The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) figure most of us learned growing up. That traditional number dates back to the 1850s and, while it was accurate for its time, large-scale modern studies show that human body temperature has been steadily dropping for over a century.
Where 98.6°F Came From
In 1851, a German physician named Carl Wunderlich collected over a million temperature readings from 25,000 patients in Leipzig. From that enormous dataset, he established 37°C (98.6°F) as the standard for normal body temperature, with a range of 97.2°F to 99.5°F. That number stuck for more than 150 years.
There’s reason to think Wunderlich’s readings ran slightly high even for his era. His thermometers were bulky instruments that required 15 to 20 minutes under the arm to get a reading, and he measured from the armpit rather than the mouth. But the bigger issue isn’t measurement error. Humans genuinely seem to have cooled down since then.
Why Human Body Temperature Has Dropped
A 2020 Stanford study analyzed temperature data from three large groups spanning 157 years: Civil War veterans (measured from 1860 to 1940), a national health survey from the 1970s, and modern Stanford hospital patients (2007 to 2017). The finding was striking: body temperature has declined by about 0.05°F per decade of birth, consistently, across the entire time period. Men born in the early 1800s ran roughly 1°F warmer than men today. Women showed a similar pattern, dropping about 0.6°F since the 1890s.
Two main explanations account for this cooling trend. First, lower rates of chronic infection and inflammation. In Wunderlich’s day, tuberculosis, syphilis, and untreated gum disease were widespread, all of which keep the immune system active and body temperature elevated. Modern medicine has dramatically reduced these baseline sources of inflammation. Second, changes in metabolic rate. Your resting metabolism generates heat the way an idling engine does, and some researchers believe that improved overall health and higher average body mass have shifted metabolic rates downward.
What Counts as a Normal Range
There is no single “normal” temperature. Your reading shifts throughout the day, typically lowest in the early morning and highest in the late afternoon. A healthy adult’s oral temperature generally falls somewhere between 97°F and 99°F (36.1°C to 37.2°C). Anything in that window is unremarkable.
Several factors push your temperature around within that range:
- Menstrual cycle: After ovulation, basal body temperature rises by 0.4°F to 1°F (0.2°C to 0.6°C) and stays elevated until the next period. This shift is small but consistent enough to be used for fertility tracking.
- Exercise: Intense physical activity can push core temperature up to 101°F to 104°F (38.3°C to 40°C). Your brain works to keep you within a degree or two of baseline, but during hard exertion it can’t always keep up.
- Age: Older adults tend to run cooler than younger adults, which means a fever in someone over 65 may register at a lower number than you’d expect.
- Time of day: Your temperature can swing by about 1°F between early morning and late afternoon, even when you’re perfectly healthy.
Where You Measure Matters
The number on your thermometer depends on where you take the reading. Oral temperature is the most common reference point, but other sites give consistently different results. Rectal and ear readings typically run 0.5°F to 1°F higher than oral. Armpit readings run 0.5°F to 1°F lower than oral. So an armpit reading of 97.5°F and a rectal reading of 99°F can both reflect the same actual body temperature.
If you’re comparing a reading to a fever threshold, make sure you’re using the right cutoff for your measurement site. An armpit temperature of 99°F is considered a fever, while the same number taken orally would be within normal range.
Fever and Hypothermia Thresholds
A fever is generally defined as an oral or rectal temperature of 100.4°F (38°C) or higher. For armpit readings, the threshold is lower: 99°F (37.2°C). Most adults with temperatures above 103°F (39.4°C) will look and feel noticeably ill.
On the low end, hypothermia begins when core body temperature drops below 95°F (35°C). Mild hypothermia covers the range from 90°F to 95°F, where you’ll experience shivering, confusion, and impaired coordination. Moderate hypothermia sets in between 82°F and 90°F, and severe hypothermia occurs below 82°F, at which point shivering actually stops and the situation becomes life-threatening.
The Bottom Line on 98.6°F
If your thermometer reads 97.5°F or 98.2°F and you feel fine, that is fine. The old standard of 98.6°F was a reasonable average for the 1850s, but it no longer reflects how warm modern humans actually run. A reading anywhere from about 97°F to 99°F is well within the range you’d expect in a healthy person, depending on time of day, activity level, and where on your body you measured.