The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us grew up hearing. That familiar figure dates back to the 1860s and, while it served medicine well for over a century, modern research shows human body temperature has been slowly declining. What counts as “normal” also shifts depending on your age, the time of day, where you measure, and whether you menstruate.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published his landmark work in 1868 after analyzing several million temperature readings from an estimated 25,000 patients. He reported 37°C (98.6°F) as the mean of that enormous data set, and the number stuck for more than 150 years.
There’s a catch, though. Wunderlich measured temperatures under the arm, not in the mouth. And his thermometers were calibrated 2.9°F to 3.4°F higher than thermometers used today. That means his instruments were reading hot compared to modern equipment, which likely pushed his average upward from the start.
What Modern Research Actually Shows
A large Stanford University study published in the journal eLife analyzed temperature data spanning nearly two centuries of Americans. The findings were striking: men born in the early 1800s ran temperatures about 0.59°C (roughly 1°F) higher than men today, with a steady decline of about 0.03°C per decade. Women showed a similar pattern, dropping about 0.32°C since the 1890s at nearly the same rate.
The takeaway is that normal oral temperature in the U.S. now averages below 37°C. Several factors likely explain the decline, including lower rates of chronic infection, reduced inflammation, less physically demanding lifestyles, and better climate-controlled environments. Whatever the cause, 98.6°F no longer represents the true midpoint for most people.
Normal Ranges by Age
Body temperature isn’t one number. It’s a range, and that range shifts significantly with age.
- Children (birth to age 10): 95.9°F to 99.5°F (35.5°C to 37.5°C), measured orally.
- Adults and older children (ages 11 to 65): 97.6°F to 99.6°F (36.4°C to 37.6°C).
- Older adults (over 65): 96.4°F to 98.5°F (35.8°C to 36.9°C).
Young children tend to run warmer because their metabolisms are higher relative to their body size. Older adults skew cooler, sometimes significantly so. This matters because a reading of 99°F might be completely normal in a five-year-old but could signal a brewing infection in someone who’s 75 and typically reads 97°F. Knowing your own baseline is more useful than comparing to any single “normal” number.
How Temperature Changes Throughout the Day
Your body temperature follows a predictable daily cycle tied to your circadian rhythm. It starts rising in the last hours of sleep, just before you wake up, and generally peaks in the late afternoon or early evening. Then it drops as nighttime approaches, reaching its lowest point in the early morning hours. Many people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon drowsiness.
This daily swing can span a full degree or more, meaning a reading of 97.5°F at 6 a.m. and 98.8°F at 5 p.m. could both be perfectly normal for the same person on the same day. If you’re tracking your temperature for any reason, measuring at the same time each day gives you the most consistent picture.
Hormonal Effects on Body Temperature
People who menstruate see a reliable temperature shift each cycle. After ovulation, basal body temperature (your temperature at complete rest, typically measured first thing in the morning) rises by less than half a degree Fahrenheit. The increase varies from person to person, ranging from as little as 0.4°F to as much as 1°F. This bump stays elevated through the luteal phase and drops back down when menstruation begins.
This pattern is consistent enough that some people use daily temperature tracking as a fertility awareness tool, since the rise confirms that ovulation has occurred. It also means a slightly elevated reading in the second half of the menstrual cycle is normal and not a sign of illness.
Why Measurement Location Matters
The number on your thermometer depends heavily on where you take the reading. Oral temperature is the most common reference point, but other methods give slightly different results.
- Rectal: reads 0.5°F to 1°F higher than oral.
- Ear (tympanic): reads 0.5°F to 1°F higher than oral.
- Armpit (axillary): reads 0.5°F to 1°F lower than oral.
- Forehead (temporal): reads 0.5°F to 1°F lower than oral.
So if an armpit reading shows 97.5°F, the oral equivalent is roughly 98°F to 98.5°F. Rectal readings are considered the most accurate, especially for infants and young children, because they’re less affected by breathing, recent drinks, or ambient air temperature. For everyday use in adults, oral and forehead thermometers are the most practical options.
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 broadly and is the standard used in clinical settings, schools, and workplace screening. Anything between your normal baseline and 100.4°F is sometimes called a “low-grade” temperature elevation, which can result from exercise, warm clothing, hot weather, or mild illness.
Keep in mind that older adults may have a genuine fever at a lower reading than 100.4°F because their baseline runs cooler. A temperature of 99.5°F in someone who normally reads 96.8°F represents a larger jump than the same reading in a younger adult. Context and personal baseline matter more than any single cutoff.