Normal body temperature for most adults falls around 97.5°F to 97.9°F (36.4°C to 36.5°C) when measured orally, not the 98.6°F (37°C) number you probably grew up hearing. That familiar figure dates back to a German physician’s measurements in the 1860s, and human body temperature has been steadily dropping since then. The true “normal” depends on where you measure, your age, the time of day, and what you’ve been doing.
Why 98.6°F Is Outdated
The 98.6°F standard comes from Carl Reinhold August Wunderlich, who published millions of temperature readings from patients in the mid-1800s. For over 150 years, that number stuck. But a large study published in eLife, analyzing temperature data from Americans born between the early 1800s and 1997, found that body temperature has declined by about 0.05°F (0.03°C) per decade. Men born in the early 19th century ran temperatures roughly 1.06°F (0.59°C) higher than men today. Women showed a similar drop of about 0.58°F (0.32°C) since the 1890s.
The reasons aren’t entirely clear, but researchers point to lower rates of chronic infection, reduced inflammation, more climate-controlled environments, and changes in metabolic rate over time. Whatever the cause, the decline is consistent and real. A critical reappraisal of Wunderlich’s original data suggested that 98.6°F should be abandoned entirely, proposing that the upper limit of normal oral temperature in healthy adults under 40 is closer to 99.0°F (37.2°C) in the early morning and 99.9°F (37.7°C) overall.
Normal Ranges by Measurement Site
Where you place the thermometer changes the number you’ll see. A systematic review of healthy adults found the following average ranges (defined as the mean plus or minus two standard deviations):
- Oral (under the tongue): 96.3°F to 99.3°F (35.73°C to 37.41°C)
- Rectal: 97.4°F to 100.0°F (36.32°C to 37.76°C)
- Ear (tympanic): 96.4°F to 99.5°F (35.76°C to 37.52°C)
- Armpit (axillary): 95.0°F to 98.5°F (35.01°C to 36.93°C)
Rectal readings run the highest because they measure your core temperature more directly. Armpit readings are the lowest, typically about 0.8°F (0.43°C) below rectal and 0.45°F (0.25°C) below oral. This matters when you’re checking for a fever: a perfectly normal armpit reading of 97.5°F might correspond to an oral reading closer to 98.0°F. If you’re using an armpit thermometer, a reading of 99.5°F (37.5°C) or higher is a reliable indicator that an oral thermometer would show a fever.
How Age Affects Your Baseline
Body temperature tends to run lower as you get older. In adults under 60, the average across all measurement sites is about 98.0°F (36.69°C). For adults 60 and older, that drops to about 97.7°F (36.5°C), and the variation from person to person widens. Looking at oral measurements specifically, younger adults average 98.1°F (36.74°C) while older adults average 97.6°F (36.42°C), a gap of about half a degree Fahrenheit.
This lower baseline in older adults is clinically important. Someone over 60 with a temperature of 99.5°F might be mounting a significant immune response even though they haven’t crossed the standard fever threshold. If you’re older and feeling unwell, a temperature that seems borderline could be more meaningful than the same reading in a younger person.
For infants, temperature regulation works differently. Babies under three months old with any rectal temperature of 100.4°F (38°C) or higher need immediate medical attention, regardless of how the baby appears.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm driven by your internal clock. It bottoms out in the early morning hours, typically between 4 a.m. and 6 a.m., and peaks in the late afternoon or early evening. The swing between your lowest and highest point can be roughly 1°F (0.5°C) or more, which means a reading of 97.2°F at 6 a.m. and 98.4°F at 5 p.m. could both be completely normal for the same person on the same day.
This is why a single temperature reading doesn’t tell the full story. If you’re monitoring your temperature because you feel off, try to compare readings taken at the same time of day.
Other Factors That Shift Your Reading
Exercise raises body temperature because working muscles produce heat as a byproduct. Your body compensates through sweating and increased blood flow to the skin, but during and shortly after a workout, your temperature can climb well above baseline. Wait at least 20 to 30 minutes after physical activity before checking your temperature if you want an accurate resting number.
Hormonal cycles also play a role. In women who menstruate, body temperature rises roughly 0.5°F to 1°F after ovulation and stays elevated through the second half of the cycle. This shift is reliable enough that it’s used as a fertility tracking method. Hot drinks, cold drinks, and recent meals can also temporarily skew an oral reading.
When a Temperature Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold applies whether you’re measuring orally, rectally, or with an ear thermometer. Temperatures between your personal baseline and 100.4°F are sometimes called “low-grade fevers,” though this isn’t a formal medical category.
In adults, fevers of 103°F (39.4°C) or higher typically come with visible signs of illness, including chills, body aches, fatigue, and loss of appetite. A temperature that high in an adult signals the immune system is working hard and warrants close monitoring. For children and especially infants, fever thresholds that trigger concern are lower, and the younger the child, the more urgently a fever should be evaluated.
Finding Your Own Normal
Because “normal” spans a range of about two to three degrees depending on where you measure, the most useful thing you can do is figure out your own baseline. Take your temperature at the same time of day, using the same method, over several days when you’re feeling well. Most people will find their oral temperature lands somewhere between 97.2°F and 98.4°F, with higher readings in the afternoon.
Knowing your personal baseline makes it easier to spot a meaningful change. A jump from your usual 97.5°F to 99.8°F is a roughly two-degree rise that tells you something is happening, even though 99.8°F wouldn’t technically meet the standard fever definition.