The average human body temperature is closer to 97.9°F (36.6°C) than the familiar 98.6°F (37°C) most of us grew up hearing. That textbook number dates back to 1868 and no longer reflects what thermometers actually read in healthy people today. Body temperature has been slowly but measurably dropping for over a century, and it also shifts throughout the day, varies by age and sex, and changes depending on where on the body 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 a landmark study in 1868 analyzing several million temperature readings from roughly 25,000 patients. He reported that the mean of his enormous dataset was 37°C, which converts to 98.6°F, and declared it the normal physiological temperature of humans.
There’s a catch, though. Wunderlich took his measurements under the armpit, not in the mouth, and used thermometers that ran 2.9°F to 3.4°F higher than modern instruments. So his raw numbers don’t translate cleanly to what you’d see on a thermometer today. Despite this, 98.6°F stuck in medical textbooks and popular culture for more than 150 years.
Researchers have been chipping away at the number for decades. A 1992 study measuring oral temperatures in healthy young adults found that 98.6°F was not the overall mean, not the median, and not even the most frequently recorded temperature. Multiple large studies published between 2017 and 2023 have reinforced the same point: it’s time to let go of 98.6°F as the single “normal” number.
Human Body Temperature Is Dropping
A large study published in eLife analyzed temperature data spanning from the 1860s to the present and found that average body temperature has decreased steadily by about 0.05°F (0.03°C) per decade of birth. Men born in the early 1800s ran roughly 1.06°F (0.59°C) warmer than men today. Women’s temperatures have dropped about 0.58°F (0.32°C) since the 1890s, following a nearly identical rate of decline.
The leading explanation is that chronic, low-grade inflammation has decreased dramatically since the Industrial Revolution. Better sanitation, antibiotics, dental care, and reduced exposure to infectious diseases like tuberculosis and malaria mean our immune systems are less constantly activated than our ancestors’ were. Since inflammation raises body temperature, less of it over a lifetime results in a cooler baseline. Improvements in climate-controlled environments may also play a role, as the body doesn’t need to work as hard to regulate its temperature.
What Counts as Normal Now
Rather than one fixed number, normal body temperature falls within a range. For a healthy adult measured orally, that range is roughly 97.0°F to 99.0°F (36.1°C to 37.2°C), with the average landing around 97.9°F to 98.2°F. Your personal baseline can sit anywhere in that window and be perfectly healthy.
Several factors shift where you fall within that range:
- Time of day: Temperature drops to its lowest point during sleep, begins rising in the last hours before you wake, dips slightly again between 2 p.m. and 4 p.m., and peaks in the late afternoon or early evening. This daily swing can be a full degree or more.
- Age: Older adults tend to run cooler than younger people. Infants and young children often run slightly warmer.
- Sex: Women’s temperatures fluctuate with the menstrual cycle, typically rising after ovulation and staying elevated until the next period begins.
- Physical activity: Exercise raises core temperature temporarily, sometimes by 1°F to 2°F depending on intensity.
How Measurement Site Changes the Reading
The number on your thermometer depends heavily on where you place it. Oral temperature is the most common reference point for adults, but each site reads slightly differently:
- 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 97.6°F and an ear reading of 99.2°F could reflect the exact same core temperature. If you’re tracking a fever, it helps to use the same method each time so you’re comparing consistent numbers. For infants, rectal thermometers remain the most accurate option.
When a Temperature Becomes a Fever
A fever is generally defined as an oral temperature of 100°F (37.8°C) or higher. For rectal readings, which run hotter, the threshold is 100.4°F (38°C). This distinction matters most for infants: any baby younger than three months with a rectal temperature of 100.4°F or higher needs prompt medical evaluation.
Low-grade fevers, in the 100°F to 102°F range, are a normal part of the immune response and often don’t require treatment in otherwise healthy adults. A temperature of 103°F (39.4°C) or higher is considered more significant and worth a call to your doctor. In children, the threshold for concern also depends on age, how long the fever has lasted, and whether other symptoms are present.
Because everyone’s baseline is slightly different, a reading of 99.5°F might be unremarkable for one person and a sign of early illness for another. If you know your personal normal, you can spot a meaningful rise sooner than relying on a universal cutoff. Taking your temperature a few times when you’re feeling well gives you that reference point.