🕷️ Crawler Inspector

- Health Conditions ### Health Conditions - [Alzheimer's & Dementia](https://www.medicalnewstoday.com/alzheimers-and-dementia) - [Anxiety](https://www.medicalnewstoday.com/anxiety) - [Asthma & Allergies](https://www.medicalnewstoday.com/asthma-and-allergies) - [Atopic Dermatitis](https://www.medicalnewstoday.com/program/investigating-atopic-dermatitis) - [Breast Cancer](https://www.medicalnewstoday.com/breast-cancer) - [Cancer](https://www.medicalnewstoday.com/cancer) - [Cardiovascular Health](https://www.medicalnewstoday.com/cardiovascular-health) - [COVID-19](https://www.medicalnewstoday.com/coronavirus?correlationId=2b137d5a-dcfb-464e-9ea6-72c5a54b4e92) - [Diabetes](https://www.medicalnewstoday.com/diabetes) - [Endometriosis](https://www.medicalnewstoday.com/endometriosis) - [Environment & Sustainability](https://www.medicalnewstoday.com/environment-and-sustainability) - [Exercise & Fitness](https://www.medicalnewstoday.com/exercise-and-fitness) - [Eye Health](https://www.medicalnewstoday.com/eye-health) - [Headache & Migraine](https://www.medicalnewstoday.com/headache-and-migraine) - [Health Equity](https://www.medicalnewstoday.com/health-equity) - [HIV & AIDS](https://www.medicalnewstoday.com/hiv-and-aids) - [Human Biology](https://www.medicalnewstoday.com/human-biology) - [Leukemia](https://www.medicalnewstoday.com/leukemia) - [LGBTQIA+](https://www.medicalnewstoday.com/LGBTQIA) - [Men's Health](https://www.medicalnewstoday.com/mens-health) - [Mental Health](https://www.medicalnewstoday.com/mental-health) - [Multiple Sclerosis (MS)](https://www.medicalnewstoday.com/multiple-sclerosis) - [Nutrition](https://www.medicalnewstoday.com/nutrition) - [Parkinson's Disease](https://www.medicalnewstoday.com/parkinsons-disease) - [Psoriasis](https://www.medicalnewstoday.com/psoriasis) - [Sexual Health](https://www.medicalnewstoday.com/sexual-health) - [Ulcerative Colitis](https://www.medicalnewstoday.com/program/investigating-ulcerative-colitis) - [Women's Health](https://www.medicalnewstoday.com/womens-health) - Health Products ### Health Products [All](https://www.medicalnewstoday.com/products-and-reviews) - [Nutrition & Fitness](https://www.medicalnewstoday.com/products-and-reviews/nutrition-and-fitness-products) - [Vitamins & Supplements](https://www.medicalnewstoday.com/products-and-reviews/vitamin-and-supplement-products) - [CBD](https://www.medicalnewstoday.com/products-and-reviews/cbd-products) - [Sleep](https://www.medicalnewstoday.com/products-and-reviews/sleep-products) - [Mental Health](https://www.medicalnewstoday.com/products-and-reviews/mental-health-services-and-products) - [At-Home Testing](https://www.medicalnewstoday.com/products-and-reviews/at-home-testing-products) - [Men’s Health](https://www.medicalnewstoday.com/products-and-reviews/mens-health-products) - [Women’s Health](https://www.medicalnewstoday.com/products-and-reviews/womens-health-products) - Discover ### News - [Latest News](https://www.medicalnewstoday.com/news) - [Medicare 2026 Costs](https://www.medicalnewstoday.com/articles/medicare-costs-3-key-changes-in-2026) ### Original Series - [Medical Myths](https://www.medicalnewstoday.com/content-series/medical-myths) - [Honest Nutrition](https://www.medicalnewstoday.com/content-series/honest-nutrition) - [Through My Eyes](https://www.medicalnewstoday.com/content-series/through-my-eyes) - [New Normal Health](https://www.medicalnewstoday.com/content-series/new-normal-health) ### Podcasts [All](https://www.medicalnewstoday.com/articles/in-conversation-podcast) - [Artificial sweeteners and brain aging: What we know so far](https://www.medicalnewstoday.com/articles/artificial-sweeteners-and-brain-aging-what-we-know-so-far) - [Does the Mediterranean diet hold the key to longevity?](https://www.medicalnewstoday.com/articles/does-the-mediterranean-diet-hold-the-key-to-longevity) - [AMA: Registered dietitian answers 5 key questions about fiber and weight loss](https://www.medicalnewstoday.com/articles/can-fiber-help-lose-weight-dietitian-answers-5-key-questions) - [Health misinformation and disinformation: How to avoid it](https://www.medicalnewstoday.com/articles/health-misinformation-and-disinformation-how-to-avoid-it) - [Brain health, sleep, diet: 3 health resolutions for 2025](https://www.medicalnewstoday.com/articles/brain-health-sleep-diet-3-health-resolutions-for-2025) - [5 things everyone should know about