The average human body temperature is about 97.9°F (36.5°C), not the 98.6°F (37°C) figure most of us learned growing up. That old number dates back to the 1800s and no longer reflects what researchers consistently find in modern populations. Your own “normal” can fall anywhere within a range, and several everyday factors shift it up or down throughout the day.
Where 98.6°F Came From
In the mid-1800s, a German physician named Carl Wunderlich measured armpit temperatures from about 25,000 people and calculated an average of 98.6°F (37°C). That number became the medical standard for more than a century. But Wunderlich’s subjects lived in an era when tuberculosis, syphilis, chronic gum disease, and other infections were widespread and often untreated. Persistent inflammation raises body temperature, which means his average was likely skewed upward from the start.
Why the Average Has Dropped
A Stanford University study analyzing temperature records spanning 157 years found that average oral temperature has fallen steadily, about 0.05°F (0.03°C) per decade. 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. The new average sits closer to 97.5°F (36.4°C) for oral readings.
Two factors likely explain the shift. First, modern medicine has dramatically reduced the chronic infections and inflammation that were common in Wunderlich’s time. Second, overall metabolic rate appears to have dropped. One theory ties this to higher average body mass, which some studies link with a lower resting metabolic rate. Better nutrition, climate-controlled homes, and improved health all contribute to a cooler baseline.
The Normal Range for Adults
There is no single “normal” number. A large systematic review of healthy adults found that average oral temperature runs about 98.1°F (36.7°C) for people under 60. The normal oral range spans roughly 96.3°F to 99.3°F (35.7°C to 37.4°C) when you account for natural variation across individuals. Rectal readings tend to be the highest, averaging about 98.8°F (37.1°C), while armpit readings are the lowest, averaging around 96.9°F (36.0°C).
Adults 60 and older consistently run cooler. Their average oral temperature drops to about 97.6°F (36.4°C), and armpit readings average just 96.4°F (35.8°C). This matters because a temperature that looks “normal” in an older adult could actually represent a fever relative to their lower baseline.
How Where You Measure Changes the Number
The thermometer’s location makes a noticeable difference. On average, armpit readings come in about 0.5°F (0.25°C) lower than oral readings and nearly 0.8°F (0.43°C) lower than rectal readings. Ear (tympanic) thermometers land somewhere in between, averaging about 98.3°F (36.8°C) in adults under 60.
If you’re comparing a reading to the 100.4°F fever threshold (more on that below), keep the measurement site in mind. An armpit temperature of 99.5°F may indicate more of a fever than it appears because that same reading taken orally would likely be closer to 100°F.
Why Your Temperature Fluctuates All Day
Your body temperature follows a predictable circadian rhythm, rising and falling by about 1°F to 1.5°F (0.5°C to 0.8°C) over the course of 24 hours. It bottoms out in the early morning hours, typically between 4 and 6 a.m., then climbs through the day and peaks in the late afternoon or early evening, usually around 10 hours after you wake up. So a reading of 97.2°F at 7 a.m. and 98.6°F at 5 p.m. can both be perfectly normal for the same person.
Ambient temperature also plays a small role. In one study, core body temperature reached 98.4°F (36.9°C) by evening when room temperature gradually rose throughout the day, compared to 97.7°F (36.5°C) when room temperature gradually fell. The effect is modest, but it helps explain why you might get slightly different readings depending on your environment.
Sex, Size, and Individual Variation
Women tend to run slightly warmer than men on average, and their temperatures fluctuate more due to hormonal cycles. Core temperature rises by about 0.5°F to 1°F after ovulation and stays elevated until menstruation begins. However, a large part of the apparent sex difference in temperature regulation comes down to body size. When researchers matched men and women for body mass and body surface area, the differences in skin temperature and metabolic heat production largely disappeared. Smaller people lose heat faster because they have more surface area relative to their volume.
Height and weight also independently influence baseline temperature. Younger people tend to run warmer than older people, and physically active individuals often have slightly different temperature patterns than sedentary ones. All of this means your personal “normal” could easily be half a degree above or below someone else’s, and both are healthy.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold applies to oral or rectal readings. A temperature between 99°F and 100.4°F is sometimes called a low-grade fever, though it can also just reflect normal daily variation, exercise, or a warm environment.
Context matters more than a single number. If your baseline runs around 97.5°F and you suddenly read 99.8°F, your body has mounted a significant response even though you haven’t crossed the official fever line. This is especially relevant for older adults, whose baseline tends to be lower. Paying attention to how a reading compares to your own typical temperature gives you better information than measuring it against a universal cutoff.