Normal adult body temperature averages about 97.9°F (36.6°C), not the 98.6°F most of us grew up hearing. The typical range for a healthy adult falls between 97.3°F and 98.2°F, though individual readings anywhere from 97°F in the early morning to 100°F in the evening can be perfectly normal.
Why 98.6°F Is Outdated
The 98.6°F figure comes from a German physician named Carl Wunderlich, who published a massive study in 1868 based on several million temperature readings from roughly 25,000 patients. He declared 37°C (98.6°F) the mean normal body temperature, and the number stuck for over 150 years. But there’s a problem: Wunderlich measured temperatures under the armpit using thermometers that were calibrated 2.9°F to 3.4°F higher than instruments used today. His data, while impressive in scale, doesn’t translate cleanly to modern readings.
Researchers at Stanford Medicine have since tracked a steady decline in average body temperature across more than a century of records. Men born in the early 1990s run about 1.06°F cooler than men born in the early 1800s. Women born in the 1990s average 0.58°F cooler than women born in the 1890s. That works out to a drop of roughly 0.05°F per decade. The reasons likely include lower rates of chronic infection, reduced inflammation, and changes in living conditions like widespread climate control.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It fluctuates in a predictable daily cycle, running lowest in the early morning (closer to 97°F for many people) and peaking in the late afternoon or evening (potentially reaching 99°F or slightly above). This means a reading of 99°F at 5 p.m. can be completely normal, while the same reading at 6 a.m. might warrant attention.
Physical activity, hot drinks, heavy clothing, and even emotional stress can temporarily push your temperature up. For women of reproductive age, body temperature rises slightly after ovulation, typically less than half a degree Fahrenheit, and stays elevated until the next menstrual period begins. This shift is small but consistent enough that some people use it to track their fertility cycle.
What Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold applies regardless of where you take the reading, though the number you see on the thermometer depends on the method. The Mayo Clinic considers an armpit reading of 99°F or higher a fever, while oral, ear, and forehead readings use the 100.4°F cutoff.
Temperatures between 99°F and 100.3°F are sometimes called “low-grade fevers,” though this isn’t an official clinical category. For most adults, a reading in this range after physical activity or on a warm day is nothing unusual. A reading that consistently stays above 100.4°F, especially with other symptoms like chills, body aches, or fatigue, signals that your immune system is actively fighting something.
Older Adults Run Cooler
As you age, your baseline body temperature tends to drop. Several things contribute to this: you lose insulating fat under the skin (especially in your hands and feet), your metabolism slows, and your skin becomes drier, all of which increase heat loss. Certain common medications, including beta blockers and drugs used for thyroid conditions, can lower temperature further.
This matters because a fever in an older adult may not look like a fever on the thermometer. Someone whose baseline runs at 96.8°F could have a significant infection at 99°F, well below the standard 100.4°F threshold. Harvard Health Publishing recommends that older adults learn their own personal baseline when feeling well, rather than relying on a universal number. If your temperature rises more than about 2°F above your personal average, that’s worth treating as a potential fever even if the reading looks “normal.”
When Temperature Is Too Low
On the other end of the spectrum, a body temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia ranges from 90°F to 95°F and typically causes shivering, confusion, and poor coordination. Moderate hypothermia (82.4°F to 90°F) brings more severe confusion and drowsiness, while severe hypothermia (below 82.4°F) is life-threatening. Older adults are especially vulnerable because of their naturally lower baselines and reduced ability to generate heat.
Getting an Accurate Reading
The temperature you get depends partly on where you measure. Oral readings (under the tongue) are the most common for adults at home. Ear (tympanic) and forehead (temporal artery) thermometers are faster but can vary slightly from oral readings. Armpit readings tend to run lower than oral temperatures. There’s no reliable formula for converting between sites, so the most useful approach is to always measure the same way. That gives you a consistent baseline to compare against.
For the most accurate oral reading, avoid eating, drinking, or exercising for at least 15 minutes beforehand. Place the thermometer under your tongue toward the back of your mouth and keep your lips closed around it. Digital thermometers typically beep when the reading is stable, usually within 30 to 60 seconds.
The single most practical thing you can do is check your temperature a few times when you’re feeling perfectly healthy. Take readings at different times of day. That personal baseline is far more useful than any universal number, because “normal” varies enough from person to person that a chart can only get you so far.