illness, triggering your brain to raise your body temperature. This helps your immune system fight off the illness or infection, but a very high temperature can be dangerous.

Fever is not the same as hyperthermia, which means your body temperature goes up due to the outside temperature. Hyperthermia is not caused by your brain and immune system. An example would be heat stroke.

Fever grades in adults

In an adult, a fever may be elevated, high or very high:

Fever in babies and children

According to the American Academy of Pediatrics (AAP), normal temperature in children varies with a child’s age, time of day and level of activity.

Fever is defined as a temperature of 38°C (100.4°F).

How to measure fever

To take your temperature, a digital thermometer is recommended instead of a mercury thermometer, especially in children, because mercury from a broken thermometer may be toxic.
There are three types of digital thermometers. There are many variations, so it is important to read and follow the thermometer instructions carefully. They all must be turned on and they will beep to tell you when to read them:

Related questions

When and how to treat a fever

Since ancient times, fevers have been associated with illness. It was not until the invention of the thermometer that normal temperature and fever temperatures were defined. Over time, the definitions have gone up and down slightly.

At this time, there is a debate on when to treat a fever.

There are no guidelines for when to treat a fever. Treatment may depend on the cause and other symptoms, but most experts agree that a very high fever should be treated.

When to call your doctor for fever

For children, call the doctor if:

For adults and older children, call if a fever is 104°F (40°C) or higher or if there are symptoms that include seizure, stiff neck, confusion, trouble breathing or any severe pain.

References
  1. U.S. National Library of Medicine Bookshelf. How is body temperature regulated and what is fever? InformedHealth.org. 2020. Available at: https://www.ncbi.nlm.nih.gov/books/NBK279457/. [Accessed January 24, 2022].
  2. U.S. National Library of Medicine Bookshelf. Physiology of Fever. StatPearls. August 2021. Available at: https://www.ncbi.nlm.nih.gov/books/NBK562334/. [Accessed January 24, 2022].
  3. American Academy of Pediatrics (AAP). Fever Without Fear. April 2016. Available at: https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-Without-Fear.aspx. [Accessed January 24, 2022].
  4. American Academy of Pediatrics (AAP). How to take your child’s temperature. Available at: https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/How-to-Take-a-Childs-Temperature.aspx. [Accessed January 24, 2022].
  5. Wise J. Rectal thermometer should be used for accurate temperature reading, analysis finds. BMJ 2015;351:h6125. https://www.bmj.com/content/351/bmj.h6125.full
  6. Geneva II, Cuzzo B, Fazili T, Javaid W. Normal Body Temperature: A Systematic Review. Open Forum Infect Dis. 2019 Apr; 6(4): ofz032. https://dx.doi.org/10.1093%2Fofid%2Fofz032.
  7. Ray JJ, Schulman CI. Fever: suppress it or let it ride? J Thorac Dis. 2015 Dec; 7(12): E633-E636. https://dx.doi.org/10.3978%2Fj.issn.2072-1439.2015.12.28.
  8. U.S. National Library of Medicine MedlinePlus. Fever. December 2016. Available at: https://medlineplus.gov/fever.html. [Accessed January 24, 2022].

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