Regular body temperature for most adults falls somewhere between 97°F (36.1°C) and 99°F (37.2°C), not the single number most of us learned growing up. The long-accepted standard of 98.6°F dates back to the 1860s, and newer evidence suggests the true average has dropped since then. Your own “normal” depends on where you measure, the time of day, your age, and other biological factors.
Where 98.6°F Came From
The 98.6°F benchmark traces back to a German physician named Carl Wunderlich, who measured armpit temperatures from roughly 25,000 people in the mid-1800s and landed on that figure as the average. It stuck for more than 150 years, becoming the number printed on thermometers and taught in schools. But a growing body of modern data shows it’s no longer the best estimate.
A large 2020 analysis reviewed temperature recordings spanning 157 years across three time periods: Civil War-era veterans in the late 1800s, a national health survey from the early 1970s, and a Stanford research database covering 2007 to 2017 with over 150,000 people. Over that span, the average oral temperature gradually fell by more than one degree Fahrenheit. Based on those findings, Harvard Health Publishing suggested that something closer to 97.5°F may better reflect the modern average. The reasons aren’t fully settled, but lower rates of chronic infection and changes in living conditions (like widespread climate control) are among the leading explanations.
How Temperature Varies by Measurement Site
The number on your thermometer shifts depending on where you take it. Oral temperature, the most common method for adults, centers around 98.6°F (37°C) as a reference point. Other sites read higher or lower:
- Rectal: 0.5 to 1°F (0.3 to 0.6°C) higher than oral. This is considered the most accurate method for infants.
- Ear (tympanic): 0.5 to 1°F (0.3 to 0.6°C) higher than oral.
- Armpit (axillary): 0.5 to 1°F (0.3 to 0.6°C) lower than oral.
There’s no exact formula to convert between these sites, so it’s worth noting which method you used when tracking your temperature over time or reporting it to a healthcare provider.
Why Your Temperature Changes Throughout the Day
Body temperature follows a natural 24-hour cycle. It dips to its lowest point in the early morning, typically between 6 and 8 a.m., and climbs to a peak in the evening, usually around 7 to 9 p.m. This swing can easily account for a difference of a degree or more, which means a reading of 97.3°F when you wake up and 98.8°F after dinner can both be perfectly normal for the same person.
This pattern matters if you’re checking your temperature to decide whether you have a fever. A reading taken first thing in the morning will naturally sit lower than one taken in the afternoon, so a “borderline” number in the early morning is more meaningful than the same number at night.
Other Factors That Shift Your Baseline
Time of day isn’t the only influence. Several biological variables push your temperature up or down on any given reading.
Hormonal cycles play a measurable role. During ovulation, basal body temperature rises by roughly half a degree Fahrenheit (0.3°C), which is why temperature tracking is sometimes used as a fertility tool. Physical activity also raises core temperature temporarily, so checking right after exercise will give you an inflated number. Even a hot drink before an oral reading can skew the result.
Age makes a difference as well. Older adults tend to run slightly cooler than younger adults, which can make it harder to detect a fever in this group. Infants and young children, on the other hand, tend to run a bit warmer and their temperatures fluctuate more easily in response to activity, clothing, or a warm room. For children, a rectal temperature of 100.4°F (38°C) or higher, an oral temperature of 100°F (37.8°C) or higher, or an armpit temperature of 99°F (37.2°C) or higher is considered a fever.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or greater. That threshold applies broadly across clinical settings and is the standard used for screening travelers, evaluating illness, and guiding treatment decisions. It refers to a core temperature measurement, so if you’re using an armpit thermometer, keep in mind that it naturally reads lower and you may need to account for that gap.
Temperatures in the 99 to 100.3°F range are sometimes called a “low-grade fever,” though the term isn’t formally standardized. For many people this zone overlaps with the upper end of their normal daily variation, especially in the evening. Context matters: a reading of 99.5°F after a workout is different from the same number when you’re resting and feel unwell.
Finding Your Own Normal
Because the “normal” range spans nearly two degrees and shifts with time of day, age, and hormones, 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, using the same thermometer and the same body site each time. Note the time of day. After several readings you’ll have a personal reference point that’s far more useful than any single textbook number. That way, when you do feel off, you’ll know what “elevated” actually looks like for you rather than comparing against a 160-year-old average that may not apply.