The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us learned growing up. That familiar standard dates back more than 150 years, and newer research shows human body temperature has been steadily dropping since then. A normal reading for a healthy adult typically falls somewhere between 97°F and 99°F (36.1°C to 37.2°C), depending on the time of day, where on the body you measure, and individual factors like age and sex.
Where 98.6°F Came From
The 98.6°F benchmark traces back to 1868, when a German physician named Carl Wunderlich published an analysis of several million temperature readings taken from roughly 25,000 patients. He calculated the average and declared it the normal physiological temperature. For more than a century, that number stuck.
There’s a catch, though. Wunderlich took armpit (axillary) temperatures using thermometers that were calibrated 2.9°F to 3.4°F higher than modern instruments. In other words, his equipment was consistently reading too high by today’s standards. That measurement error alone accounts for a significant chunk of the difference between his number and what researchers find now.
What Modern Research Shows
A large study published in eLife analyzed temperature data spanning nearly two centuries of Americans and found that average body temperature has dropped by about 0.03°C per decade of birth. Men born in the early 1800s ran temperatures roughly 0.59°C (about 1°F) higher than men today. Women showed a similar decline of about 0.32°C since the 1890s. Overall, the average human body temperature in high-income countries is now about 1.6% lower than it was in the pre-industrial era.
The reasons aren’t entirely clear, but the leading theory points to reduced chronic inflammation. People in the 1800s lived with more untreated infections, worse dental health, and higher rates of conditions like tuberculosis. Widespread inflammation raises baseline temperature. Modern sanitation, antibiotics, and improved living conditions have likely cooled us down over generations.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable daily cycle tied to your circadian rhythm. Your lowest temperature occurs during sleep, then starts climbing in the final hours before you wake. It generally peaks in the late afternoon or early evening. Most people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon energy slump. The total swing from your daily low to high can be about 1°F (0.6°C), so a reading of 97.5°F in the morning and 98.5°F in the evening can both be perfectly normal for the same person.
How Age and Hormones Affect Temperature
Babies and young children tend to run warmer than adults. Their higher metabolic rate and greater surface area relative to body weight both play a role. In children, a rectal or ear temperature of 100.4°F (38°C) or higher is considered a fever.
Older adults tend to run cooler. Baseline temperatures gradually decline with age, which means a reading that looks “normal” on paper could actually represent a fever in someone over 65. This is one reason infections in older people sometimes go undetected longer.
Menstrual cycles also create a predictable shift. During the second half of the cycle (the luteal phase, after ovulation), the hormone progesterone pushes basal body temperature up by 0.3 to 0.5°C (roughly 0.5 to 0.9°F). This rise is consistent enough that some people use daily temperature tracking as a way to identify ovulation.
Where You Measure Matters
Different spots on the body give different readings, and understanding the offsets helps you interpret your number correctly. Oral temperature is the most common reference point for adults, so the differences are usually described relative to an oral reading:
- Rectal: 0.5 to 1°F (0.3 to 0.6°C) higher than oral
- 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
- Forehead (temporal artery): generally close to oral, though accuracy varies by device
Rectal readings are considered the most accurate, especially for infants and young children. Armpit readings are the least precise but the easiest to take. If you’re using an armpit thermometer at home, keep in mind that your actual core temperature is likely about half a degree to a full degree Fahrenheit higher than what the display shows.
When Temperature Signals a Problem
For adults, most clinicians consider an oral temperature at or above 100.4°F (38°C) a fever. A reading between 99°F and 100.3°F is often called a low-grade fever, though some of those readings may simply reflect normal daily variation or recent physical activity. Context matters: a temperature of 99.5°F after exercise on a warm day is very different from the same reading at rest in the morning.
On the low end, hypothermia begins when core temperature drops below 95°F (35°C). Mild hypothermia (95°F to 89.6°F) causes shivering, confusion, and poor coordination. Moderate hypothermia (89.6°F to 82.4°F) brings drowsiness, slurred speech, and a slowing heart rate. Severe hypothermia, below 82.4°F (28°C), is a medical emergency where the heart and breathing can stop.
Finding Your Own Baseline
Because “normal” covers such a wide range, knowing your personal baseline is more useful than comparing yourself to a population average. Take your temperature at the same time of day, using the same method, over several days when you’re feeling well. Most people find their typical reading lands somewhere between 97.2°F and 98.6°F when measured orally in the morning. Once you know your own pattern, a reading that’s 1°F or more above your usual baseline is a more reliable signal that something is off than simply comparing to 98.6°F.