Normal body temperature for most adults falls between 97.0°F (36.1°C) and 99.0°F (37.2°C) when measured orally, with an average closer to 97.9°F (36.6°C) rather than the commonly cited 98.6°F. That old number dates back to the 1860s and no longer reflects what modern thermometers consistently find. Your own “normal” can shift by more than a full degree throughout the day, and it varies based on your age, where you measure, and several other factors.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician named Carl Wunderlich, who published a massive study in 1868 analyzing over one million temperature readings from roughly 25,000 patients. He identified 37.0°C (98.6°F) as the average and it stuck for over 150 years. But his methods were very different from what we use today. His thermometers were bulky, required 15 to 20 minutes to stabilize, and he measured under the armpit rather than in the mouth. Modern thermometers are faster, more accurate, and typically used orally or in the ear.
When researchers have revisited this question with current tools and populations, the numbers come in lower. A large Stanford analysis tracking body temperatures across birth cohorts found that average temperatures have dropped by about 0.05°F per decade since the early 1800s, totaling roughly a 1°F decline overall. A separate study of more than 35,000 British patients found the mean oral temperature to be 36.6°C (97.9°F). The reasons for this long-term decline aren’t fully settled, but reduced rates of chronic infection and inflammation in modern populations are leading explanations.
What “Normal” Looks Like by Time of Day
Your body temperature isn’t a fixed number. It follows a predictable daily cycle driven by your internal clock. Temperature is lowest in the early morning hours, often dipping to around 97.4°F (36.3°C), and climbs to its peak in the late afternoon, reaching as high as 99.6°F (37.6°C). That means a reading of 99.2°F at 5 p.m. could be perfectly healthy, while the same number at 6 a.m. might suggest something is off.
Most people also experience a small dip between 2 p.m. and 4 p.m. before temperatures rise again toward the evening peak. Body temperature then begins dropping at night as the body prepares for sleep, and it starts rising again in the final hours before waking.
How Age Affects Body Temperature
Children tend to run warmer than adults and produce higher fevers when they’re sick. This is partly because their metabolic rate is higher relative to body size. In contrast, older adults and people who are frail often run cooler than average and may not mount a noticeable fever even when fighting an infection. For these groups, a temperature that looks “normal” on a chart could actually signal a problem if it’s significantly above their personal baseline.
Temperature Varies by Measurement Site
Where you place the thermometer changes the number you get. Oral readings (under the tongue) are the most common reference point, and the offsets from other sites are fairly consistent:
- Rectal: 0.5°F to 1°F (0.3°C to 0.6°C) higher than oral
- Ear (tympanic): 0.5°F to 1°F (0.3°C to 0.6°C) higher than oral
- Armpit (axillary): 0.5°F to 1°F (0.3°C to 0.6°C) lower than oral
- Forehead (temporal): 0.3°F to 1°F (0.5°C to 0.6°C) lower than oral
This matters when you’re comparing a number to a fever threshold. A forehead reading of 99.5°F doesn’t mean the same thing as a rectal reading of 99.5°F. If you’re using an armpit or forehead thermometer, your actual core temperature is likely somewhat higher than what you see on the display.
Other Factors That Shift Your Baseline
Beyond time of day and measurement site, several things can push your temperature up or down without signaling illness. Exercise raises body temperature noticeably and can keep it elevated for a while afterward. Stress, alcohol, disrupted sleep, and travel across time zones can all influence readings.
For women of reproductive age, ovulation causes a small but measurable rise in basal body temperature, typically less than half a degree Fahrenheit (0.3°C). This shift is subtle enough that it requires a sensitive thermometer and consistent morning measurements to detect, but it’s real and sustained through the second half of the menstrual cycle. Certain medications can also affect baseline temperature.
When Temperature Signals a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies to oral readings and is the standard used in most clinical settings. Below that, in the 99°F to 100.3°F range, is sometimes called a low-grade fever, though opinions vary on exactly where “normal” ends and “elevated” begins. This gray zone is why knowing your own baseline matters.
For oral temperatures in adults and children over 12, fever severity breaks down roughly like this:
- Mild fever: up to 100.3°F (37.9°C)
- Moderate fever: 100.4°F to 103.9°F (38°C to 39.9°C)
- High fever: 104°F (40°C) and above
If you’re measuring in the ear, those thresholds shift upward by roughly half a degree to a full degree. For armpit readings, they shift downward by a similar amount.
When Temperature Drops Too Low
On the other end of the spectrum, a body temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia ranges from 95°F down to about 89.6°F (35°C to 32°C) and typically causes shivering, confusion, and clumsiness. Moderate hypothermia, between 89.6°F and 82.4°F (32°C to 28°C), brings more severe confusion and drowsiness, and shivering may actually stop. Severe hypothermia, below 82.4°F (28°C), is a medical emergency where the heart and organs are at risk of failure.
Hypothermia is most associated with cold exposure, but it can also develop in older adults or frail individuals in modestly cool indoor environments, especially if they’re not eating enough or have certain medical conditions. Low body temperature can occasionally accompany infection in these same groups, making it a less obvious but important warning sign.