The normal temperature for a human body is about 97.9°F (36.6°C), not the 98.6°F (37.0°C) you probably learned growing up. That familiar number dates back to a German study from the 1860s, and modern research consistently shows it’s a bit too high. A large systematic review covering 36 studies found the average across all adults and measurement methods was 97.9°F (36.59°C), with a normal range spanning roughly 97.1°F to 98.6°F (36.16°C to 37.02°C).
Why 98.6°F Is Outdated
The 98.6°F standard comes from the work of Carl Wunderlich, a 19th-century physician who took millions of temperature readings and declared 37.0°C the human average. For over a century, that number stuck. But a landmark reanalysis at the University of Maryland found that the true mean oral temperature was closer to 98.2°F (36.8°C), and the upper limit of normal was 99.9°F (37.7°C) rather than the traditional 100.4°F.
Several factors likely explain the shift. Thermometer technology has improved dramatically since the 1860s. Population health has also changed: lower rates of chronic infection, better dental health, and reduced inflammation may all contribute to slightly cooler average readings in modern populations.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable 24-hour cycle. It bottoms out in the early morning, typically around 6 AM, and peaks in the late afternoon between 4 and 6 PM. The difference between your daily low and high averages about 0.9°F (0.5°C), though in some people it can swing by as much as 1.6°C over the course of a day.
This means a reading of 99.1°F at 5 PM can be perfectly normal, while the same number at 6 AM might signal a low-grade fever. The time of day matters when interpreting any single reading.
How Age Affects Normal Temperature
Younger adults tend to run slightly warmer than older adults. In the systematic review, people under 60 averaged 98.0°F (36.69°C), while those 60 and older averaged 97.7°F (36.5°C). This decline with age is well documented and appears to reflect a gradual slowing of metabolic rate.
Children, especially infants and toddlers, tend to run warmer still. Normal ranges for young children are slightly broader. For babies under 3 months, normal ear temperature falls between about 96.4°F and 99.4°F (35.8°C to 37.4°C). For toddlers between 3 months and 3 years, the normal range extends from about 95.7°F up to 99.6°F (35.4°C to 37.6°C). For children older than 3 and adults, readings up to about 99.9°F (37.7°C) are generally within normal limits.
Other Factors That Shift Your Baseline
Sex makes a small difference: women’s temperatures average very slightly lower than men’s across large studies, though the gap is so small (a tenth of a degree) that it’s clinically meaningless. However, the menstrual cycle creates a more noticeable shift. After ovulation, basal body temperature rises by 0.4°F to 1.0°F (0.22°C to 0.56°C) and stays elevated until the next period. This is the principle behind temperature-based fertility tracking.
Exercise, stress, alcohol, poor sleep, illness, certain medications, and even jet lag can all nudge your temperature up or down. Two readings taken hours apart on the same day in the same person can differ by a full degree without anything being wrong.
Where You Measure Matters
Different spots on the body give different numbers. Rectal readings run highest, averaging about 98.7°F (37.04°C). Oral readings land around 97.9°F (36.57°C). Ear (tympanic) readings tend to be close to oral, around 97.95°F (36.64°C). Armpit readings are the lowest, averaging about 96.7°F (35.97°C).
As a rough guide for converting between methods:
- Rectal and ear readings are about 0.5°F to 1.0°F (0.3°C to 0.6°C) higher than an oral reading.
- Armpit readings are about 0.5°F to 1.0°F (0.3°C to 0.6°C) lower than an oral reading.
Rectal temperature is considered the most accurate reflection of core body temperature, which is why it’s the standard for infants. For older children and adults, oral readings are the most common reference point.
When a Temperature Becomes a Fever
Most clinicians define a fever as an oral temperature at or above 100.4°F (38.0°C). Below that, a reading between about 99.5°F and 100.3°F (37.5°C to 37.9°C) is often called a low-grade fever, a sign of mild immune activation that’s usually not concerning on its own.
In adults, fevers below 103°F (39.4°C) are typically not dangerous. Above that level, it’s worth contacting a healthcare provider. For children, the threshold for concern is a bit higher: 104°F (40°C). Untreated fevers above 105.8°F (41°C) become genuinely dangerous, as organs begin to malfunction at sustained high temperatures.
When a Temperature Is Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. The stages break down by severity:
- Mild hypothermia: 90°F to 95°F (32°C to 35°C). You’ll shiver intensely, and your coordination and judgment start to decline.
- Moderate hypothermia: 82°F to 90°F (28°C to 32°C). Shivering may actually stop, confusion worsens, and heart rhythm can become irregular.
- Severe hypothermia: Below 82°F (28°C). This is a medical emergency with a risk of cardiac arrest.
What “Normal” Really Means for You
Normal body temperature is a range, not a single number. For most healthy adults, an oral reading somewhere between 97.1°F and 99.0°F (36.2°C to 37.2°C) at a random point during the day is unremarkable. Your personal baseline may sit at the lower or higher end of that range, and it will shift depending on the time of day, your activity level, and where on your body you take the reading.
If you want to know your own baseline, take your temperature at the same time of day, using the same method, for several days in a row when you’re feeling well. That personal average is more useful than any textbook number when you’re trying to decide whether a future reading is actually elevated.