menopause](https://www.medicalnewstoday.com/articles/5-things-everyone-should-know-about-menopause) - Tools ### General Health - [Drugs A-Z](https://www.medicalnewstoday.com/drugs) - [Health Hubs](https://www.medicalnewstoday.com/articles/content-hubs) - [Newsletter](https://www.medicalnewstoday.com/newsletter) - [Medicare Plans by State](https://www.medicalnewstoday.com/about/health-medicare-state-coverage) ### Health Tools - [Find a Doctor](https://connect.medicalnewstoday.com/) - [BMI Calculators and Charts](https://www.medicalnewstoday.com/articles/323586) - [Blood Pressure Chart: Ranges and Guide](https://www.medicalnewstoday.com/articles/327077) - [Breast Cancer: Self-Examination Guide](https://www.medicalnewstoday.com/articles/316898) - [Sleep Calculator](https://www.medicalnewstoday.com/articles/sleep-calculator) ### Quizzes - [RA Myths vs Facts](https://www.medicalnewstoday.com/articles/science-fact-or-fiction-separating-ra-myths-from-facts#1) - [Type 2 Diabetes: Managing Blood Sugar](https://www.medicalnewstoday.com/articles/managing-blood-sugar-type-2-diabetes-quiz#1) - [Ankylosing Spondylitis Pain: Fact or Fiction](https://www.medicalnewstoday.com/articles/complementary-therapies-for-as-quiz#1) - Connect ### About Medical News Today - [Who We Are](https://www.medicalnewstoday.com/about) - [Our Editorial Process](https://www.medicalnewstoday.com/articles/process) - [Content Integrity](https://www.medicalnewstoday.com/about/content-integrity) - [Conscious Language](https://www.medicalnewstoday.com/about/conscious-language) ### Find Community - [Bezzy Breast Cancer](https://www.bezzybc.com/lp/2/?verzenio&utm_source=mnt&utm_medium=nav-bar-connect) - [Bezzy MS](https://www.bezzyms.com/?utm_source=mnt&utm_medium=nav-bar-connect) - [Bezzy Migraine](https://www.bezzymigraine.com/?utm_source=mnt&utm_medium=nav-bar-connect) - [Bezzy Psoriasis](https://www.bezzypsoriasis.com/?utm_source=mnt&utm_medium=nav-bar-connect) ### Follow Us Subscribe # What is considered a fever? How to recognize one in every age group  [Medically reviewed]() by [Cynthia Taylor Chavoustie, MPAS, PA-C]() — Written by [MaryAnn De Pietro, CRT]() — [Updated on May 30, 2025]() - [Temperature of a fever](https://www.medicalnewstoday.com/articles/fever-temperature#temperature-of-a-fever) - [Average temperature](https://www.medicalnewstoday.com/articles/fever-temperature#average-temperature) - [Causes](https://www.medicalnewstoday.com/articles/fever-temperature#causes) - [How to measure](https://www.medicalnewstoday.com/articles/fever-temperature#how-to-measure) - [When to seek help](https://www.medicalnewstoday.com/articles/fever-temperature#when-to-seek-help) - [Summary](https://www.medicalnewstoday.com/articles/fever-temperature#summary) The temperature that constitutes a fever varies by age and the method of measuring it. Generally, doctors consider any body temperature over 100.4°F (38°C) a fever. A fever is a body temperature that is higher than usual due to an infection or illness. 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. ## [What temperature is considered a fever?]()  [Share on Pinterest](https://www.pinterest.com/pin/create/button/?url=https%3A%2F%2Fwww.medicalnewstoday.com%2Farticles%2Ffever-temperature&media=https%3A%2F%2Fpost.medicalnewstoday.com%2Fwp-content%2Fuploads%2Fsites%2F3%2F2021%2F01%2FGettyImages-1245148140_header-1024x575.jpg&description=Fever%20symptoms%3A%20What%20temperature%20is%20considered%20a%20fever%3F "Share on Pinterest") Geber86/Getty Images Generally, doctors consider a temperature of [100\.4°F (38°C)](https://www.nhs.uk/conditions/fever-in-adults/) or above to be a fever. However, several factors can influence the threshold, including: ### Individual differences The typical [body temperature](https://www.medicalnewstoday.com/articles/323819/) for an adult is around [98\.6°F (37°C)](https://www.ncbi.nlm.nih.gov/books/NBK562334/), although this can vary slightly from person to person. Some people are naturally a little hotter or colder than this. Any temperature that is persistently higher than usual for a given individual could be a sign of a fever, particularly if they have other symptoms that suggest an infection, such as: - [chills](https://www.medicalnewstoday.com/articles/chills) - [fatigue](https://www.medicalnewstoday.com/articles/248002) - [body aches](https://www.medicalnewstoday.com/articles/319985) - [headache](https://www.medicalnewstoday.com/articles/headache-and-fever) - [cough](https://www.medicalnewstoday.com/articles/327142) - [stuffy](https://www.medicalnewstoday.com/articles/313808) or [runny nose](https://www.medicalnewstoday.com/articles/325248) - burning sensation when urinating - [diarrhea](https://www.medicalnewstoday.com/articles/158634) - skin discoloration ### Age The temperature that doctors consider a fever for adults and children is often the same, at [100\.4°F (38°C)](https://www.nhs.uk/conditions/fever-in-children/). However, some guidelines state that the threshold for children over 3 months old is slightly higher, at [101\.3°F (38.5°C)](https://www.ncbi.nlm.nih.gov/books/NBK279455/) or more. [Some guidelines](https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/) also set the threshold a little lower for older adults. ### Differences by method There are several methods of taking a person’s body temperature with a thermometer, including: - rectally - orally - in the ear - in the armpit - on the forehead Body temperature measurements can vary by as much as [0\.9°F to 1.8°F](https://journals.lww.com/tnpj/Fulltext/2018/04000/Noninfectious_causes_of_fever_in_adults.6.aspx) (0.5°C to 1°C) depending on the anatomic site. A [2021 review](https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/) notes that what experts consider a fever in each case varies slightly. A medical textbook, Harrison’s Principles of Internal Medicine, defines it as follows: | Method | High temperature | |---|---| | Rectal | 99\.5°F to 100.9°F (37.5°C to 38.3°C) | | Oral | more than 99°F (37.2°C) in the morning or 99.9°F (37.7°C) in late afternoon | | Armpit | more than 99°F (37.2°C) | A rectal temperature is the most accurate measure of core body temperature, but other methods are less invasive and more convenient in many cases. ## [What is a typical body temperature?]() The average body temperature is about [98\.6°F (37°C)](https://www.ncbi.nlm.nih.gov/books/NBK562334/). However, body temperatures can vary between individuals and may fluctuate by as much as 0.9°F (0.5°C) over the course of a day. ## [Possible causes of fever]() The causes of a fever include: ### Common infections Various [viral and bacterial infections](https://www.medicalnewstoday.com/articles/bacterial-vs-viral-infections/) can trigger a fever, such as: - [flu](https://www.medicalnewstoday.com/articles/15107) - [strep throat](https://www.medicalnewstoday.com/articles/312433) - [urinary tract infections](https://www.medicalnewstoday.com/articles/189953) - [gastroenteritis](https://www.medicalnewstoday.com/articles/acute-gastroenteritis-treatment) - [COVID-19](https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/symptoms.html) ### Autoimmune conditions and inflammation Conditions that affect the immune system in other ways can also result in fever. For example, [autoimmune conditions](https://www.medicalnewstoday.com/articles/311852/) occur when the immune system mistakenly attacks healthy tissue. This attack can involve an elevated body temperature. Examples of conditions that may cause this include: - [rheumatoid arthritis](https://www.medicalnewstoday.com/articles/323361) - [systemic lupus](https://www.medicalnewstoday.com/articles/323653) - [rheumatic fever](https://www.medicalnewstoday.com/articles/176648) ### Cancer Some [cancers](https://www.medicalnewstoday.com/articles/323648) can cause fever, including [lymphoma](https://www.medicalnewstoday.com/articles/146136). Health experts believe that cancer cells produce chemical signals that increase body temperature, leading to a fever. ## [How to take a temperature]() A thermometer can measure body temperature. However, do not use a glass or [mercury](https://www.medicalnewstoday.com/articles/320563/) thermometer. Mercury is toxic and could cause poisoning if the thermometer breaks. Instead, doctors usually suggest using a digital thermometer. People can use these orally or in the armpit. ### Oral temperature To use a digital thermometer orally, 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 before taking an oral temperature. The temperature of the food and drink may alter the results. ### Armpit temperature For babies, it can be easier to measure their temperature via the armpit. To do this: 1. Lie them down or hold them on the knees. 2. Put the thermometer in their armpit and gently hold their arm against their body. 3. Hold in place according to the thermometer’s instructions. Usually, this is around [15 seconds](https://www.nhs.uk/baby/health/how-to-take-your-babys-temperature/). 