The average human body temperature is about 98.6°F (37°C), but that number is outdated. Modern research puts the true average closer to 98.2°F (36.8°C), and healthy people regularly fall anywhere between 97°F and 99°F (36.1°C to 37.2°C). Your own “normal” depends on the time of day, how you measure it, your age, and your sex.
Where the 98.6°F Number Came From
The 98.6°F figure 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 identified 37°C (98.6°F) as the average for healthy adults, and the number stuck for more than 150 years.
The problem is that Wunderlich’s methods don’t hold up well by modern standards. His thermometers were bulky, took 15 to 20 minutes to settle on a reading, and were placed under the arm rather than in the mouth or ear. Today’s instruments are faster, more accurate, and typically used at different sites. When researchers at JAMA re-examined the question, they found the actual mean oral temperature was 98.2°F (36.8°C), not 98.6°F.
Why Average Body Temperature Has Dropped
It’s not just better thermometers. A Stanford Medicine study found that average body temperature in the United States has genuinely decreased over time. The leading explanation is a population-wide decline in inflammation. Inflammation triggers proteins that raise metabolism and generate heat. As modern medicine reduced chronic infections and inflammatory diseases, baseline temperatures dropped along with them.
Climate-controlled living plays a role too. In the 19th century, homes had inconsistent heating and no air conditioning. Your body had to burn more energy just to stay at a stable internal temperature. Today, central heating and cooling do that work for you, lowering your resting metabolic rate and, with it, your temperature.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. It’s lowest in the early morning hours, then begins rising during the last stretch of sleep, just before you wake up. It peaks in the late afternoon or early evening, then drops again as nighttime approaches and your body prepares for sleep. Most people also experience a small dip between 2 p.m. and 4 p.m., which partly explains that familiar afternoon slump.
This daily swing can span a full degree or more. That means a reading of 97.8°F at 7 a.m. and 99°F at 5 p.m. could both be perfectly normal for the same person. Because of this variability, researchers have argued that no single number should be treated as the universal “normal.” Instead, the JAMA review suggested that 98.9°F (37.2°C) in the early morning and 99.9°F (37.7°C) later in the day represent more useful upper limits for healthy adults under 40.
How Measurement Site Affects the Reading
Where you place the thermometer changes the number you get. Rectal and ear readings run closest to your true core temperature. Oral readings tend to be slightly lower. Armpit readings are the least reliable and often read lower still.
For context, a fever is generally defined as:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F to 100.4°F (37.8°C to 38°C) or higher
- Armpit: 99°F (37.2°C) or higher
If you’re using an armpit thermometer and the result seems off, it’s worth rechecking with a different method. Armpit readings are considered the least accurate of the common options.
Differences by Sex
Women and men regulate temperature a bit differently. Women’s skin temperature reacts more strongly to changes in the surrounding environment. In controlled experiments, every 1°C rise in room temperature increased women’s mean skin temperature by 0.35°C compared to 0.28°C for men. Men showed stronger thermoregulation during sudden temperature swings, adjusting core temperature more aggressively when conditions changed quickly. These differences are most noticeable at the extremities, especially the hands, which is one reason women’s hands often feel colder.
Hormonal cycles also shift baseline temperature in women. Body temperature rises slightly after ovulation, typically by 0.5°F to 1°F, and stays elevated until the next period begins. This is well-established enough that temperature tracking has long been used as one method of monitoring fertility.
Age Matters Too
Young children tend to run warmer than adults because their metabolisms are higher relative to body size. Older adults tend to run cooler. This is important because a lower baseline means a smaller rise in temperature can signal a real problem. An older person with an infection might reach only 99°F or 100°F, a reading that wouldn’t raise alarms in a younger adult but could indicate a fever relative to their personal norm.
If you’re trying to figure out whether a temperature reading is “normal,” the most useful reference point isn’t a universal number. It’s your own baseline, taken at the same time of day, with the same type of thermometer, when you’re feeling well. That personal average gives you a far more reliable benchmark than 98.6°F ever could.