Normal body temperature for a human is generally cited as 98.6°F (37°C), but modern research shows the true average is closer to 97.5–97.9°F (36.4–36.6°C). The familiar 98.6 number dates back to the 1860s and, while still a useful reference point, is higher than what most healthy people actually register today. A normal reading can fall anywhere from 97°F to 99°F (36.1°C to 37.2°C) depending on the person, the time of day, and where on the body the temperature is taken.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published a landmark study in 1868 analyzing over one million armpit temperature readings from roughly 25,000 patients. He identified 37°C (98.6°F) as the average temperature of healthy adults. That number became medical gospel and has stayed in textbooks ever since.
There’s a catch, though. Wunderlich’s thermometers were bulky instruments that required 15 to 20 minutes under the arm to get a stable reading. Armpit measurements also tend to run lower than oral ones, which means his methods and equipment introduced inaccuracies that weren’t well appreciated at the time. The figure was never meant to be a single “correct” temperature for every person, but that’s how it came to be used.
The Modern Average Is Lower
A large Stanford-led study published in 2020 compared temperature data across three time periods: Civil War veterans measured between 1860 and 1940, a national health survey from the early 1970s, and over 150,000 patients seen at Stanford between 2007 and 2017. The researchers found that average body temperature has been dropping steadily, by about 0.05°F (0.03°C) per decade of birth. Men born in the early 1800s ran roughly 1°F hotter than men today. Women showed a similar decline of about 0.6°F since the 1890s.
A separate analysis of more than 35,000 patients in the UK found a mean oral temperature of 97.9°F (36.6°C), reinforcing the idea that 98.6 is no longer the best estimate. The reasons behind the decline aren’t fully settled, but researchers point to reduced rates of chronic infection, lower levels of inflammation, and more temperature-controlled living environments compared to the 19th century.
What Counts as a Fever
The CDC defines a clinical fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies to rectal, ear, and forehead readings. For oral measurements, many clinicians use 100°F (37.8°C) as the cutoff, and for armpit readings, 99°F (37.2°C). Anything between your personal baseline and those cutoffs is sometimes called a low-grade fever, but there’s no single universally agreed-upon number for that range.
On the other end, a core temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia, with a core temperature between roughly 89.6°F and 95°F (32–35°C), is generally tolerated without lasting harm. Below that, the risks climb sharply.
Why Readings Vary by Measurement Site
Where you place the thermometer matters more than most people realize. Rectal temperatures read closest to true core body temperature and tend to run about 0.5–1°F higher than oral readings. Armpit (axillary) temperatures run about 0.5–1°F lower than oral. Ear and forehead thermometers fall somewhere in between but are more sensitive to technique, like how well the sensor is aimed at the eardrum or how sweaty the forehead is.
For adults, an oral reading under the tongue is the most practical balance between accuracy and convenience. For infants and young children, rectal measurement remains the most reliable method.
Factors That Shift Your Temperature
Body temperature isn’t a fixed number. It follows a daily cycle, dipping to its lowest point in the early morning hours and peaking in the late afternoon or evening. That swing can easily span a full degree Fahrenheit over the course of a day, which means a reading of 97.3°F at 6 a.m. and 98.4°F at 5 p.m. can both be perfectly normal for the same person.
Age plays a role too. Older adults tend to run cooler baselines, which can make fevers harder to detect. A temperature of 99°F in a 75-year-old may represent a more significant immune response than the same reading in a 30-year-old.
For people who menstruate, body temperature shifts predictably across the cycle. After ovulation, basal temperature rises by 0.4°F to 1°F (0.22–0.56°C) and stays elevated through the second half of the cycle. This is the basis for the temperature-tracking method of fertility awareness. Physical activity, hot or cold drinks consumed shortly before measurement, and heavy clothing can also nudge readings up or down temporarily.
How to Get an Accurate Reading
If you want a number you can trust, take your temperature at the same time of day each time, ideally before eating or drinking anything. Wait at least 15 minutes after consuming hot or cold beverages before using an oral thermometer. Place the tip under the tongue toward the back of the mouth and keep your lips sealed around it until the thermometer signals it’s done.
Taking a few readings over several days when you feel well gives you a personal baseline, which is more useful than comparing yourself to the old 98.6 benchmark. Knowing your own normal makes it easier to spot when something is off, especially if your baseline runs on the low side.