A regular body temperature for most adults is around 97.9°F (36.6°C), not the 98.6°F (37°C) figure you probably grew up hearing. That long-standing number dates back to a German study published in 1868, and modern research shows it no longer reflects reality. Normal temperatures in healthy adults typically fall between 97.3°F and 98.2°F, and your personal baseline depends on your age, time of day, and even your menstrual cycle.
Why 98.6°F Is Outdated
The 98.6°F standard came from over 150 years ago, when chronic infections like tuberculosis and syphilis were widespread and largely untreated. Those ongoing infections produced low-grade inflammation that pushed average body temperatures higher. As public health improved and chronic infection rates dropped, so did the population’s average temperature.
A Stanford Medicine analysis of more than 618,000 oral temperature readings from adult patients between 2008 and 2017 found that today’s average sits closer to 97.9°F. The research team also discovered that average body temperature in the U.S. has been falling by roughly 0.05°F per decade since the 1800s. The decline appears to reflect a broader drop in basal metabolic rate, which tracks with longer lifespans and changes in body size over the past century and a half.
What Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies to rectal, ear, and forehead readings. Oral readings run slightly lower, so a fever by mouth is generally considered 100°F (37.8°C) or above. Armpit readings are lower still, with 99°F (37.2°C) serving as the cutoff.
If you don’t have a thermometer handy, skin that feels warm to the touch, visible facial flushing, or chills can all suggest a fever. Keep in mind that fever-reducing medications can mask a true reading, so a normal number on the thermometer doesn’t always mean the fever is gone.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable daily rhythm tied to your internal clock. Your temperature begins rising in the last hours of sleep, just before you wake up. It peaks sometime in the late afternoon or early evening, then gradually falls as bedtime approaches. Most people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon slump.
This daily swing means a temperature taken first thing in the morning will be noticeably lower than one taken after dinner. A reading of 97.5°F at 7 a.m. and 98.5°F at 5 p.m. could both be perfectly normal for the same person on the same day.
How Hormones Shift Your Baseline
In women of reproductive age, body temperature fluctuates by about 0.5 to 0.8°F over the course of the menstrual cycle. Temperature dips slightly just before ovulation, when estrogen is high, then rises during the second half of the cycle as progesterone climbs. This is the principle behind fertility tracking methods that use a daily morning temperature reading to estimate ovulation. Estrogen generally promotes heat loss and a cooler core, while progesterone pushes temperatures up.
Where You Measure Matters
Different spots on the body give different readings, and the gaps are consistent enough that you need to know which method you’re using before deciding whether a number is normal or elevated.
- Rectal: Considered the most accurate reflection of core temperature. Readings run about 0.5 to 1°F higher than oral.
- Oral (under the tongue): The most common method for adults. This is where the 97.9°F modern average comes from.
- Armpit: Convenient but less reliable. Readings typically run about 1°F lower than oral, so a normal armpit temperature may sit closer to 97°F.
- Ear and forehead: Quick and non-invasive. These aim to approximate core temperature and generally align closer to rectal readings, though accuracy varies by device.
If you’re tracking your temperature over time, consistency matters more than the method itself. Use the same thermometer in the same location at roughly the same time of day to get readings you can meaningfully compare.
Other Factors That Affect Your Reading
Exercise raises core temperature, sometimes significantly. During physical activity, your body generates excess heat and relies on sweating and increased blood flow to the skin to cool down. A reading taken right after a workout won’t reflect your resting baseline.
Cold environments trigger the opposite response. Your body constricts blood vessels near the skin to conserve heat and may generate warmth through shivering. Hot environments push blood toward the surface and ramp up sweat production. In both cases, your core temperature stays remarkably stable unless the exposure is extreme or prolonged, but skin temperature and readings from surface-level methods like armpit or forehead thermometers can shift noticeably.
Age plays a role too. Older adults tend to run cooler than younger people, which means a temperature of 99°F in someone over 70 may be more significant than the same reading in a 30-year-old. The wide normal range of 97°F to 99°F (36.1°C to 37.2°C) reported across studies reflects real person-to-person variation, not measurement error.
Finding Your Personal Normal
Because normal temperature varies so much from person to person, the most useful thing you can do is learn your own baseline. Take your temperature a few times over the course of a normal, healthy week, at different times of day, using the same thermometer and method each time. That gives you a personal reference point, so when you feel off, you have something meaningful to compare against rather than relying on a 19th-century average that probably doesn’t apply to you.