The correct body temperature isn’t one fixed number. The long-held standard of 98.6°F (37°C) is outdated. Research from Stanford Medicine shows that the average human body temperature today is closer to 97.9°F (36.6°C), and healthy adults typically fall anywhere between 97.3°F and 98.2°F. Your personal normal depends on your age, the time of day, where on your body you measure, and even your sex.
Why 98.6°F Is No Longer the Standard
The 98.6°F figure dates back to 1868, when a German physician named Carl Wunderlich published an analysis of millions of temperature readings from roughly 25,000 patients. He declared 37°C (98.6°F) the mean of his dataset, and that number stuck for more than 150 years.
There are good reasons to let it go. Wunderlich measured temperatures under the armpit using thermometers that were calibrated 2.9°F to 3.4°F higher than modern instruments, making direct comparisons unreliable. A landmark 1992 study confirmed that 98.6°F wasn’t the overall mean, the median, or even the most commonly recorded oral temperature in healthy adults. And Stanford researchers have found that average body temperature in the U.S. has been dropping by about 0.05°F per decade since the 1800s, likely due to lower rates of chronic infection and inflammation in modern populations.
Despite all of this, 98.6°F persists in popular knowledge and even in some clinical settings. Researchers have attributed this partly to a psychological phenomenon called belief perseverance, where people cling to a familiar number even when evidence contradicts it.
What’s Actually Normal for Adults
For most adults, a normal oral temperature falls between 97.3°F and 98.2°F (36.3°C to 36.8°C), with an average around 97.9°F. But “normal” really means your own baseline, which can shift throughout the day and across your lifespan. Two healthy people can have resting temperatures that differ by nearly a full degree and both be perfectly fine.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. It’s lowest in the early morning, typically between 6 and 8 a.m., and peaks in the evening around 7 to 9 p.m. This swing is significant enough to affect medical decisions. For example, a man over 65 has roughly a 30% chance of registering a fever at 9 a.m. but a 55% chance of the same fever showing up at 8 p.m., simply because his baseline is higher later in the day.
This is worth keeping in mind if you’re monitoring a temperature at home. A reading of 99°F in the morning is more meaningful than the same reading at night, when your body is naturally warmer.
How Age and Hormones Shift the Baseline
Older adults tend to run cooler than younger people, which means a temperature that looks “normal” on a thermometer could actually represent a fever in someone over 65. This lower baseline is one reason infections in older adults sometimes go undetected early on.
Young children, on the other hand, tend to have slightly higher resting temperatures. For infants and toddlers, fever thresholds are defined more precisely because the stakes are higher: a rectal temperature of 100.4°F (38°C) or above is considered a fever in children.
Hormonal cycles also play a role. After ovulation, a woman’s basal body temperature rises by roughly 0.3°F to 0.5°F and stays elevated until the next menstrual period begins. This shift is small but consistent enough that it’s used as a method of tracking fertility.
Where You Measure Matters
Not all spots on the body give the same reading. Rectal temperatures run the highest and are considered the most accurate reflection of core body temperature, which is why they’re the standard for infants. Oral readings come in slightly lower. Armpit (axillary) readings are the lowest and least precise of the three.
The fever thresholds for children illustrate the gap clearly. A fever is defined as 100.4°F when measured rectally, 100°F orally, and 99°F under the armpit. The same child at the same moment would produce three different numbers depending on the thermometer’s location. Forehead (temporal artery) and ear readings fall closer to the rectal range but can be affected by sweat, ambient temperature, or earwax.
If you’re comparing readings over time, use the same method and the same site each time. Switching between an armpit thermometer and an oral one will make your trend line meaningless.
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 to adults and children alike, though for oral readings in children, 100°F is the cutoff.
Context matters more than a single number, though. If your personal baseline runs around 97.5°F, a reading of 99.5°F represents a two-degree jump, which could signal that your immune system is responding to something even though you haven’t technically hit the fever threshold. Paying attention to how you feel alongside the number gives a more complete picture than the thermometer alone.
Low-grade temperatures between 99°F and 100.4°F can result from exercise, warm environments, heavy clothing, or the natural evening peak. A single reading in that range without other symptoms is rarely cause for concern.
How to Get an Accurate Reading
For the most reliable result, take your temperature after sitting still for at least 15 minutes. Eating, drinking, or exercising right before a reading can skew it. If you’re using an oral thermometer, avoid hot or cold beverages for 15 to 30 minutes beforehand.
Digital thermometers are the standard for home use. Glass mercury thermometers are no longer recommended due to the risk of breakage and mercury exposure. For young children, a rectal digital thermometer provides the most accurate reading. For older children and adults, oral or forehead thermometers are practical and reasonably accurate when used consistently.
If you’re tracking a temperature over the course of an illness, measure at the same time each day, using the same device and the same body site. This gives you a reliable trend rather than a collection of numbers you can’t compare.