Normal body temperature is generally cited as 98.6°F (37°C), but the true average for most adults today falls closer to 97.5°F to 97.9°F. That familiar 98.6°F number dates back over 150 years and reflects a population that ran slightly warmer than we do now. In practice, a healthy body temperature can range anywhere from 97°F to 99°F depending on the time of day, your age, and how you measure it.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published his landmark work in 1868 after analyzing several million temperature readings from an estimated 25,000 patients. He reported that the mean of his enormous data set landed at 37°C, which converts to 98.6°F. That number stuck and became the textbook definition of “normal” for more than a century.
A large Stanford University study published in eLife found that human body temperature has been steadily dropping since the 1800s, declining roughly 0.05°F per decade. Men born in the early 19th century ran about 1.06°F warmer than men today, and women have seen a similar decline since the 1890s. The likely reasons include lower rates of chronic infection and inflammation, better living conditions, and widespread climate control. So while 98.6°F was probably accurate for Wunderlich’s era, it no longer reflects the modern average.
The Normal Range for Adults
For a healthy adult, an oral temperature between 97°F and 99°F is considered normal. Most people hover somewhere around 97.5°F to 98.3°F during ordinary daytime activity. Your personal baseline might sit at the low or high end of that range and still be perfectly healthy.
Body temperature isn’t fixed throughout the day. It follows your circadian rhythm, dropping to its lowest point in the early morning hours and rising through the afternoon. Most people also experience a small dip between 2 p.m. and 4 p.m. The difference between your daily low and high can be about 1°F, which is why a reading of 97.3°F at 6 a.m. and 98.4°F at 5 p.m. are both normal for the same person.
How Age Affects Temperature
Older adults tend to run cooler than younger people. After age 65, baseline temperatures often drop below 97.5°F, which means a fever can be present at a lower number than you’d expect. Infants and young children, by contrast, tend to run slightly warmer than adults, and their temperatures also fluctuate more throughout the day. For babies, rectal temperature is the most reliable measurement method.
Other Factors That Shift Your Temperature
Physical activity raises core temperature, sometimes significantly during intense exercise. Hormonal cycles also play a role: after ovulation, basal body temperature typically rises by less than half a degree Fahrenheit and stays elevated until the next menstrual period. This small but consistent shift is the basis of temperature-based fertility tracking.
Hot or cold environments, heavy clothing, recent meals, and even emotional stress can all nudge your reading up or down. Certain medications that affect metabolism or blood flow can shift your baseline, too.
How Measurement Method Changes the Number
The “normal” ranges you see are based on oral (under the tongue) readings. If you use a different method, expect some variation:
- Rectal: reads 0.5°F to 1°F higher than oral
- Ear (tympanic): reads 0.5°F to 1°F higher than oral
- Armpit (axillary): reads 0.5°F to 1°F lower than oral
- Forehead (temporal): reads 0.5°F to 1°F lower than oral
This matters more than most people realize. An armpit reading of 98.0°F and a rectal reading of 98.0°F represent very different internal temperatures. If you’re monitoring for a fever, keep the method consistent and know which offset applies.
When Temperature Signals a Problem
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies regardless of measurement site, though you should account for the offsets above when interpreting your reading. A low-grade fever, between roughly 99°F and 100.3°F, is common with minor infections and often resolves on its own.
On the other end of the spectrum, hypothermia sets in when core body temperature drops below 95°F. At that point the body loses heat faster than it can produce it, and normal muscle and brain function start to deteriorate. Symptoms include intense shivering, confusion, slurred speech, and drowsiness. Hypothermia is a medical emergency that requires immediate warming and professional care.
Extremely high temperatures, above 104°F, are also dangerous. At that level the body’s cooling systems are overwhelmed, and prolonged exposure to those internal temperatures can damage organs. Fevers in this range need prompt medical attention regardless of the cause.
Finding Your Personal Baseline
Because the “normal” range spans two full degrees and varies by individual, knowing your own baseline is more useful than comparing to a single number. Take your temperature at the same time of day, using the same method, on several days when you feel well. Most people find their usual reading lands between 97.2°F and 98.6°F. Once you know your baseline, spotting a meaningful elevation becomes much easier, especially for older adults whose resting temperature may sit well below the traditional 98.6°F mark.