The normal temperature for humans is around 97.9°F (36.6°C), not the 98.6°F (37°C) that most of us learned growing up. That old number dates back to a German study from 1851 and has stuck around in textbooks ever since. But modern research shows human body temperature actually spans a range, and the true average has been dropping for over a century.
Why 98.6°F Is Outdated
The famous 98.6°F standard came from German physician Carl Wunderlich, who collected millions of temperature readings in the mid-1800s. For his era, the number was probably accurate. But a large-scale analysis published in eLife, drawing on temperature data from Americans born between the early 1800s and the late 1990s, found that average body temperature has been falling steadily: about 0.05°F per decade of birth. Men born in the early 19th century ran temperatures roughly 1.06°F higher than men today. Women showed a similar decline of about 0.58°F since the 1890s.
The likely reasons? People today have lower rates of chronic infection and inflammation than people did 150 years ago, thanks to antibiotics, improved sanitation, and better dental care. Since the immune system generates heat when it fights infection, less background inflammation means a cooler baseline. Changes in metabolic rate, climate-controlled environments, and overall body composition probably contribute too.
The Actual Normal Range
There is no single “normal” body temperature. In a study of adults tracked over two weeks, individual averages ranged from 95.4°F (35.2°C) to 99.3°F (37.4°C). That’s a spread of nearly 4°F across healthy people. Your own personal baseline might sit anywhere within that window and still be completely normal.
Core temperature (the temperature of your brain, heart, and central organs) is typically regulated to about 97.9°F, with a 95% confidence interval of 96.3°F to 99.1°F. So if you take your temperature on a random afternoon and get 97.5°F or 99.0°F, neither reading is cause for concern on its own.
Readings Differ by Measurement Site
Where you place the thermometer changes the number you get. Rectal readings run highest because they measure closest to your core. Oral readings fall slightly lower, and armpit readings are lower still. Here’s how the sites compare:
- Rectal or ear: Runs about 0.5°F to 1°F higher than oral. A fever threshold here is 100.4°F (38°C).
- Oral (under the tongue): The most common method for adults. Fever starts at 100°F (37.8°C).
- Armpit: Reads about 1°F lower than oral. Fever threshold is 99°F (37.2°C), though armpit readings are the least reliable of the three.
If you’re comparing temperatures over time, use the same site and the same thermometer each time. Mixing methods makes it impossible to spot meaningful changes.
What Shifts Your Temperature Throughout the Day
Your body temperature isn’t static. It follows a daily rhythm, dipping lowest in the early morning (often around 4 a.m.) and peaking in the late afternoon or early evening. That swing can be a full degree or more, which is why a reading of 99°F at 5 p.m. might be perfectly normal while the same number at 6 a.m. could signal something is off.
Exercise pushes core temperature up significantly. During intense physical activity, well-trained athletes can reach internal temperatures of 106.7°F (41.5°C) without lasting harm. Heat exhaustion typically sets in when core temperature climbs to between 101.3°F and 104°F (38.5°C to 40°C) during exertion, especially in hot environments.
For women, the menstrual cycle creates a predictable temperature pattern. After ovulation, basal body temperature rises by 0.4°F to 1°F (0.22°C to 0.56°C) and stays elevated through the second half of the cycle. This shift is small but consistent enough that some people use it to track fertility.
How Age Affects Body Temperature
Older adults tend to run cooler than younger people. As you age, you lose insulating fat under the skin (particularly in your hands and feet), and your metabolism slows, both of which reduce heat production. This means a temperature that looks “normal” on paper might actually represent a fever in someone over 65. An older adult with a serious infection might never hit the standard 100.4°F fever threshold, so even modest temperature increases deserve attention in that age group.
Young children, on the other hand, tend to run slightly warmer than adults and can spike higher fevers in response to minor infections. Their temperature regulation systems are still maturing, which makes them more reactive to illness, activity, and overdressing.
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 rectal, ear, and forehead readings. For oral measurements, 100°F (37.8°C) is the cutoff. These numbers haven’t changed despite the downward shift in average body temperature.
A temperature between 99°F and 100.3°F is sometimes called a “low-grade fever,” though it may simply reflect normal daily variation, recent exercise, or hormonal changes. Context matters more than any single number. If you feel fine and your temperature is 99.2°F after a walk on a warm day, there’s likely nothing to worry about. The same reading paired with chills, body aches, or fatigue tells a different story.
On the low end, temperatures below 95°F (35°C) are classified as hypothermia and require immediate attention. This is more common in older adults, people exposed to cold environments, and those with certain metabolic conditions.