A fever is generally defined as a body temperature of 100.4°F (38°C) or higher, measured orally. This is the threshold used by the CDC and most healthcare providers. But the number isn’t as fixed as it sounds, because your “normal” baseline, the time of day, your age, and where you take the reading all shift what counts as elevated.
Why 100.4°F Is the Standard
The 100.4°F cutoff is a clinical standard, but it comes with context. The old benchmark of 98.6°F as “normal” dates back to 1868, and it’s no longer accurate for most people. Researchers at Stanford Medicine analyzed over 618,000 temperature readings from adult outpatients and found that today’s average body temperature is closer to 97.9°F. Normal readings ranged from 97.3°F to 98.2°F. Average body temperature has dropped about 0.05°F per decade since the 19th century, likely because modern living conditions have reduced chronic inflammation.
So for many people, a reading of 100.4°F represents a jump of roughly 2.5 degrees above their personal baseline, not the 1.8 degrees you’d calculate from 98.6°F. That’s why the 100.4°F threshold still works as a practical dividing line, even though the “normal” number has shifted downward.
Low-Grade vs. High Fever
Not every elevated temperature is a full fever. Many providers consider a body temperature between 99.5°F (37.5°C) and 100.3°F (37.9°C) to be a low-grade fever. You might feel slightly warm or “off” but not outright sick. Low-grade fevers often accompany mild viral infections, ovulation, or physical exertion and typically resolve on their own.
Once you cross 100.4°F, you’re in standard fever territory. Temperatures above 103°F in adults are considered high and worth monitoring closely. At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At that level, your organs start to struggle. The brain, heart, kidneys, and liver can all sustain damage, and without rapid treatment, hyperpyrexia can cause brain swelling, permanent organ damage, or death.
Your Temperature Changes Throughout the Day
Your body doesn’t sit at one temperature all day. It follows a natural cycle, dipping to its lowest point in the early morning (around 4 to 6 a.m.) and peaking in the early evening (around 8 p.m.). The difference between your daily low and high can range from 0.5°F to nearly 2°F. This means a reading of 99.5°F at 9 p.m. might be completely normal for you, while that same reading at 6 a.m. could signal the early stages of an illness.
Age, sex, height, and weight also play a role. Stanford’s research found that these factors, combined with time of day, accounted for about 25% of the temperature variation within a single person. Your personal normal might run consistently warmer or cooler than someone else’s.
Where You Measure Matters
Different parts of the body give different readings, and the fever threshold shifts accordingly. The 100.4°F standard applies to oral readings (under the tongue). Here’s how other methods compare:
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher indicates a fever. Rectal readings tend to run about 0.5 to 1°F higher than oral, so they use the same cutoff.
- Oral: 100°F (37.8°C) or higher. Some sources use 100.4°F, others use 100°F as the threshold, reflecting slight differences in clinical guidelines.
- Armpit (axillary): 99°F (37.2°C) or higher. Armpit readings run lower than oral, so the threshold drops to compensate.
If you’re using a forehead or ear thermometer and get a borderline reading, switching to an oral measurement gives you a more reliable comparison to the standard cutoff.
Fever Thresholds for Children
For infants and young children, the same 100.4°F (38°C) threshold applies, but the stakes are higher. The American Academy of Pediatrics uses a rectal temperature of 100.4°F or above as the definition of fever in infants. For babies under 2 months old, any fever at that level is treated seriously because their immune systems are still developing, and infections can progress quickly.
Rectal thermometers are the most accurate method for infants. Armpit readings are sometimes used as a quick screen, but if an armpit reading suggests fever, a rectal measurement should follow to confirm.
Fever Thresholds for Older Adults
People over 65 are a special case. Baseline body temperature tends to run lower with age, and the immune system’s fever response weakens. An older adult can have a serious infection without ever reaching 100.4°F. CDC guidelines for long-term care residents define fever more broadly: a single oral temperature above 100°F, repeated readings above 99°F, or any increase of more than 2°F above that person’s known baseline.
This means that for an older adult who normally runs at 97.5°F, a reading of 99.6°F could be clinically significant, even though it falls well below the standard fever cutoff. If you’re monitoring an older family member, knowing their typical temperature gives you a much better reference point than the generic 100.4°F threshold.