A temperature of 100.4°F (38°C) or higher is generally considered a fever in both adults and children. That’s the threshold most hospitals, pediatricians, and public health guidelines use. But the number that counts as a fever actually shifts depending on where you measure, how old you are, and what your normal baseline looks like.
Fever Thresholds by Measurement Method
Not all thermometers give you the same reading, even at the same moment. A rectal thermometer typically reads 0.5 to 1°F higher than an oral one, while an armpit or forehead thermometer usually reads 0.5 to 1°F lower. That means the same fever can show up as different numbers depending on the method you use.
Here’s what counts as a fever for each type:
- Oral (under the tongue): 100.4°F (38°C) or higher
- Rectal: 100.4°F (38°C) or higher
- Ear (tympanic): 100.4°F (38°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
- Forehead (temporal): roughly 0.5 to 1°F lower than an oral reading, so around 99°F or higher
Rectal readings are considered the most accurate, which is why they’re the standard for infants. Armpit readings are the least precise and tend to underestimate your actual core temperature.
Low-Grade vs. High Fevers
Many providers consider a body temperature between 99.5°F and 100.3°F a low-grade fever. It’s elevated above normal but below the standard 100.4°F cutoff. Low-grade fevers are common with mild viral infections and often resolve on their own without treatment.
Once a fever climbs above 100.4°F, it’s considered a true fever. A mild fever under 101°F usually doesn’t need medication. In adults, fevers below 103°F are typically not dangerous on their own. Untreated fevers above 105.8°F, however, can become genuinely dangerous and require urgent care.
Why Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate response your immune system launches to fight infection. When your body detects bacteria, viruses, or other invaders, immune cells release signaling molecules that travel through the bloodstream to a temperature-control center deep in the brain. That center works like a thermostat. Normally it’s set around 98.6°F, but during an infection, those immune signals trigger the release of a chemical messenger that pushes the set point higher.
Once the set point rises, your body treats its current temperature as “too cold.” Blood vessels near the skin constrict to trap heat, your metabolism ramps up to generate more warmth, and you start shivering. That’s why you can feel freezing cold even as your temperature climbs. The process continues until your blood temperature matches the new, higher set point. This elevated temperature makes your body a less hospitable environment for many pathogens and can speed up certain immune responses.
Fever Thresholds for Babies and Children
For infants, the threshold is the same 100.4°F, but the urgency is very different. Any baby under 3 months old with a rectal temperature of 100.4°F or higher needs immediate medical evaluation. At that age, even a modest fever can signal a serious bacterial infection, though the odds are reassuring: fewer than 0.05% of febrile infants develop meningitis, the most concerning possibility. Urinary tract infections are more common, showing up in about 10% of feverish infants.
For babies between 3 and 6 months, a rectal temperature above 102°F warrants a call to the pediatrician. The same goes if the baby seems unusually sluggish, irritable, or uncomfortable even with a lower fever. Between 7 and 24 months, a temperature above 102°F that persists for more than a day without other symptoms is worth a call. For older children, a fever lasting more than three days, or one accompanied by repeated vomiting, a severe headache, confusion, or a seizure, is a reason to seek care promptly.
Fever in Older Adults
Older adults run cooler than younger people. The average resting temperature in nursing home residents is about 97.7°F, a full degree below the traditional 98.6°F baseline. That means a reading of 100.4°F may actually represent a larger spike in an older person than it would in a 30-year-old.
The most reliable approach for older adults is to compare against their personal baseline. A rise of 1.4°F above their usual well temperature is considered a fever, even if the thermometer doesn’t hit 100.4°F. If no baseline is established, 99°F or higher is a reasonable rule of thumb. This matters because blunted fever responses can mask serious infections in older adults, making it easy to dismiss a “low” reading that actually signals trouble.
When a Fever Needs Attention
In adults, a temperature of 103°F or higher is the point where you should contact a healthcare provider. But the temperature alone isn’t always what matters most. Certain symptoms alongside a fever point to something more serious, regardless of the number on the thermometer:
- Stiff neck with pain when bending the head forward (a possible sign of meningitis)
- Rash or unusual sensitivity to bright light
- Confusion, altered speech, or strange behavior
- Persistent vomiting
- Difficulty breathing or chest pain
- Seizures or convulsions
- Severe headache or abdominal pain
Any of these paired with a fever calls for immediate medical attention. A fever from a hot car, rather than an infection, is also a medical emergency at any temperature. For seizures lasting longer than five minutes, call 911.
What “Normal” Temperature Actually Looks Like
The familiar 98.6°F figure dates back to the 1800s, and modern data suggests it’s a rough average at best. Body temperature fluctuates throughout the day, running lowest in the early morning and peaking in the late afternoon. Physical activity, hormonal cycles, heavy meals, and even the weather can shift your reading by half a degree or more. Some perfectly healthy people consistently run at 97.5°F, while others sit closer to 99°F.
This natural variation is why a single reading just above or below 100.4°F doesn’t tell the whole story. If you know your typical baseline, you’re in a much better position to judge whether a reading is genuinely elevated. Taking your temperature a few times when you’re feeling well gives you that reference point.