Normal body temperature is widely quoted as 98.6°F, but modern research puts the actual average closer to 97.9°F. That longstanding 98.6°F number dates back more than 150 years to the work of a German physician named Carl Wunderlich, and while it served as a useful benchmark for generations, large-scale studies now show that healthy adults typically fall in a range of 97.3°F to 98.2°F.
Why 98.6°F Is Outdated
The 98.6°F standard became medical gospel in the mid-1800s, but it no longer reflects what thermometers actually show. Researchers at Stanford Medicine analyzed more than 618,000 oral temperature readings from adult outpatients seen between 2008 and 2017. They found the overall average was 97.9°F, not 98.6°F. Their earlier work showed that average body temperature in the U.S. has been dropping by about 0.05°F per decade since the 19th century, likely because improvements in health, sanitation, and living conditions have reduced chronic inflammation in the general population.
So if your thermometer reads 97.5°F or 98.1°F when you feel perfectly fine, that’s not a sign something is wrong. It’s well within the modern normal range.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable 24-hour cycle tied to your internal clock. Your lowest reading typically occurs in the early morning hours, then begins climbing during the last stretch of sleep, right before you wake up. It peaks in the late afternoon or early evening, then starts falling again as nighttime approaches. Many people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon drowsiness.
This daily swing can easily span a full degree or more, meaning a reading of 97.4°F at 7 a.m. and 98.4°F at 5 p.m. could both be perfectly normal for the same person.
Where You Measure Matters
Different parts of the body give different readings, and knowing the offsets helps you interpret your number correctly. Oral readings (under the tongue) are the most common reference point. Compared to an oral reading:
- Rectal temperatures run 0.5°F to 1°F higher.
- Ear (tympanic) temperatures also run 0.5°F to 1°F higher.
- Armpit (axillary) temperatures run 0.5°F to 1°F lower.
This means an armpit reading of 97.2°F and a rectal reading of 98.8°F could reflect the exact same core temperature. Rectal readings are considered the most accurate, which is why they’re the standard for infants and young children. For everyday use in adults, oral thermometers give a reliable enough picture as long as you haven’t just had something hot or cold to drink.
Factors That Shift Your Baseline
Beyond time of day and measurement site, several personal factors influence where your temperature naturally sits.
Age
Older adults tend to run cooler than younger adults. Their baseline can sit well below 98°F even when healthy, which means a temperature that looks “normal” on paper might actually represent a fever for someone in their 70s or 80s. Young children, by contrast, tend to run slightly warmer.
Hormonal Cycles
In people who menstruate, body temperature follows a predictable pattern across the cycle. After ovulation, basal body temperature rises by 0.4°F to 1°F and stays elevated until the next period begins. This shift is reliable enough that tracking morning temperature is a well-established method for identifying fertile windows.
Physical Activity and Environment
Exercise raises core temperature, sometimes significantly, and it can take 30 minutes or more to return to baseline afterward. Hot weather, heavy clothing, and even a hot bath can temporarily push readings up. For the most consistent measurement, check your temperature after sitting quietly for a few minutes.
When a Reading Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F or higher. For oral thermometers, many clinicians use 100°F as the threshold, and for armpit readings, 99°F. These cutoffs apply broadly, but they’re worth interpreting in context. If your personal baseline runs around 97.5°F, a jump to 99.5°F represents a two-degree increase and may signal your immune system is responding to something, even though it falls below the official fever line.
In children, the Mayo Clinic defines fever as a rectal, ear, or forehead temperature of 100.4°F or higher, an oral temperature of 100°F or higher, or an armpit temperature of 99°F or higher. The same number can mean different things depending on where you place the thermometer, so it helps to use the same method consistently and know which threshold applies to it.
Finding Your Personal Normal
Because normal spans a range rather than landing on a single number, the most useful thing you can do is learn your own baseline. Take your temperature a few times over the course of a week when you feel well, at different times of day, using the same thermometer and the same method. Most people will find their readings cluster in the 97°F to 98.5°F range. Once you know where you typically sit, a reading that’s 1.5 to 2 degrees above that baseline is a more meaningful signal than comparing yourself to the old 98.6°F benchmark.