A normal body temperature for most adults is around 97.9°F (36.6°C), not the 98.6°F (37°C) figure you probably grew up hearing. That old number dates back to a study from 1868, and large-scale research has since shown that average human body temperature has been dropping by about 0.05°F per decade. A Stanford Medicine study analyzing over 618,000 temperature readings found that normal adult oral temperatures now range from 97.3°F to 98.2°F, with 97.9°F as the overall average.
Why 98.6°F Is Outdated
The 98.6°F standard came from a German physician named Carl Wunderlich, who measured thousands of patients in the mid-1800s. For over a century, that number was treated as gospel. But people in the 19th century had higher rates of chronic infection, untreated dental disease, and tuberculosis, all of which cause low-level inflammation that raises body temperature slightly. As public health improved, average body temperature drifted downward.
Researchers at Stanford found this decline has been steady and consistent across generations. Better living conditions, widespread use of anti-inflammatory medications, and lower rates of chronic infection all likely contribute. So if your thermometer reads 97.5°F and you feel fine, that’s completely normal.
Your Temperature Changes Throughout the Day
Body temperature isn’t static. It follows your circadian rhythm, rising and falling in a predictable pattern. Your lowest reading typically comes in the early morning hours, during the last stretch of sleep, then climbs as you wake and move through the day. Most people hit their daily peak in the late afternoon or early evening.
There’s also a subtle dip between about 2 p.m. and 4 p.m. for most people, which may partly explain that familiar afternoon slump. The total swing from your daily low to your daily high can be about 1°F, so a reading of 97.4°F at 7 a.m. and 98.4°F at 5 p.m. could both be perfectly normal for the same person on the same day.
Factors That Shift Your Baseline
Several things can push your temperature up or down independent of illness. During the second half of the menstrual cycle (the luteal phase, after ovulation), body temperature rises by roughly 0.7°F (0.4°C) compared to the first half. This shift is reliable enough that some people use it to track fertility.
Exercise raises core temperature significantly. Well-trained athletes exercising in the heat can reach internal temperatures of 106.7°F (41.5°C) without harm. Even moderate activity can bump your reading by a degree or more, so checking your temperature right after a workout won’t give you an accurate baseline.
Dehydration also plays a role. For every 1% of body weight lost through fluid loss, core temperature during physical activity rises by about 0.27°F (0.15°C). Hot drinks, heavy clothing, and even recent meals can cause temporary increases too.
How Age Affects Normal Temperature
Babies and young children tend to run slightly warmer than adults, partly because their metabolisms are faster relative to their body size. For infants, a rectal temperature of 100.4°F (38°C) or higher is considered a fever. Older adults tend to run cooler than younger adults, which means a temperature that looks “normal” on a thermometer could actually represent a fever in someone over 65. If you’re older and feel unwell, a reading of 99°F may be more significant than it sounds.
Where You Measure Matters
Not all thermometer readings are equal. Different spots on the body give different numbers, and the offsets are consistent enough to be useful:
- Rectal and ear (tympanic): 0.5 to 1°F higher than an oral reading
- Armpit (axillary): 0.5 to 1°F lower than an oral reading
- Forehead (temporal): 0.5 to 1°F lower than an oral reading
So if your forehead thermometer reads 97.5°F, your oral temperature would likely be around 98°F to 98.5°F. Rectal readings are the most accurate for infants, while oral digital thermometers remain the most reliable option for adults at home. Forehead scanners are convenient but can be thrown off by sweating, direct sunlight, or cold outdoor air.
When a Temperature Becomes a Fever
For adults, an oral temperature of 100°F (37.8°C) or higher is generally considered a fever. At 103°F (39.4°C), it’s worth contacting a healthcare provider. For children, the thresholds depend on age and how you’re measuring. A rectal temperature of 100.4°F (38°C) or higher counts as a fever in babies under three months, and that warrants prompt medical attention even if the baby seems otherwise fine. For children between 3 and 24 months, a rectal temperature above 102°F (38.9°C) is the threshold that raises concern.
Keep in mind that fever itself isn’t a disease. It’s your immune system’s response to infection or inflammation, and moderate fevers often help your body fight off illness more effectively.
When a Temperature Is Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. The stages break down like this:
- Mild hypothermia: 90°F to 95°F (32.2°C to 35°C), causing shivering, confusion, and clumsiness
- Moderate hypothermia: 82.4°F to 90°F (28°C to 32.2°C), where shivering may actually stop and confusion worsens
- Severe hypothermia: below 82.4°F (28°C), which is life-threatening
Hypothermia doesn’t require extreme cold. Older adults in poorly heated homes, people who are wet in windy conditions, or anyone with prolonged cold exposure can develop it gradually. If your thermometer consistently reads below 96°F in a normal indoor environment, that’s worth investigating.
Finding Your Personal Normal
Given that “normal” spans a range and varies by time of day, age, and individual biology, it helps to know your own baseline. Take your temperature a few times over the course of a week when you’re feeling well, using the same thermometer and the same body site each time. Morning readings will be lower, afternoon readings higher. After a few days, you’ll have a personal reference point that makes it much easier to tell when something is actually off.