A temperature of 98 degrees Fahrenheit is not a fever. It falls well within the normal range for a healthy adult, which spans roughly 97°F to 99°F. In fact, 98°F is closer to what modern research considers the true average human body temperature than the familiar 98.6°F benchmark most of us grew up with.
Why 98.6°F Isn’t the Magic Number
The idea that 98.6°F is “normal” dates back to 1868, when a German physician named Carl Wunderlich published a massive study of over a million temperature readings from 25,000 patients. He declared 37.0°C (98.6°F) the mean temperature of healthy adults, and the number stuck for more than a century.
Modern research tells a different story. A well-known reanalysis from Stanford found that the actual mean oral temperature is closer to 98.2°F. Wunderlich’s original readings were taken under the armpit using bulky thermometers that needed 15 to 20 minutes to settle, a method and tool quite different from what’s used today. The researchers concluded that 98.6°F “should be abandoned as a concept having any particular significance” for normal body temperature. So if your thermometer reads 98°F, you’re sitting right near the modern average.
What Temperature Actually Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That’s the threshold most doctors, hospitals, and public health agencies use. A reading of 98°F is a full 2.4 degrees below that line.
The Mayo Clinic breaks it down by measurement site:
- Oral, rectal, ear, or forehead: 100.4°F or higher
- Armpit: 99°F or higher
Armpit readings run about 0.5°F to 1°F lower than oral readings, which is why the threshold is set lower for that method. If you took an armpit reading of 98°F, that’s roughly equivalent to an oral temperature of 98.5°F to 99°F, still normal.
The 99°F to 100.3°F Gray Zone
Temperatures between 99°F and 100.3°F sometimes get called a “low-grade fever,” but they don’t meet the clinical definition. Your body temperature naturally fluctuates throughout the day. It tends to be lowest in the early morning and peaks in the late afternoon or evening. Physical activity, hormonal cycles, a warm drink, or even heavy clothing can nudge your reading up by a degree without anything being wrong.
This is why a single temperature reading isn’t always meaningful on its own. A person who normally runs at 97.4°F might feel off at 99°F, while someone who naturally runs warmer might feel fine at the same number. Context matters more than any single cutoff.
Where You Measure Changes the Number
Not all thermometer placements give identical readings. Rectal and ear thermometers tend to read 0.5°F to 1°F higher than an oral thermometer. Armpit thermometers read 0.5°F to 1°F lower. So 98°F on an armpit thermometer is not the same as 98°F on a rectal thermometer, even if the number looks identical.
If you’re comparing your reading to the 100.4°F fever threshold, make sure you know which site you used. Most guidelines assume an oral reading unless stated otherwise.
Age Affects What’s Normal
Older adults tend to run lower baseline temperatures than younger people. A reading of 98°F in an older adult could actually be on the warmer side of their personal normal. This is important because elderly individuals can have a serious infection without ever reaching 100.4°F. For them, even a modest rise from their usual temperature can be significant.
At the other end of the spectrum, babies under three months old need immediate medical attention for any rectal temperature of 100.4°F or higher, even if they seem otherwise fine. Their immune systems are still developing, and fever in that age group can signal something that needs quick evaluation. A reading of 98°F in an infant, however, is perfectly normal.
When a Low Temperature Is a Concern
While 98°F is normal, temperatures that drop well below the typical range can be a problem. Hypothermia begins when core body temperature falls below 95°F. Persistent readings below 97°F without an obvious cause (like being in a cold environment) are worth paying attention to, particularly in older adults or people with certain medical conditions like an underactive thyroid.