A temperature of 100.4°F (38°C) or higher is considered a fever for both adults and children. That threshold applies when measured rectally, orally, or with an ear or forehead thermometer. Armpit readings run lower, so 99°F (37.2°C) or higher in the armpit also qualifies as a fever.
Why 100.4°F Is the Standard
For decades, 98.6°F was treated as the universal “normal” body temperature. That number dates back to 1868. More recent research paints a different picture. A Stanford Medicine study analyzing over 618,000 oral temperature readings found that the average body temperature in the U.S. today is closer to 97.9°F, with most healthy adults falling between 97.3°F and 98.2°F. The slow decline, roughly 0.05°F per decade since the 19th century, is likely tied to improvements in overall health and lower levels of chronic inflammation.
Despite this shift in what “normal” looks like, 100.4°F remains the clinical cutoff for a fever. That’s because a reading at or above that level reliably signals the body’s immune system has kicked into a higher gear, regardless of where your personal baseline sits.
How Measurement Location Changes the Number
Not every thermometer placement gives you the same reading. Different parts of the body run at slightly different temperatures, so the number that counts as a fever depends on where you take it.
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral (under the tongue): 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
Armpit readings are the least accurate of the three. If you get a borderline result from an armpit measurement, it’s worth rechecking with an oral or forehead thermometer to confirm. Rectal readings are the gold standard for accuracy, especially in infants, because they reflect core body temperature most closely.
Fever Thresholds in Babies and Young Children
The same 100.4°F cutoff applies to infants and children, but the stakes are higher at younger ages. The American Academy of Pediatrics flags any rectal temperature of 100.4°F or above in babies between 8 and 60 days old as something that needs prompt medical evaluation, even if the baby looks well. A young infant’s immune system is still developing, and fevers in this age range can sometimes signal serious infections that aren’t obvious from outward symptoms alone.
For older babies and toddlers, a fever of 100.4°F is common with routine viral illnesses and isn’t automatically a cause for alarm. How your child is acting, whether they’re drinking fluids, staying alert, and responsive, matters as much as the number on the thermometer.
Low-Grade vs. High Fever
Not all fevers carry the same weight. A temperature between 99°F and 100.3°F is often called a low-grade fever. It can show up after exercise, in the late afternoon (body temperature naturally peaks in the evening), or during mild illness. Many people with a low-grade fever feel mostly fine.
Once you cross 100.4°F, it’s a true fever, and the body is actively fighting something. Temperatures between 100.4°F and 103°F are typical of common infections like the flu, colds, or urinary tract infections. You may feel achy, chilled, fatigued, or sweaty as your body works to create an environment that’s hostile to whatever pathogen triggered the response.
A fever above 104°F (40°C) is considered high and warrants a call to your doctor. At that level, the risk of complications rises, and it’s important to identify the underlying cause rather than just managing the symptom.
Your Temperature Fluctuates Throughout the Day
A single reading doesn’t tell the whole story. Body temperature naturally rises and falls over a 24-hour cycle. It tends to be lowest in the early morning, sometimes dipping below 97.5°F, and highest in the late afternoon or evening. This means a temperature of 99.5°F at 7 a.m. is more significant than the same reading at 5 p.m., when your body is already running warmer.
Other factors push your baseline around too. Physical activity, heavy meals, hormonal shifts during the menstrual cycle, and even warm clothing can bump your reading up by a degree or so without any illness involved. If you’re checking for a fever, try to measure at rest, after sitting quietly for a few minutes.
When a Fever Needs Urgent Attention
Most fevers in otherwise healthy adults resolve on their own within a few days. But certain combinations of fever plus other symptoms point to something more serious. Seek immediate medical care if a fever comes with any of the following:
- Seizure or loss of consciousness
- Confusion or difficulty staying alert
- A stiff neck
- Trouble breathing
- Severe pain anywhere in the body
- Significant swelling or inflammation
- Pain during urination or foul-smelling urine
A fever lasting more than three days, or one that keeps returning after it seemed to break, is also worth investigating. The fever itself isn’t the danger. It’s a signal, and sometimes what it’s signaling needs attention beyond rest and fluids.