Normal body temperature is not actually 98.6°F. That number, while still widely cited, comes from a study published in 1868. Modern research based on over 618,000 temperature readings puts the average closer to 97.9°F, with most healthy adults falling somewhere between 97.3°F and 98.2°F when measured orally.
Where 98.6°F Came From
The 98.6°F standard traces back to German physician Carl Wunderlich, who analyzed over one million temperature readings from roughly 25,000 patients in the mid-1800s. His work was groundbreaking for its time, but his methods had significant limitations. The thermometers he used were bulky, had to be read while still in place, and required 15 to 20 minutes to reach a stable reading. He also measured temperatures under the armpit rather than in the mouth, which consistently reads lower. Despite all this, his number stuck and became medical gospel for over 150 years.
Research from Stanford Medicine has since shown that average body temperature in the U.S. has been dropping by about 0.05°F per decade since the 1800s. The likely explanation: improvements in health, hygiene, and living conditions have reduced the chronic, low-level inflammation that was common in earlier centuries. People today simply run cooler than people did 150 years ago.
What “Normal” Looks Like Today
Your normal temperature is personal. While 97.9°F is the modern average for oral readings, the range of 97.3°F to 98.2°F captures where most healthy adults land. Some people consistently sit at the lower end, others at the higher end, and both are perfectly fine. A combination of age, sex, height, weight, and time of day accounts for about 25% of the variation within any one person and 7% of the differences between people.
Why Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily cycle tied to your internal clock. It hits its lowest point in the early morning, typically between 4 a.m. and 6 a.m., and peaks in the early evening around 8 p.m. The difference between your daily low and high can range from 0.5°F to 1.9°F. So if you take your temperature at 6 a.m. and get 97.4°F, then take it again at 7 p.m. and get 98.8°F, both readings can be completely normal for you.
This is worth keeping in mind if you’re monitoring for a fever. A reading of 99°F at 6 a.m. is more meaningful than the same reading at 8 p.m., when your body is naturally warmer.
Readings Vary by Where You Measure
The number on your thermometer depends on where you place it. Oral readings (under the tongue) are the standard reference point, and other methods read slightly higher or lower:
- Rectal: 0.5°F to 1°F higher than oral
- Ear (tympanic): 0.5°F to 1°F higher than oral
- Armpit (axillary): 0.5°F to 1°F lower than oral
- Forehead (temporal): 0.5°F to 1°F lower than oral
This means a “normal” armpit reading might be around 97.4°F, while a normal rectal reading could be closer to 99°F. Neither is cause for alarm. The key is knowing which method you used and adjusting your expectations accordingly. For infants and very young children, rectal thermometers are the most accurate option.
Factors That Shift Your Baseline
Age plays a clear role. Older adults tend to run cooler than younger people, which can make fevers harder to detect. A temperature of 99°F in a 75-year-old may represent the same immune response as 101°F in a 30-year-old.
Hormonal cycles matter too. During the menstrual cycle, basal body temperature rises after ovulation by anywhere from 0.4°F to 1°F. This shift is reliable enough that some people use it as a fertility tracking method.
Physical activity temporarily raises core temperature, sometimes significantly. Shivering in cold environments can increase heat production to four times its normal level. Your body constantly adjusts blood flow to your skin, either widening vessels to release heat when you’re warm or narrowing them to conserve heat in the cold. As long as the surrounding temperature stays within a comfortable range, these vascular adjustments alone are enough to keep your core steady. Once conditions become more extreme, your body adds sweating or shivering to the equation.
When Temperature Signals a Problem
A fever is generally defined as an oral temperature of 100.4°F or higher. The zone between 99.5°F and 100.3°F is often considered a low-grade fever, though some people naturally sit in that range without being sick.
For children, the fever threshold depends on the measurement method. A rectal, ear, or forehead temperature of 100.4°F or higher qualifies as a fever, as does an oral reading of 100°F or above, or an armpit reading of 99°F or above.
On the other end of the spectrum, a core temperature below 95°F is classified as hypothermia and requires immediate attention. This is most common after prolonged cold exposure but can also affect older adults in insufficiently heated homes.
For adults, fevers under 103°F are generally not dangerous on their own. In children, temperatures above 104°F warrant a call to a healthcare provider. At 105.8°F and above, organs begin to malfunction regardless of age, making this a medical emergency.
How to Get an Accurate Reading
If you’re checking your temperature at home, consistency matters more than the specific method you choose. Use the same type of thermometer, measure at the same site, and try to take readings at a similar time of day. Avoid checking right after exercise, a hot drink, or a bath, all of which can temporarily skew the number.
It also helps to know your own baseline. Taking your temperature a few times when you’re healthy gives you a personal reference point. If you normally run at 97.5°F, a reading of 99.5°F represents a two-degree jump, which is more informative than comparing it to an outdated population average.