A regular body temperature for a healthy adult falls between 97°F (36.1°C) and 99°F (37.2°C). The long-standing standard of 98.6°F (37°C) is still widely cited, but modern research shows that most people actually run a bit cooler than that number suggests.
Why 98.6°F Isn’t Quite Right Anymore
The 98.6°F benchmark dates back to the mid-1800s, when a German physician established it based on thousands of readings. It stuck as the textbook number for over a century. But a large-scale study published in eLife, drawing on nearly 190,000 temperature measurements spanning from the Civil War era to 2017, found that average human body temperature has been dropping steadily: about 0.05°F per decade. Men born in the early 1800s ran roughly 1°F warmer than men today, and women’s temperatures have dropped by about 0.6°F since the 1890s.
The reasons aren’t entirely settled, but reduced rates of chronic infection, lower levels of inflammation, and changes in living conditions (like climate-controlled homes) are all plausible explanations. The practical takeaway: if your thermometer consistently reads 97.5°F or 98.2°F, that’s perfectly normal.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable 24-hour cycle, dipping to its lowest point in the early morning (around 4 to 6 a.m.) and peaking in the early evening (around 8 p.m.). The difference between your daily low and high can be anywhere from 0.5°F to nearly 2°F. So a reading of 97.3°F before breakfast and 98.8°F after dinner can both be completely normal for the same person.
Other everyday factors push your temperature around as well. Physical exercise raises it, sometimes significantly, especially if you’re dehydrated. Eating a meal bumps it up slightly. Hormonal shifts during the menstrual cycle cause a small but measurable rise of about 0.5°F after ovulation, which is why tracking basal body temperature is used as a fertility planning tool.
How Age Affects Normal Temperature
Older adults tend to run cooler than younger people. This matters because a temperature that looks “normal” on a thermometer might actually represent a mild fever in someone over 65. A reading of 99°F in an older adult could signal an infection even though it wouldn’t raise any flags in a 30-year-old.
Young children, on the other hand, tend to run slightly warmer and their temperatures fluctuate more readily. Babies and toddlers can spike a fever quickly in response to minor infections, which is why pediatric fever thresholds are defined carefully by measurement method.
Where You Measure Matters
Different spots on the body give different readings, and knowing the gap helps you interpret your number correctly. Rectal temperatures read closest to true core body temperature. Oral readings tend to run about 0.4°F lower than rectal. Armpit (axillary) readings are the least precise and typically come in about 1°F below rectal.
For children, a fever is defined as:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
If you’re comparing readings over time, try to use the same method and the same time of day. Switching between an armpit reading in the morning and an oral reading at night makes it hard to spot real changes.
When a Temperature Becomes a Fever
Most healthcare providers define a fever as an oral temperature at or above 100.4°F (38°C). The range between 99.5°F and 100.3°F is often considered a low-grade fever, a gray zone where your body is mounting a response but hasn’t hit full fever territory yet.
For adults, fevers below 103°F (39.4°C) are generally not dangerous on their own. Above that level, it’s worth contacting a provider. For children, the threshold for concern is a fever above 104°F (40°C). Any temperature above 105.8°F (41°C), in anyone, can be dangerous if untreated.
On the low end, a body temperature below 95°F (35°C) is classified as hypothermia and is a medical emergency. This is most common in cold weather exposure, but it can also happen indoors in older adults with poor heating or certain medical conditions.
Getting an Accurate Reading
For the most reliable result, take your temperature after you’ve been sitting quietly for at least 15 minutes. Avoid measuring right after exercise, a hot shower, or a hot or cold drink. If you’re using an oral thermometer, keep your mouth closed around it for the full recommended time (usually 30 to 60 seconds for digital models).
If you’re trying to establish your personal baseline, take a few readings at the same time of day over the course of a week. You’ll likely find your “normal” clusters around a specific number that may or may not be 98.6°F, and that’s perfectly fine. Knowing your own baseline makes it much easier to tell when something is genuinely off.