A normal adult body temperature isn’t a single number. It’s a range, and it’s lower than most people think. Modern research from Stanford Medicine puts the average at about 97.9°F (36.6°C), with healthy adults falling anywhere between 97.3°F and 98.2°F. The familiar 98.6°F figure is outdated.
Why 98.6°F Is No Longer Accurate
The 98.6°F standard comes from a German physician named Carl Wunderlich, who published temperature data from roughly 25,000 patients in 1868. He took over a million measurements, and the overall average landed at 98.6°F. That single number lodged itself in medical textbooks and public memory for more than 150 years.
There were problems with that original data, though. Wunderlich measured temperatures under the armpit, not orally, and his thermometers were calibrated about 2.9°F to 3.4°F higher than the instruments used today. So his readings were consistently inflated compared to what a modern thermometer would show.
On top of the equipment differences, human body temperature has genuinely dropped over time. Research from Stanford found that average body temperature in the U.S. has declined by about 0.05°F per decade since the 1800s. The likely reasons include lower rates of chronic infection and inflammation in modern populations, along with changes in metabolism and living conditions. The result is that today’s healthy adults simply run cooler than people did a century and a half ago.
Your Temperature Changes Throughout the Day
Body temperature isn’t static. It follows a daily cycle tied to your internal clock. Your temperature is lowest in the early morning hours, then begins rising during the last phase of sleep, just before you wake up. It peaks in the late afternoon or early evening, often reaching half a degree or more above your morning reading. A small dip also occurs in most people between 2 p.m. and 4 p.m.
This means a reading of 97.4°F at 7 a.m. and 98.4°F at 5 p.m. could both be perfectly normal for the same person. If you’re checking your temperature for any reason, the time of day matters. A late-afternoon reading will almost always be higher than one taken first thing in the morning.
Where You Measure Makes a Difference
Different parts of the body give slightly different readings. Here’s what to expect from each method in a healthy adult:
- Oral (under the tongue): about 97.9°F to 98.4°F (36.6°C to 36.9°C)
- Armpit (axillary): about 97.9°F (36.6°C), typically the lowest reading
- Ear (tympanic): about 98.8°F (37.1°C), slightly higher than oral
- Rectal: about 98.8°F (37.1°C), similar to ear measurements
Rectal and ear readings run roughly 0.5°F to 1°F higher than oral readings, while armpit readings tend to run slightly lower. Knowing which method you’re using helps you interpret the number correctly. A reading of 99.0°F from an ear thermometer, for example, is not the same as 99.0°F taken orally.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies to oral readings. Anything between your personal baseline and 100.4°F is sometimes called a “low-grade fever,” though it may simply reflect normal daily fluctuation, physical exertion, or a warm environment.
On the other end of the spectrum, a core body temperature below 95°F (35°C) is classified as hypothermia. This is a medical emergency that requires immediate attention.
Temperature in Older Adults
Baseline body temperature doesn’t change dramatically with age, but older adults have a harder time regulating it. The more clinically important issue is that older people may not mount a strong fever when they’re sick. Their bodies sometimes can’t push temperature up the way a younger person’s would during an infection. A reading of 99°F in an 80-year-old could signal something that would produce a 101°F fever in a younger adult.
For this reason, fever in an older person is a significant warning sign, even if the number seems modest. It’s often the first and sometimes the only symptom of an illness for several days.
What Your Personal Normal Looks Like
Rather than comparing yourself to a universal standard, it helps to know 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 the same method each time. You’ll likely find your readings cluster within a range of about one degree. That range is your personal normal.
Once you know your baseline, a meaningful deviation is easier to spot. A jump of 1.5°F or more above your typical reading is more informative than comparing any single number to 98.6°F, a figure that was never meant to be a bright line between healthy and sick.