Your body temperature is supposed to be around 97.9°F (36.6°C), not the 98.6°F (37°C) number you probably grew up hearing. That old standard comes from data published in 1868, and more recent research shows the average has dropped since then. What’s more, there’s no single “correct” number. Healthy adults range from about 97°F to 99°F depending on the time of day, their age, and how they’re measuring.
Why 98.6°F Is Outdated
The 98.6°F benchmark was established over 150 years ago by a German physician who averaged thousands of armpit readings. It stuck around as medical gospel for generations. But researchers at Stanford Medicine have found that average body temperature in the U.S. has been dropping by roughly 0.05°F per decade since the 19th century. Their analysis of modern data puts the overall adult average at 97.9°F, with normal readings falling between 97.3°F and 98.2°F.
The likely explanation is that people today are healthier than they were in the 1800s. Better nutrition, sanitation, antibiotics, and dental care mean less chronic infection and lower levels of background inflammation. Since inflammation generates heat, a population with less of it simply runs cooler.
Your Temperature Changes Throughout the Day
Body temperature isn’t static. It follows a daily cycle tied to your internal clock. You hit your lowest point during the middle of the night while you’re sleeping, and your highest point in the late afternoon or early evening as your metabolism peaks. This swing can easily account for a full degree of difference, which is why a reading of 97.5°F in the morning and 98.5°F after dinner can both be perfectly normal for the same person.
Exercise raises your temperature as your muscles generate heat. Hormonal shifts matter too: people who menstruate tend to run slightly warmer in the second half of their cycle, after ovulation, due to changes in how the body regulates heat. Older adults generally run a bit cooler than younger people, so a temperature that looks “normal” on paper could actually represent a meaningful rise for someone in their 70s or 80s.
Where You Measure Matters
The number on your thermometer depends on where you place it. Rectal readings are the most accurate reflection of your core temperature. Oral thermometers give similar accuracy and are far more practical for everyday use. Armpit (axillary) readings tend to run lower, and forehead scanners can be thrown off by sweating, cold air, or direct sunlight. Ear thermometers are convenient but can give unreliable results if the ear canal is angled differently, has wax buildup, or is infected.
There’s no reliable formula to convert between measurement sites. Adding or subtracting a degree to “adjust” a forehead reading to match an oral one doesn’t hold up consistently. The best approach is to use the same method each time so you can spot changes that are meaningful for you.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. The Mayo Clinic uses the same threshold for oral, rectal, ear, and forehead readings, with a slightly lower cutoff of 99°F (37.2°C) for armpit measurements since that site reads cooler.
A fever isn’t a malfunction. It’s your immune system deliberately raising your internal thermostat to help fight off infection. Your brain resets its target temperature upward in response to chemical signals from immune cells, and your body generates extra heat through shivering and blood vessel constriction to reach that new set point. This is different from overheating due to environmental causes like heat stroke, where your cooling system simply can’t keep up with external heat. In heat stroke, the thermostat hasn’t been reset. It’s been overwhelmed.
That distinction matters because the two situations call for very different responses. A fever typically resolves on its own or with basic care as the infection clears. Heat stroke is a medical emergency that requires rapid external cooling.
What Counts as Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. The severity breaks down by degree:
- Mild (above 93.2°F): shivering, clumsiness, and confusion. The body can usually recover with warm clothing and shelter.
- Moderate (86°F to 93.2°F): shivering may stop, thinking becomes sluggish, and drowsiness sets in. Active warming is needed.
- Severe (below 86°F): loss of consciousness and dangerous heart rhythms. This requires emergency medical intervention.
Hypothermia doesn’t only happen in extreme cold. Older adults, very young children, and people who are wet, exhausted, or malnourished can develop it in surprisingly mild conditions.
Finding Your Personal Baseline
Because “normal” spans a range of roughly two degrees, knowing your own baseline is more useful than memorizing a single number. Take your temperature a few times over several days using the same thermometer and the same method, ideally at the same time of day. Most people will find their readings cluster around a consistent point somewhere between 97°F and 98.5°F. Once you know your personal normal, a jump of one degree or more above that baseline is a more reliable signal that something is off than comparing against a universal standard that may not apply to you.