A person’s normal body temperature isn’t a single number. While 98.6°F (37°C) has been taught as the standard for over 150 years, modern research consistently shows that the true average runs closer to 97.5°F to 97.9°F (36.4°C to 36.6°C) for most adults. Normal temperatures range from about 97°F to 99°F (36.1°C to 37.2°C), and your own baseline depends on your age, the time of day, where on your body the reading is taken, and several other factors.
Why 98.6°F Is Outdated
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published a landmark study in 1868 based on millions of temperature readings from roughly 25,000 patients. He declared 37°C (98.6°F) the mean of his massive data set, and medicine accepted it as fact for the next century and a half.
There’s a problem, though. Wunderlich measured temperatures under the armpit using thermometers that were calibrated 2.9°F to 3.4°F higher than modern instruments. So his readings were likely off from the start. Beyond the equipment issues, multiple studies from the 1990s onward have tested whether 98.6°F actually holds up as a meaningful number. A well-known 1992 study measured oral temperatures in healthy young adults and found that 98.6°F was not the overall mean, the median, or even the most frequently recorded temperature. It was just one unremarkable point in a broad range.
A Stanford University analysis published in eLife found that body temperatures in the U.S. have been falling steadily since the 1800s, dropping about 0.05°F per decade. Men born in the early 19th century had temperatures roughly 1°F higher than men today. Women showed a similar decline of about 0.6°F since the 1890s. One leading theory is that reduced rates of chronic infection (particularly tuberculosis, which was widespread in Wunderlich’s era) and lower levels of background inflammation have cooled us down over the generations. A large British study of over 35,000 patients found the modern mean oral temperature to be 97.9°F (36.6°C).
What Counts as Normal by Age
Infants and young children tend to run warmer than adults because their metabolisms are faster and their bodies are less efficient at regulating heat. A rectal reading of up to 100.3°F in a baby can still fall within normal bounds depending on the time of day and activity level, though anything at 100.4°F (38°C) or above is generally considered a fever in children under three months.
Healthy adults typically sit somewhere between 97°F and 99°F (36.1°C to 37.2°C) when measured orally. Older adults tend to run at the lower end of that range, and some may have resting temperatures below 97°F. This matters because an elderly person with an infection might register a temperature that looks “normal” on paper but is actually elevated for them. A shift of even one degree above a person’s usual baseline can be more meaningful than hitting a specific number.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable 24-hour cycle. It hits its lowest point in the early morning, roughly two hours before you wake up (around 5 to 7 AM), and peaks in the evening about two hours before you fall asleep (around 8 to 10 PM). The gap between your daily low and daily high is typically 0.4°F to 1.4°F. So a reading of 97.3°F at 6 AM and 98.7°F at 9 PM could both be perfectly normal for the same person on the same day.
This rhythm is driven by your internal clock and happens regardless of activity. It’s one reason a temperature check in the evening can look mildly elevated even when nothing is wrong.
How Measurement Location Affects the Reading
Where you take your temperature changes the number you get, sometimes by a full degree or more.
- Rectal: The closest approximation to core body temperature. A fever is defined as 100.4°F (38°C) or higher.
- Oral (under the tongue): The most common method for adults. A fever starts at 100°F (37.8°C).
- Armpit (axillary): Reads the lowest and is the least accurate. A fever starts at 99°F (37.2°C), but readings are easily skewed by clothing, sweat, or a loose fit.
- Ear (tympanic): Quick and convenient, but earwax buildup or a curved ear canal can throw off accuracy. Fever threshold is 100.4°F (38°C).
- Forehead (temporal artery): Non-invasive and widely used in clinics. Fever threshold is 100.4°F (38°C).
If you’re checking a child’s temperature and an armpit reading seems borderline, confirming with a rectal or oral measurement gives a more reliable answer.
Other Factors That Shift Your Baseline
The menstrual cycle is one of the most well-documented influences on body temperature. Before ovulation, resting temperature typically sits between 96°F and 98°F. After ovulation, a rise in progesterone pushes it up by 0.4°F to 1°F, into the 97°F to 99°F range. That elevated temperature holds steady through the second half of the cycle. If pregnancy occurs, it stays high. If not, temperature drops just before the period starts. This shift is reliable enough that some people use daily temperature tracking as a fertility awareness method.
Physical activity raises temperature temporarily, sometimes to 101°F or higher during intense exercise. Hot or cold environments, recent meals, and even emotional stress can nudge readings up or down. Certain medications, including those that affect thyroid function or hormone levels, can also shift your baseline. Body composition plays a role too: higher metabolic rates generally produce more heat.
When a Temperature Becomes a Fever
For adults, an oral temperature of 100°F (37.8°C) or higher is generally considered a fever. At 103°F (39.4°C) or above, the fever is considered high enough to warrant medical attention regardless of other symptoms.
The thresholds are more specific for young children. An infant under three months with a rectal temperature of 100.4°F (38°C) or higher needs prompt evaluation, even if the baby seems fine otherwise. For children between 3 and 24 months, the concern level rises at rectal temperatures above 102°F (38.9°C), particularly if the fever persists or is accompanied by irritability, poor feeding, or lethargy.
Because everyone’s baseline is slightly different, tracking your own normal temperature when you’re healthy gives you a more useful reference point than relying on 98.6°F. A person who normally runs at 97.2°F and suddenly reads 99.5°F has experienced a larger shift than someone whose baseline is 98.4°F reaching the same number. That shift, not the absolute reading, is often what matters most.