A good body temperature for most adults falls between 97.3°F and 98.2°F (36.3°C to 36.8°C), with an overall average around 97.9°F (36.6°C). That’s notably lower than the 98.6°F number you probably grew up hearing. The old standard dates back to the 1860s, and modern research has consistently found it’s too high to represent the true average.
Why 98.6°F Is Outdated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 analyzing over a million temperature readings from roughly 25,000 patients. The number stuck for more than 150 years. But Wunderlich measured temperatures under the armpit using bulky thermometers that took 15 to 20 minutes to stabilize, a method and technology very different from what we use today.
Researchers at Stanford Medicine have found that the average body temperature in the United States has been dropping by about 0.05°F per decade since the 19th century, landing at roughly 97.9°F today. The reasons likely include lower rates of chronic infection, better living conditions, and changes in overall inflammation levels. When a major study tested Wunderlich’s number against modern data, 98.6°F wasn’t the mean, the median, or even the most frequently recorded temperature. It didn’t fall within the statistical confidence range at all.
What Counts as Normal
Normal body temperature isn’t a single number. It’s a range, and your personal baseline may sit a bit higher or lower than someone else’s. For adults, oral readings between about 97°F and 99°F (36.1°C to 37.2°C) are generally considered normal. The more precise modern average clusters between 97.3°F and 98.2°F.
Your temperature shifts throughout the day in a predictable pattern. It tends to be lowest in the early morning, dips slightly again between 2 p.m. and 4 p.m., and starts to decrease in the evening as your body prepares for sleep. These fluctuations can easily account for a degree or more of variation over 24 hours, so a reading of 97.5°F before breakfast and 98.8°F in the late afternoon can both be perfectly normal for the same person.
Factors That Shift Your Baseline
Several things can push your temperature up or down without anything being wrong:
- Menstrual cycle: Body temperature rises after ovulation due to increased progesterone. The jump is typically between 0.4°F and 1°F, which is why temperature tracking is used as a fertility awareness tool. Before ovulation, most people run between 96°F and 98°F. Afterward, it shifts to 97°F to 99°F.
- Physical activity: Exercise raises your core temperature, sometimes significantly. Even moving around, talking, or being awake for several minutes before taking a reading can skew results.
- Sleep and stress: Poor sleep, emotional stress, alcohol, and travel across time zones can all affect your resting temperature.
- Age: Older adults tend to run cooler than younger people, which can make fevers harder to detect. Infants and young children typically run slightly warmer.
Where You Measure Matters
The number on your thermometer depends on where you take the reading. Oral temperature (under the tongue) is the most common reference point, and the ranges above are based on oral readings. Other methods read consistently higher or lower:
- Rectal: 0.5°F to 1°F higher than oral
- Ear (tympanic): 0.5°F to 1°F higher than oral
- Armpit (axillary): 0.5°F to 1°F lower than oral
- Forehead (temporal): 0.5°F to 1°F lower than oral
This means a forehead reading of 97.2°F and a rectal reading of 98.8°F could reflect the exact same core temperature. Knowing which method you used is essential before deciding whether a reading looks unusual.
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 method, though you should account for the offsets above. A reading between 99°F and 100.4°F is sometimes called a low-grade fever, though opinions vary on exactly where “elevated” ends and “fever” begins.
For infants 8 to 60 days old, a rectal temperature of 100.4°F or higher is treated as a medical priority. Babies in this age range can’t mount the same immune response as older children, so even a temperature that would be minor in an adult warrants prompt evaluation.
On the low end, hypothermia begins when body temperature drops below 95°F (35°C). Mild hypothermia covers the range from 95°F down to about 89.6°F, moderate hypothermia runs from 89.6°F to 82.4°F, and severe hypothermia is anything below 82.4°F. These thresholds matter most in cold-weather exposure, but older adults and people with certain medical conditions can develop mild hypothermia even indoors.
Finding Your Personal Normal
Because the “normal” range spans nearly two degrees, knowing your own baseline is more useful than comparing to a population average. Take your temperature at the same time of day, using the same method, over several days when you feel well. Most people will find their readings cluster within a narrow band. Once you know that band, a reading that’s 1.5°F or more above your personal average is a more reliable signal of illness than simply checking whether you’ve crossed 100.4°F.
If your resting temperature consistently sits below 97°F or above 99°F when measured orally, and you feel fine, that may simply be your normal. The old idea of a single “correct” body temperature has given way to a more personal understanding: your healthy temperature is a range, it shifts throughout the day, and it may not match anyone else’s.