For most adults, a body temperature at or above 100.4°F (38°C) is considered a fever. That said, the traditional “normal” of 98.6°F is outdated, and your personal baseline may be lower, which affects how you interpret a reading. Here’s what the numbers actually mean.
Normal Body Temperature Isn’t 98.6°F Anymore
The 98.6°F standard dates back to the 1800s. Modern data tells a different story. Researchers at Stanford Medicine analyzed over 618,000 oral temperature readings from adults and found that today’s average body temperature is closer to 97.9°F. The normal range for adults in that study spanned from 97.3°F to 98.2°F. Average body temperature in the U.S. has dropped by roughly 0.05°F per decade since the 19th century, likely because improvements in health and living conditions have reduced chronic inflammation.
This matters because if your personal baseline runs around 97.5°F, a reading of 99.5°F represents a bigger shift than it would for someone who normally sits at 98.2°F. Knowing your own typical temperature gives you a more accurate reference point.
What Counts as a Fever
The widely used clinical threshold for a fever in adults is 100.4°F (38°C) when measured rectally or with an ear thermometer. For an oral reading, 100°F (37.8°C) or higher is generally considered a fever. An armpit reading of 99°F (37.2°C) or higher also qualifies, since armpit measurements tend to run about a degree lower than oral ones.
Temperatures between 99°F and 100.4°F are often described as a low-grade fever. This range sits above normal but below the standard fever cutoff, and it can signal that your immune system is responding to something, even if you don’t feel particularly sick. A reading above 100.4°F is a clear fever. Once a temperature hits 103°F (39.4°C) or higher, it warrants a call to your doctor.
Why Your Reading Changes Throughout the Day
Body temperature follows a built-in daily rhythm controlled by your internal clock. It’s lowest in the early morning, often dipping below 97.5°F, and peaks in the late afternoon or early evening. This swing can be a full degree or more, which means a reading of 99°F at 5 p.m. may be perfectly normal for you even though the same number at 6 a.m. would be unusual.
Physical activity, heavy clothing, a hot environment, and recent meals can also push your temperature up temporarily. Age, sex, height, and weight all play a role too. Stanford researchers found that these factors, combined with the time of day, accounted for about 25% of the variation in an individual’s normal temperature readings.
Where You Measure Makes a Difference
Not all thermometer placements give the same number. Rectal and ear (tympanic) readings run the highest and are considered the most accurate reflection of core body temperature. Oral readings typically come in about 0.4°F lower than rectal readings. Armpit (axillary) measurements are the least precise and tend to read about a degree lower than oral.
If you’re using a forehead (temporal artery) thermometer, expect results similar to ear and rectal readings. The key is consistency: use the same method each time so you can compare readings meaningfully. If you switch from an oral thermometer to an armpit one mid-illness, the numbers won’t line up.
Fever Thresholds Are Lower for Older Adults
Adults over 65 tend to have lower baseline body temperatures than younger people. This means a reading that looks borderline or even “normal” could actually represent a significant fever in an older person. Research published in the Journal of the American Medical Directors Association found that for adults aged 75 to 84, an oral temperature of just 99.1°F (37.2°C) can indicate a fever. For those 85 and older, the threshold drops even further to about 98.4°F (36.9°C).
This is clinically important because older adults may not mount the same high fevers that younger people do when fighting an infection. They’re also more likely to have atypical symptoms, so a modest temperature increase that would be dismissed in a 30-year-old could be the earliest sign of something serious in an 80-year-old. If you’re monitoring an older family member, keep these lower cutoffs in mind.
When a Fever Becomes Urgent
A temperature of 103°F (39.4°C) or higher in an adult calls for medical attention. Below that, most fevers in otherwise healthy adults are uncomfortable but not dangerous. They’re your body’s way of making conditions less hospitable for viruses and bacteria.
Regardless of the number on the thermometer, seek immediate care if a fever comes with any of the following: a severe headache, stiff neck, rash, confusion or altered speech, persistent vomiting, difficulty breathing, chest pain, abdominal pain, pain during urination, unusual sensitivity to bright light, or seizures. These symptoms alongside even a moderate fever can point to conditions that need prompt evaluation.