A fever in adults and children is defined as a body temperature of 100.4°F (38°C) or higher. That number applies to rectal, ear, and forehead readings. Oral temperatures read slightly lower, so a fever by mouth starts around 100°F (37.8°C).
Why 100.4°F, Not 98.6°F
The familiar 98.6°F “normal” comes from a German physician who published over a million temperature readings in 1868. It was a useful average for its time, but research from Stanford Medicine analyzing more than 618,000 oral temperature readings found that the actual average body temperature today is closer to 97.9°F. The decline, roughly 0.05°F per decade since the 1800s, likely reflects improvements in health and living conditions that have reduced chronic inflammation.
Normal body temperature also isn’t a single fixed number. It varies by person based on age, sex, height, and weight. It shifts throughout the day, too: your temperature is lowest during sleep, rises in the morning hours just before you wake, dips slightly again between 2 p.m. and 4 p.m., and peaks in the late afternoon or early evening. A reading of 99°F in the late afternoon might be perfectly normal for you, while the same reading at 6 a.m. could signal a real fever. Context matters more than any single number.
How Readings Differ by Thermometer Location
Where you take your temperature changes the number on the thermometer. Rectal and ear readings run about 0.5°F to 1°F higher than oral readings. Armpit and forehead scans run about 0.5°F to 1°F lower than oral readings. That means a 99.5°F armpit reading and a 101°F rectal reading could reflect the same actual body temperature.
For children under 3, rectal thermometers are considered the most accurate. The fever thresholds by measurement site for kids break down like this:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
Fever Thresholds for Older Adults
Adults over 65 tend to run cooler at baseline than younger adults. Using the standard 100.4°F cutoff can miss real infections in this group. Research from the University of Maryland found that lowering the fever threshold for older adults to 99°F (37.2°C) increased the ability to detect bacterial infections from 40% to 83%, while still correctly ruling out non-fevers 89% of the time. If you’re caring for an older adult and see a temperature of 99°F or above, especially with new confusion, fatigue, or loss of appetite, treat it with the same seriousness you’d give a higher fever in a younger person.
When a Fever Becomes Dangerous
Most fevers in the 100.4°F to 103°F range are the body’s normal immune response to infection and resolve on their own. The brain has a built-in thermostat that rarely lets temperature climb to dangerous levels from infection alone.
At 104°F (40°C) and above, you should contact a doctor. At 106.7°F (41.5°C), the fever enters a category called hyperpyrexia, which is a medical emergency. Temperatures this extreme are rare with ordinary infections and more commonly result from heat stroke, reactions to anesthesia, or certain drug reactions.
The temperature number alone doesn’t tell the full story. A fever of 102°F with confusion, stiff neck, trouble breathing, seizures, or severe pain anywhere in the body warrants immediate medical attention regardless of how “low” the number looks. Persistent fever lasting several days also deserves a call to your doctor, even if it never spikes especially high.
Fever in Babies Under 3 Months
Any rectal temperature of 100.4°F or higher in a baby younger than 3 months is treated seriously, even if the baby looks perfectly fine. Newborns have immature immune systems that can harbor dangerous infections without showing obvious signs of illness. The American Academy of Pediatrics has specific evaluation guidelines for infants 8 to 60 days old with fever, with the youngest babies (under 21 days) receiving the most cautious workup. A fever at this age always warrants a prompt call to your pediatrician or a visit to the emergency department.
For older babies and toddlers, a fever alone is less alarming. How the child acts matters more than the number: a child with 103°F who is drinking fluids, making eye contact, and playing between bouts of fussiness is generally in better shape than a child with 101°F who is limp, unresponsive, or refusing all fluids.