4. Read the display. ## [When to seek help]() 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, people should get immediate help if a person has: - a high grade fever of [102\.4°F (39.1°C)](https://www.ncbi.nlm.nih.gov/books/NBK562334/) or more - severe headache - pain or swelling in any area of the body - difficulty breathing - [a stiff neck](https://www.medicalnewstoday.com/articles/meningitis-stiff-neck) - [a rash](https://www.medicalnewstoday.com/articles/324726/) that does not disappear under a glass - [confusion](https://www.medicalnewstoday.com/articles/confusion), [delirium](https://www.medicalnewstoday.com/articles/326684/), or [loss of consciousness](https://www.medicalnewstoday.com/articles/182524) - signs of severe [dehydration](https://www.medicalnewstoday.com/articles/153363), such as a lack of [sweat](https://www.medicalnewstoday.com/articles/266427/) or [urination](https://www.medicalnewstoday.com/articles/325398) ## [Summary]() The temperature that constitutes a fever varies slightly by age and where on the body someone takes the temperature. Usually, though, 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 a 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 contact a doctor. - [Urinary Tract Infection](https://www.medicalnewstoday.com/categories/urinary-tract-infection) - [Emergency Medicine](https://www.medicalnewstoday.com/categories/emergency-medicine) - [Immune System / Vaccines](https://www.medicalnewstoday.com/categories/immune-system) - [Infectious Diseases / Bacteria / Viruses](https://www.medicalnewstoday.com/categories/infectious_diseases) - [Parenthood](https://www.medicalnewstoday.com/categories/parenthood) - [COVID-19](https://www.medicalnewstoday.com/categories/covid-19) ### How we reviewed this article: Sources Medical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our [editorial policy](https://www.medicalnewstoday.com/articles/process). - Balli S, et al. (2023). Physiology, fever. <https://www.ncbi.nlm.nih.gov/books/NBK562334/> - High temperature (fever) in adults. (2023). <https://www.nhs.uk/conditions/fever-in-adults/> - High temperature (fever) in children. (2024). <https://www.nhs.uk/conditions/fever-in-children/> - How to take your baby's temperature. (2023). <https://www.nhs.uk/baby/health/how-to-take-your-babys-temperature/> - How is body temperature regulated and what is fever? (2022). <https://www.ncbi.nlm.nih.gov/books/NBK279457/> - Mackowiak PA, et al. (2021). Defining fever. <https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/> - Steele GM, et al. (2018). Noninfectious causes of fever in adults. <https://journals.lww.com/tnpj/Fulltext/2018/04000/Noninfectious_causes_of_fever_in_adults.6.aspx> Share this article  [Medically reviewed]() by [Cynthia Taylor Chavoustie, MPAS, PA-C]() — Written by [MaryAnn De Pietro, CRT]() — [Updated on May 30, 2025]() ## Latest news - [Two days of oatmeal may help reduce 'bad' cholesterol levels, study finds](https://www.medicalnewstoday.com/articles/two-days-oatmeal-reduce-ldl-cholesterol-levels-study) - [Is sleep the missing piece in mental health?](https://www.medicalnewstoday.com/articles/is-sleep-the-missing-piece-in-mental-health) - [Is this why dementia affects more women? How menopause alters the brain](https://www.medicalnewstoday.com/articles/dementia-menopause-brain-health-gray-matter-memory-loss) - [Low dose aspirin does not reduce cancer risk in older adults, long-term study finds](https://www.medicalnewstoday.com/articles/low-dose-aspirin-does-not-reduce-cancer-risk-in-older-adults) - [Can treating rheumatoid arthritis before it sets in help delay its onset?](https://www.medicalnewstoday.com/articles/pretreatment-rheumatoid-arthritis-onset-delay-4-years) ### Related Coverage - [What causes a fever and what can I do about it?](https://www.medicalnewstoday.com/articles/9895?utm_source=ReadNext) Medically reviewed by [University of Illinois](https://www.medicalnewstoday.com/reviewers) [A fever is when the body's temperature rises above 100.4°F or 38°C. It is the body's response to something wrong in one part or another, and it may…](https://www.medicalnewstoday.com/articles/9895?utm_source=ReadNext) [READ MORE](https://www.medicalnewstoday.com/articles/9895?utm_source=ReadNext) - [When is a fever in adults serious?](https://www.medicalnewstoday.com/articles/fever-in-adults?utm_source=ReadNext) Medically reviewed by [Alana