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 familiar number dates back to 1868 and no longer reflects what thermometers actually show in modern populations. A Stanford Medicine analysis of more than 618,000 temperature readings found that today’s adults typically fall somewhere between 97.3°F and 98.2°F.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published a massive study in 1868 analyzing over one million temperature readings from roughly 25,000 patients. It was groundbreaking work for its time, but the methods had significant limitations. Wunderlich’s thermometers were bulky instruments that needed 15 to 20 minutes to stabilize, and he measured temperatures under the armpit rather than in the mouth, which consistently reads lower. Modern thermometers are smaller, more accurate, and equilibrate in seconds.
The number stuck for over 150 years, partly because it’s easy to remember and partly because no one had gathered enough data to convincingly replace it. That changed in 2020 when researchers at Stanford published findings showing a clear, steady decline in average body temperature since the Industrial Revolution.
Why Body Temperature Has Dropped
Human body temperature has been falling by roughly 0.03°C (about 0.05°F) per decade since the early 1800s. Men born in the 19th century ran temperatures about 0.59°C (1.06°F) higher than men today. Women have seen a similar decline of about 0.32°C (0.58°F) since the 1890s.
The likely explanation is reduced chronic inflammation. In the 1800s, infections like tuberculosis, gum disease, and untreated wounds were widespread and persistent. Chronic inflammation raises baseline body temperature. With improvements in sanitation, antibiotics, dental care, and overall living conditions, our bodies simply run cooler than they used to. This isn’t a sign that something is wrong. It reflects a population that’s healthier on average.
What Counts as “Normal” for You
Your personal normal temperature depends on several factors, and no single number applies to everyone. The Stanford analysis found that age, sex, height, weight, and time of day account for about 25% of the variation in any individual’s temperature readings. Person to person, those same factors explain about 7% of the differences, meaning much of what sets your baseline is unique to your biology.
A few patterns hold across populations:
- Time of day: Body temperature is lowest in the early morning and peaks in the late afternoon or evening. The swing can be a full degree Fahrenheit or more.
- Age: Older adults tend to run cooler than younger adults, which can make fevers harder to detect in seniors.
- Menstrual cycle: During the second half of the cycle (the luteal phase), core temperature rises by 0.3°F to 1.0°F compared to the days right after a period. This shift is driven by progesterone and is reliable enough that some people use it for fertility tracking.
- Physical activity: Exercise pushes core temperature well above resting levels. During sustained activity in warm conditions, core temperature can reach 99.5°F to 100.4°F, which is normal and expected.
How Measurement Site Affects Your Reading
The number on your thermometer depends heavily on where you take the measurement. Oral readings are the most common reference point, and all the offsets below are relative to an oral reading:
- Rectal: 0.5 to 1.0°F higher than oral
- Ear (tympanic): 0.5 to 1.0°F higher than oral
- Armpit (axillary): 0.5 to 1.0°F lower than oral
- Forehead (temporal): 0.5 to 1.0°F lower than oral
This matters more than people realize. An armpit reading of 97.5°F and an ear reading of 99.0°F could reflect the exact same core temperature. If you’re checking for a fever, knowing which method you used is just as important as the number itself. Rectal readings are considered the most accurate, which is why they’re the standard for infants and young children.
When a Temperature Becomes a Fever
A fever is defined as a body temperature of 100.4°F (38°C) or higher. That threshold applies to oral, rectal, and ear measurements, though with different expected baselines for each site. For children, the cutoffs are the same: 100.4°F rectally, by ear, or on the forehead, 100°F orally, and 99°F under the armpit.
Given that the true average body temperature is closer to 97.9°F, a reading of 100.4°F represents roughly a 2.5-degree jump from baseline, not the 1.8-degree increase you’d calculate from the old 98.6°F figure. In practical terms, this means a temperature of 99.5°F, while technically below the fever line, is already meaningfully elevated for most people and worth paying attention to if you feel unwell.
For older adults who naturally run cooler, even a reading of 99°F could signal an infection that a younger person’s body would register at 100°F or above. Context matters: your own baseline, how you feel, and what other symptoms are present all factor into whether a temperature reading is concerning.