Normal body temperature for adults averages about 97.9°F (36.6°C), not the 98.6°F (37°C) most of us grew up hearing. That familiar number dates back to the 1860s and no longer reflects what thermometers actually show in healthy people today. Most adults fall somewhere between 97.3°F and 98.2°F, with natural variation based on age, sex, time of day, and where on the body you measure.
Where 98.6°F Came From
The 98.6°F standard traces back to 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 declared 37.0°C (98.6°F) the mean temperature of healthy adults, and that number stuck for more than 150 years.
The problem is that Wunderlich’s methods were very different from what we use now. His thermometers were bulky, had to be read while still in the armpit, and took 15 to 20 minutes to stabilize. Modern thermometers are faster, more accurate, and typically measure from the mouth or ear rather than the armpit. When researchers at Stanford tested his claim using modern tools, they found the actual mean oral temperature was 98.2°F, and 98.6°F held no special significance. It wasn’t the mean, the median, or even the most frequently recorded temperature in their study.
Why Humans Are Running Cooler
It’s not just better thermometers. Human body temperature appears to have genuinely dropped over the past two centuries. A large study published in eLife analyzed temperature records spanning birth decades from the early 1800s through the late 1990s and found a steady decline: about 0.05°F per decade in both men and women. Men born in the 1990s ran roughly 1.06°F cooler than men born in the early 1800s.
The likely explanation involves chronic inflammation. In the 19th century, untreated infections like tuberculosis, syphilis, and gum disease were widespread, and ongoing inflammation raises body temperature. As sanitation, antibiotics, and dental care improved, baseline inflammation dropped, and average temperatures fell with it. Body temperature also tracks with metabolic rate, so shifts in how our bodies burn energy over generations may play a role as well.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a daily cycle, dipping to its lowest point around 6 a.m. and peaking around 8 p.m. The difference between that low and high point typically ranges from 0.5°F to 1.9°F in healthy people. This means a reading of 97.5°F in the early morning and 98.8°F in the evening can both be perfectly normal for the same person.
This daily swing matters when you’re checking for a fever. A temperature of 99.5°F at 7 a.m. is more meaningful than the same reading at 8 p.m., because your body is naturally warmer in the evening. Researchers have suggested that 98.9°F in the early morning and 99.9°F at any time of day represent more useful upper limits of normal for adults under 40.
What Counts as a Fever
A fever is generally defined as a body temperature of 100.4°F (38°C) or higher. Below that threshold, elevated readings break down into severity levels:
- Low-grade fever: 99.1°F to 100.4°F (37.3°C to 38.0°C)
- Moderate fever: 100.6°F to 102.2°F (38.1°C to 39.0°C)
- High fever: 102.4°F to 105.8°F (39.1°C to 41.0°C)
For children, the fever threshold depends on how the temperature is taken. A rectal or ear reading of 100.4°F or higher signals a fever, while an oral reading of 100°F or an armpit reading of 99°F meets the same threshold. Rectal readings run higher than oral ones because they measure closer to the body’s core.
Where You Measure Matters
Different spots on the body give different readings, and knowing the gap helps you interpret your thermometer correctly. Rectal and ear temperatures read closest to true core temperature. Oral readings typically come in slightly lower, and armpit readings lower still. The fever cutoffs reflect this: 100.4°F rectally, 100°F orally, and 99°F under the arm all represent roughly the same internal temperature.
If you’re comparing readings over time, try to use the same method and the same time of day. Switching from an oral thermometer in the morning to an ear thermometer in the evening introduces two variables at once, making it hard to spot a real trend.
Factors That Shift Your Baseline
A combination of age, sex, height, weight, and time of day accounts for about 25% of the variation in any individual’s normal temperature. Some of the most common influences:
Age plays a significant role. Children tend to run warmer than adults, and older adults tend to run cooler. This means a temperature that looks normal in an elderly person could actually represent a fever relative to their usual baseline.
Hormonal cycles affect temperature predictably. During ovulation, basal body temperature rises by less than half a degree Fahrenheit and stays elevated for three or more days. This small but consistent shift is the basis of temperature-based fertility tracking.
Other factors that can temporarily raise or lower your reading include illness, stress, poor sleep, alcohol, shift work, travel across time zones, and certain medications. Physical activity raises temperature in the short term, which is why basal body temperature is measured first thing in the morning before getting out of bed.
When Body Temperature Drops Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. It progresses in stages:
- Mild hypothermia: 90°F to 95°F (32.2°C to 35°C), marked by shivering and confusion
- Moderate hypothermia: 82.4°F to 90°F (28°C to 32.2°C), where shivering may stop and coordination deteriorates significantly
- Severe hypothermia: below 82.4°F (28°C), a life-threatening emergency
A persistently low temperature in everyday life, say 96°F or below without cold exposure, can sometimes signal an underactive thyroid or other metabolic issue worth investigating.
Finding Your Personal Normal
Because “normal” spans a range rather than landing on a single number, it helps to know your own baseline. Take your temperature at the same time of day, using the same method, on several days when you feel well. Most people will land somewhere between 97.3°F and 98.2°F orally. Once you know your personal average, you can spot a meaningful rise more quickly, even if the thermometer hasn’t crossed the official fever line.