A body temperature of 100.4°F (38°C) or higher is generally considered a fever in both adults and children. But that number comes with important context: where you measure, what time of day it is, and what’s normal for your body all affect whether a reading truly counts as “high.”
Why 98.6°F Isn’t Really “Normal” Anymore
The 98.6°F (37°C) standard dates back to a 19th-century German study and has stuck around ever since. Modern research tells a different story. A study published in PLOS One found that the average body temperature across participants was 97.0°F (36.1°C), a full degree and a half below the old benchmark. Even more striking, 77% of participants had average temperatures at least 1°F below 98.6°F.
Individual normal temperatures in the study ranged from 95.4°F to 99.3°F (35.2°C to 37.4°C). That’s a huge spread. Someone whose baseline runs at 96.5°F could have a meaningful fever at 99°F, while someone who naturally sits at 99°F would be perfectly fine at that reading. This matters because using 98.6°F as the assumed normal for everyone can cause doctors to miss real fevers in people who run cool, or flag false alarms in people who run warm.
Your temperature also shifts throughout the day. It tends to be lowest in the early morning and rises in the late afternoon and evening. A reading of 99°F at 7 a.m. is more significant than the same number at 5 p.m.
Fever Thresholds by Measurement Method
Where you place the thermometer changes the number you get, because different body sites reflect core temperature differently. The Mayo Clinic defines fever using these cutoffs:
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
Rectal readings are closest to your actual core temperature and are the gold standard for infants. Armpit readings run about a degree lower than rectal, which is why the threshold is set lower. If you’re using an armpit thermometer, a number that looks modest can still qualify as a fever.
What Counts as a Fever in Babies and Children
The rules are stricter for young children, especially newborns. A rectal temperature of 100.4°F (38°C) in a baby under 3 months old is treated seriously and warrants an immediate call to their pediatrician. The American Academy of Pediatrics specifically addresses fever evaluation in infants between 8 and 60 days old at or above this threshold, because young immune systems can’t always fight infections effectively.
For older babies and toddlers, the concern level scales with both temperature and behavior. Between 3 and 6 months, a rectal temperature above 102°F (38.9°C) is a red flag, but so is a lower fever paired with unusual irritability or sluggishness. Between 7 and 24 months, 102°F or higher that lasts more than a day without other symptoms is worth a call. In children of any age, a fever lasting longer than three days needs medical attention regardless of the number on the thermometer.
Fever Grades: Low, Moderate, and High
Not all fevers are the same. A low-grade fever, roughly 99°F to 100.4°F orally, often signals that your immune system is doing its job. Many viral infections produce fevers in this range, and they typically resolve on their own within a few days.
A moderate fever falls in the 100.4°F to 103°F range. This is common with flu, ear infections, and other more active infections. You’ll likely feel noticeably unwell with chills, body aches, and fatigue, but for most healthy adults this range is manageable at home.
At 103°F (39.4°C) or higher, the Mayo Clinic recommends contacting a healthcare provider. Fevers at this level can cause significant discomfort, dehydration, and confusion, especially in older adults. As you age, your body becomes less efficient at regulating temperature, which means high fevers can escalate more quickly.
When a Fever Becomes Dangerous
A temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At this level, the heat itself starts damaging your body. Your brain, heart, lungs, liver, kidneys, and digestive system can all begin to malfunction. Without rapid treatment, hyperpyrexia can cause brain swelling, permanent brain damage, organ failure, coma, or death.
Hyperpyrexia is rare from ordinary infections. It’s more commonly seen with heatstroke, certain drug reactions, or damage to the part of the brain that controls temperature. If a thermometer reads anywhere near 106°F, that’s an emergency room situation regardless of other symptoms.
Factors That Shift Your Baseline
Several things influence what’s normal for you, which in turn affects what qualifies as “high.” Women’s temperatures fluctuate with their menstrual cycle, rising by about half a degree after ovulation. Physical activity raises temperature temporarily. Heavy clothing, a hot bath, or a warm drink right before measuring can all skew readings upward.
Older adults tend to have slightly lower baseline temperatures, which creates a hidden risk. A reading of 99°F might look unremarkable, but for someone whose normal is 96.8°F, that represents a significant jump. This is one reason infections in elderly people sometimes go undetected until they’re advanced.
If you’re trying to determine whether a reading is truly elevated, the most useful comparison isn’t 98.6°F. It’s your own baseline. Taking your temperature a few times when you’re healthy, at different times of day, gives you a personal reference point that’s far more reliable than any single universal number.