Biggers, M.D., MPH](https://www.medicalnewstoday.com/reviewers) [A fever is a natural response to an infection. However, it can be serious. Learn about the different types of fever and when to seek help.](https://www.medicalnewstoday.com/articles/fever-in-adults?utm_source=ReadNext) [READ MORE](https://www.medicalnewstoday.com/articles/fever-in-adults?utm_source=ReadNext) - [How dangerous is a fever in toddlers, and when to seek help](https://www.medicalnewstoday.com/articles/fever-in-toddlers?utm_source=ReadNext) Medically reviewed by [Karen Gill, M.D.](https://www.medicalnewstoday.com/reviewers) [Fever in toddlers normally resolves without treatment. However, some situations warrant medical attention. Learn more about when to seek help here.](https://www.medicalnewstoday.com/articles/fever-in-toddlers?utm_source=ReadNext) [READ MORE](https://www.medicalnewstoday.com/articles/fever-in-toddlers?utm_source=ReadNext) - [Ilaris side effects: What you should know](https://www.medicalnewstoday.com/articles/drugs-ilaris-side-effects?utm_source=ReadNext) [Ilaris, which treats conditions including Still’s disease, can cause side effects such as unexpected weight gain. Learn about side effects and how to…](https://www.medicalnewstoday.com/articles/drugs-ilaris-side-effects?utm_source=ReadNext) [READ MORE](https://www.medicalnewstoday.com/articles/drugs-ilaris-side-effects?utm_source=ReadNext) - [Ilaris dosage](https://www.medicalnewstoday.com/articles/drugs-ilaris-dosage?utm_source=ReadNext) [Ilaris is used to treat certain periodic fever conditions and Still's disease. Find out what the recommended dosages are, how the drug is given, and…](https://www.medicalnewstoday.com/articles/drugs-ilaris-dosage?utm_source=ReadNext) [READ MORE](https://www.medicalnewstoday.com/articles/drugs-ilaris-dosage?utm_source=ReadNext) © 2026 Healthline Media UK Ltd, London, UK. All rights reserved. MNT is the registered trade mark of Healthline Media. Healthline Media is an RVO Health Company. Any medical information published on this website is not intended as a substitute for informed medical advice and you should not take any action before consulting with a healthcare professional. [See additional information](https://www.medicalnewstoday.com/about/additional-information). - [About Us](https://www.medicalnewstoday.com/about) - [Contact Us](https://www.medicalnewstoday.com/about/contact-us) - [Terms of Use](https://www.medicalnewstoday.com/about/terms-of-use) - [Privacy Policy](https://www.medicalnewstoday.com/about/privacy-policy) - [Privacy Settings](https://www.medicalnewstoday.com/privacy-settings) - [Advertising Policy](https://www.medicalnewstoday.com/ad-policy) - [Health Topics](https://www.medicalnewstoday.com/directory/a-b) - [Health Hubs](https://www.medicalnewstoday.com/articles/content-hubs) - [Medical Affairs](https://www.medicalnewstoday.com/about/reviewers) - [Content Integrity](https://www.medicalnewstoday.com/about/content-integrity) - [Newsletters](https://www.medicalnewstoday.com/newsletter) - © 2026 Healthline Media UK Ltd, London, UK. All rights reserved. MNT is the registered trade mark of Healthline Media. Healthline Media is an RVO Health Company. Any medical information published on this website is not intended as a substitute for informed medical advice and you should not take any action before consulting with a healthcare professional. [See additional information](https://www.medicalnewstoday.com/about/additional-information). © 2026 Healthline Media UK Ltd, London, UK. All rights reserved. MNT is the registered trade mark of Healthline Media. Healthline Media is an RVO Health Company. Any medical information published on this website is not intended as a substitute for informed medical advice and you should not take any action before consulting with a healthcare professional. [See additional information](https://www.medicalnewstoday.com/about/additional-information). [About](https://www.healthlinemedia.com/?utm_source=medicalnewstoday.com&utm_medium=site&utm_campaign=footer&utm_content=about)[Careers](https://rvohealth.com/careers)[Advertise with us](https://www.healthlinemedia.com/advertise?utm_source=medicalnewstoday.com&utm_medium=site&utm_campaign=footer&utm_content=advertise) OUR BRANDS [Healthline](https://www.healthline.com/)[Medical News Today](https://www.medicalnewstoday.com/)[Greatist](https://greatist.com/)[Psych Central](https://psychcentral.com/)[Bezzy](https://bezzy.com/) 

