A body temperature of 100.4°F (38°C) or higher is generally considered a fever in both adults and children. That said, the number can shift slightly depending on where you measure and who you’re measuring, so the full picture is worth understanding.
The Standard Fever Threshold
The widely accepted cutoff is 100.4°F (38°C), measured rectally, in the ear, or at the temporal artery (forehead). Oral readings run slightly lower, so an oral temperature of 100°F (37.8°C) or higher also qualifies. Armpit measurements are the least precise, and a reading of 99°F (37.2°C) or higher in the armpit is considered a fever.
These thresholds apply to both adults and children, though the stakes and responses differ by age group.
Low-Grade Fever vs. High Fever
Not all fevers carry the same weight. Many healthcare providers classify a body temperature between 99.5°F (37.5°C) and 100.3°F (37.9°C) as a low-grade fever. You might feel slightly warm, a little run-down, or not notice it at all. Low-grade fevers often accompany minor viral infections and typically resolve on their own.
Once a fever climbs to 103°F (39.4°C) or higher in adults, most people look and feel noticeably sick. Above 106.7°F (41.5°C), the body enters a dangerous state called hyperpyrexia. This is a medical emergency that requires immediate treatment, as temperatures that high can damage organs and become life-threatening without rapid intervention.
Why 98.6°F Isn’t Really “Normal”
The famous 98.6°F benchmark dates back to the 1800s and has become deeply embedded in how people think about body temperature. But it’s outdated. Research from Stanford Medicine, based on more than 618,000 oral temperature readings from adult patients, found that today’s average body temperature is closer to 97.9°F. The average has actually been dropping by about 0.05°F per decade since the 19th century, likely because improvements in health and living conditions have reduced chronic inflammation across the population.
Your personal baseline also depends on your age, sex, height, and weight. Body temperature fluctuates throughout the day, too: it tends to be lowest in the early morning and highest in the late afternoon. These normal swings mean a reading of 99°F at 4 p.m. might be perfectly ordinary for you, while the same number at 6 a.m. could signal something is off.
This variability matters in real-world situations. Older adults, for instance, tend to run cooler at baseline. A temperature that technically falls below the 100.4°F threshold could still represent a significant fever for an elderly person. Stanford researcher Julie Parsonnet has described the case of an elderly family member whose serious heart infection went undiagnosed for weeks because her temperature never reached a conventional fever cutoff.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s a deliberate defensive response. When your immune system detects an infection, it releases signaling molecules that act on the hypothalamus, the part of your brain that works like a thermostat. These signals raise the hypothalamus’s “set point,” essentially telling your body that 98°F is no longer the target and something higher is.
Your body then works to reach that new target. Blood vessels near the skin constrict to reduce heat loss, which is why you might feel cold or get chills even though your temperature is rising. Shivering kicks in to generate additional heat through muscle activity. These processes continue until your blood temperature matches the new set point. It’s also why, once a fever breaks, you often sweat heavily: the set point drops back to normal, and your body needs to dump the excess heat quickly.
Where You Measure Matters
Different measurement sites give different readings because they reflect different levels of proximity to your core body temperature. Rectal readings are the most accurate, especially for infants and young children, because they measure closest to the body’s actual internal temperature. Ear (tympanic) and forehead (temporal artery) thermometers are also reliable and more comfortable for most people. Oral thermometers work well for older children and adults, though eating, drinking, or breathing through your mouth right before can skew the reading. Armpit measurements are the least reliable and consistently read lower than other methods.
Because of these differences, the fever thresholds shift by location:
- 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
If you’re comparing readings from different thermometers or different body sites, keep these offsets in mind. A 99.5°F armpit reading and a 100.4°F rectal reading may represent the same actual fever.
Fever Thresholds in Children and Infants
The same temperature numbers apply to children, but the urgency changes dramatically with age. Any fever of 100.4°F or higher in a baby under 3 months old warrants prompt medical attention, regardless of how the baby appears. Young infants have immature immune systems, and a fever can be the only early sign of a serious infection.
For older babies and toddlers, the fever number matters less than the child’s behavior. A child with a 102°F temperature who is playing normally and drinking fluids is in a very different situation than one with a 101°F fever who is listless and refusing to eat. In children, how they look and act is often a better gauge of severity than the thermometer reading alone.