The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) figure most of us grew up hearing. That well-known number dates back to 1868, and large-scale studies now show that human body temperature has been steadily dropping since then. A normal reading can fall anywhere from 97°F to 99°F depending on your age, the time of day, and how you take 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 based on over one million temperature readings from roughly 25,000 patients. It was an enormous dataset for the time, and his conclusion stuck for more than 150 years. But Wunderlich’s thermometers were bulky instruments that took 15 to 20 minutes to reach a stable reading, and he measured temperatures under the arm rather than in the mouth. Both of those factors introduce inaccuracies that likely skewed his results slightly high.
The Modern Average Is Lower
A 2020 study published in eLife analyzed body temperature records spanning nearly two centuries of American adults. The researchers found that average body temperature has dropped by about 0.03°C (0.05°F) per decade of birth. Men born in the early 1800s ran temperatures roughly 1.06°F (0.59°C) higher than men today. Women showed a similar decline of about 0.58°F (0.32°C) since the 1890s.
The most likely explanations are improvements in public health. People today have lower rates of chronic infections like tuberculosis and gum disease, which cause persistent low-level inflammation that raises body temperature. Better housing, climate control, and overall health mean our metabolic baseline has shifted downward over generations. The practical takeaway: if your thermometer reads 97.5°F or 98.0°F, that’s perfectly normal by today’s standards.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable 24-hour cycle driven by your internal clock. It bottoms out in the early morning, typically between 4 a.m. and 6 a.m., and peaks in the early evening. The difference between your daily low and high can range from 0.5°F to 1.9°F. So a reading of 97.3°F first thing in the morning and 98.8°F after dinner could both be completely normal for the same person on the same day.
Age, Sex, and Hormones Matter
Children tend to run slightly warmer than adults because their metabolisms are faster. Older adults often run cooler, sometimes well below 98°F, which can make fevers harder to detect in people over 65. A temperature that looks borderline in a younger adult might actually signal a significant infection in an older person.
The menstrual cycle also shifts baseline temperature. After ovulation, body temperature rises by about 0.5°F (0.3°C) and stays elevated through the second half of the cycle. This shift is small but consistent enough that some people use daily temperature tracking as a method of natural family planning.
How Measurement Site Affects Your Reading
Where you place the thermometer changes the number you get, sometimes by a meaningful amount. Oral readings are the standard reference point. Compared to an oral measurement:
- Rectal: 0.5°F to 1°F higher
- Ear (tympanic): 0.5°F to 1°F higher
- Armpit (axillary): 0.5°F to 1°F lower
- Forehead (temporal): 0.5°F to 1°F lower
This means a forehead reading of 97.8°F and a rectal reading of 99.4°F could reflect the exact same core temperature. If you’re monitoring for a fever, it helps to use the same method each time so you’re comparing consistent numbers. Rectal readings are the most accurate, which is why they’re recommended for infants. For everyday home use in adults, oral thermometers give the best balance of accuracy and convenience.
What Counts as a Fever
Most healthcare providers define a fever as an oral temperature at or above 100.4°F (38°C). Temperatures between 99.5°F and 100.3°F are generally considered a low-grade fever, that gray zone where your body is mounting a mild immune response but hasn’t crossed the clinical threshold. Below 99.5°F is considered normal variation for most people.
Keep in mind that your personal baseline matters. If you normally run at 97.5°F, a reading of 99.5°F represents a two-degree jump and may feel like a fever to you, even though it falls within the “normal” range on paper.
When Temperature Drops Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. The stages break down by severity:
- Mild: 89.6°F to 95°F (32°C to 35°C), marked by shivering and confusion
- Moderate: 82.4°F to 89.6°F (28°C to 32°C), where shivering stops and drowsiness sets in
- Severe: below 82.4°F (28°C), a medical emergency with risk of cardiac arrest
Older adults, very young children, and people with certain medical conditions are most vulnerable to hypothermia because their bodies are less efficient at generating or retaining heat. It doesn’t require extreme cold. Prolonged exposure to even mildly cool environments, especially with wet clothing or poor nutrition, can gradually lower core temperature into dangerous territory.