Normal 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 closer to 97.9°F. That’s notably lower than the 98.6°F figure most of us grew up hearing, which dates back to a German study from 1851 and no longer reflects modern human averages.
Why 98.6°F Is Outdated
The familiar 98.6°F standard came from physician Carl Wunderlich, who measured axillary (armpit) temperatures in thousands of patients in the mid-1800s. For over a century, that number stuck. But research from Stanford Medicine has shown that average body temperature in the United States has been dropping by about 0.05°F per decade since the Industrial Revolution. Men born in the early 19th century ran temperatures roughly 0.59°C (about 1°F) higher than men today, and women’s temperatures have dropped by a similar margin since the 1890s.
The likely explanation: people in the 1800s carried far more chronic inflammation. Tuberculosis, syphilis, and untreated gum disease were widespread, all of which elevate baseline temperature. Modern improvements in hygiene, medicine, and nutrition have lowered our metabolic rate and, with it, our resting temperature. So if your thermometer consistently reads 97.8°F or 98.0°F, that’s perfectly normal for a healthy adult today.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable daily rhythm, dipping to its lowest point in the early morning hours (usually between 4 and 6 a.m.) and peaking in the late afternoon or early evening. Your brain keeps your core temperature within about one to two degrees of your personal baseline at all times, adjusting blood flow and sweat production to stay in range.
Several things push your temperature up or down within that normal window:
- Time of day: A reading of 97.4°F at 7 a.m. and 98.4°F at 5 p.m. could both be completely normal for the same person.
- Physical activity: Exercise temporarily raises core temperature. During intense exertion, especially in heat, it can climb above 100°F before your cooling mechanisms bring it back down.
- Menstrual cycle: After ovulation, basal body temperature rises by roughly 0.3°C (about half a degree Fahrenheit) and stays elevated through the second half of the cycle.
- Age: Older adults typically run cooler than younger adults. If you’re over 65, your baseline may sit a bit below the population average, which matters when interpreting whether a reading counts as a fever.
- Recent food or drink: A hot cup of coffee or a cold glass of water can temporarily shift an oral reading by a few tenths of a degree.
How Measurement Site Affects Your Reading
Where you take your temperature matters as much as the number itself. Rectal readings run about 0.5°C (roughly 1°F) higher than oral readings because they measure closer to your body’s core. Armpit readings tend to run about 1°F lower than oral. Ear (tympanic) thermometers can be convenient but are less consistent; in one study, ear readings came in 0.5°C lower than oral temperature in about one in five subjects.
For adults, an oral thermometer gives the most practical balance of accuracy and ease. For infants and young children, rectal temperature is considered the gold standard. Forehead (temporal artery) thermometers are popular for quick screening but can be thrown off by sweat or ambient temperature. If you’re tracking your temperature over time, use the same method and the same thermometer each time to get a meaningful comparison.
When a Reading Becomes a Fever
For adults, an oral temperature of 100°F (37.8°C) or higher is generally classified as a fever. At the rectal site, that threshold is 100.4°F (38°C). A temperature of 103°F (39.4°C) or above warrants prompt medical attention regardless of age.
For children, the thresholds shift depending on age:
- Under 3 months: A rectal temperature of 100.4°F (38°C) or higher is considered urgent at this age.
- 3 to 24 months: A rectal temperature above 102°F (38.9°C) is the key threshold to watch.
- Older children: The same adult oral threshold of 100°F applies, along with an armpit threshold of 99°F (37.2°C).
Why Older Adults Need a Lower Threshold
Because seniors naturally run cooler, a temperature that looks unremarkable on paper could actually signal infection. Someone whose baseline hovers around 97.2°F reaching 99.5°F has experienced a meaningful spike, even though it falls below the standard fever cutoff. This is one reason infections in older adults sometimes go unrecognized until they become serious. If you’re over 65, or caring for someone who is, it helps to take a few baseline readings when healthy so you have a personal reference point. Bacterial and viral infections can hit harder in this age group, making early detection more valuable.
Finding Your Personal Normal
Given how much normal temperature varies from person to person, the most useful thing you can do is establish your own baseline. Take your temperature a few times over the course of a regular, healthy week, using the same thermometer and the same site each time. Note the time of day. After a handful of readings, you’ll see a pattern: your personal range will likely span about one degree from morning low to afternoon high.
That personal baseline is far more informative than any single universal number. A reading of 99.2°F might be a normal afternoon temperature for one person and an early sign of illness for another. Context, including how you feel, when you took the reading, and what your usual numbers look like, matters more than any cutoff printed on a thermometer box.