The temperature that constitutes a fever varies by age and the method of measuring it. Generally, doctors consider any body temperature over 100.4°F (38°C) a fever. A fever is a body temperature that is higher than usual due to an infection or illness. 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. Generally, doctors consider a temperature of [100\.4°F (38°C)](https://www.nhs.uk/conditions/fever-in-adults/) or above to be a fever. However, several factors can influence the threshold, including: Individual differences The typical [body temperature](https://www.medicalnewstoday.com/articles/323819/) for an adult is around [98\.6°F (37°C)](https://www.ncbi.nlm.nih.gov/books/NBK562334/), although this can vary slightly from person to person. Some people are naturally a little hotter or colder than this. Any temperature that is persistently higher than usual for a given individual could be a sign of a fever, particularly if they have other symptoms that suggest an infection, such as: - [chills](https://www.medicalnewstoday.com/articles/chills) - [fatigue](https://www.medicalnewstoday.com/articles/248002) - [body aches](https://www.medicalnewstoday.com/articles/319985) - [headache](https://www.medicalnewstoday.com/articles/headache-and-fever) - [cough](https://www.medicalnewstoday.com/articles/327142) - [stuffy](https://www.medicalnewstoday.com/articles/313808) or [runny nose](https://www.medicalnewstoday.com/articles/325248) - burning sensation when urinating - [diarrhea](https://www.medicalnewstoday.com/articles/158634) - skin discoloration Age The temperature that doctors consider a fever for adults and children is often the same, at [100\.4°F (38°C)](https://www.nhs.uk/conditions/fever-in-children/). However, some guidelines state that the threshold for children over 3 months old is slightly higher, at [101\.3°F (38.5°C)](https://www.ncbi.nlm.nih.gov/books/NBK279455/) or more. [Some guidelines](https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/) also set the threshold a little lower for older adults. Differences by method There are several methods of taking a person’s body temperature with a thermometer, including: - rectally - orally - in the ear - in the armpit - on the forehead Body temperature measurements can vary by as much as [0\.9°F to 1.8°F](https://journals.lww.com/tnpj/Fulltext/2018/04000/Noninfectious_causes_of_fever_in_adults.6.aspx) (0.5°C to 1°C) depending on the anatomic site. A [2021 review](https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/) notes that what experts consider a fever in each case varies slightly. A medical textbook, Harrison’s Principles of Internal Medicine, defines it as follows: | Method | High temperature | |---|---| | Rectal | 99\.5°F to 100.9°F (37.5°C to 38.3°C) | | Oral | more than 99°F (37.2°C) in the morning or 99.9°F (37.7°C) in late afternoon | | Armpit | more than 99°F (37.2°C) | A rectal temperature is the most accurate measure of core body temperature, but other methods are less invasive and more convenient in many cases. The average body temperature is about [98\.6°F (37°C)](https://www.ncbi.nlm.nih.gov/books/NBK562334/). However, body temperatures can vary between individuals and may fluctuate by as much as 0.9°F (0.5°C) over the course of a day. A thermometer can measure body temperature. However, do not use a glass or [mercury](https://www.medicalnewstoday.com/articles/320563/) thermometer. Mercury is toxic and could cause poisoning if the thermometer breaks. Instead, doctors usually suggest using a digital thermometer. People can use these orally or in the armpit. Oral temperature To use a digital thermometer orally, 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 before taking an oral temperature. The temperature of the food and drink may alter the results. Armpit temperature For babies, it can be easier to measure their temperature via the armpit. To do this: 1. Lie them down or hold them on the knees. 2. Put the thermometer in their armpit and gently hold their arm against their body. 3. Hold in place according to the thermometer’s instructions. Usually, this is around [15 seconds](https://www.nhs.uk/baby/health/how-to-take-your-babys-temperature/). 4. Read the display. The temperature that constitutes a fever varies slightly by age and where on the body someone takes the temperature. Usually, though, 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 a 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 contact a doctor.