The average human body temperature is often cited as 98.6°F (37°C), but that number is outdated. It comes from a massive study conducted in the 1860s, and modern research shows the true average has dropped since then. Most healthy adults today run closer to 97.5°F to 97.9°F (36.4°C to 36.6°C), with a normal range spanning roughly 97°F to 99°F (36.1°C to 37.2°C).
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Reinhold August Wunderlich, who published a landmark study in 1868 analyzing over one million temperature readings from around 25,000 patients. His work was groundbreaking for the era, but the tools he used were primitive by today’s standards. His thermometers were bulky, had to be read while still under the arm, and took 15 to 20 minutes to produce a stable reading. He also measured temperatures under the armpit rather than orally, which typically reads lower than oral temperature. Despite these limitations, 98.6°F became the textbook number and stuck for over 150 years.
Human Body Temperature Is Dropping
A large Stanford University study published in 2020 found that body temperature in the United States has been declining steadily since the Industrial Revolution. Men born in the early 1800s ran temperatures about 1.06°F (0.59°C) higher than men today, with a consistent drop of roughly 0.05°F (0.03°C) per decade. Women showed a similar pattern, declining about 0.58°F (0.32°C) since the 1890s at the same rate per decade.
The reasons aren’t entirely clear, but researchers point to reduced rates of chronic infection and inflammation compared to the 19th century. Better sanitation, antibiotics, and improved living conditions mean our immune systems are less constantly activated, which likely contributes to a lower resting temperature. Changes in metabolic rate, indoor climate control, and overall health may also play a role.
What Counts as Normal
A healthy body temperature isn’t a single number. It’s a range, generally between 97°F and 99°F (36.1°C and 37.2°C) for adults when measured orally. Several factors push your temperature up or down within that window throughout any given day.
Your body follows a circadian rhythm that affects temperature predictably. It drops to its lowest point during sleep and begins rising in the last hours before you wake up. Most people also experience a small dip between 2 p.m. and 4 p.m. By late afternoon or early evening, your temperature typically peaks. This daily swing can account for a full degree of variation, which is why a reading of 99°F in the evening may be perfectly normal while the same reading first thing in the morning could signal a mild fever.
Age matters too. Older adults tend to run cooler than younger people, which means a temperature that looks “normal” on paper could actually represent a fever in someone over 65. Children and infants, on the other hand, tend to run slightly warmer and also spike fevers more readily.
Ovulation and Hormonal Shifts
For people who menstruate, body temperature shifts predictably across the cycle. After ovulation, basal body temperature (the temperature taken first thing in the morning before getting out of bed) rises by less than half a degree Fahrenheit (about 0.3°C). It stays elevated through the second half of the cycle and drops again when menstruation begins. This shift is small but consistent enough that some people use daily temperature tracking as a method of natural family planning.
Where You Measure Changes the Reading
Not all thermometer placements give the same number, and the differences are predictable. Oral temperature is the most common reference point. Compared to an oral reading:
- Rectal reads 0.5°F to 1°F (0.3°C to 0.6°C) higher
- Ear (tympanic) reads 0.5°F to 1°F (0.3°C to 0.6°C) higher
- Armpit (axillary) reads 0.5°F to 1°F (0.3°C to 0.6°C) lower
- Forehead (temporal) reads 0.5°F to 1°F (0.3°C to 0.6°C) lower
This means an armpit temperature of 97°F and an ear temperature of 99°F could both reflect the same core body temperature. If you’re checking for a fever, it helps to know which site your thermometer uses so you can interpret the number correctly.
When Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold is widely used in clinical settings, at airports during health screenings, and as a general guideline for deciding whether illness symptoms warrant closer attention. Temperatures between 99°F and 100.3°F are sometimes called “low-grade fever,” though that term doesn’t have a strict clinical definition.
Keep in mind that the 100.4°F threshold assumes an oral or equivalent reading. If you’re using an armpit thermometer, you’d expect a fever reading to appear lower, while a rectal thermometer would show it higher. Exercise, heavy clothing, hot drinks, and even the time of day can all nudge a reading up without meaning you’re sick. A single reading is less useful than a pattern: if your temperature is consistently elevated above your personal baseline, that’s more informative than any one number on the thermometer.