Normal body temperature for an adult averages about 97.9°F to 98.2°F (36.6°C to 36.8°C) when measured orally, not the 98.6°F (37°C) figure most of us grew up hearing. That old number dates back to a study from 1851, and modern research consistently shows it’s too high. Your temperature also shifts throughout the day and depends on where you measure it, your age, and your sex.
Why 98.6°F Is Outdated
The 98.6°F standard comes from German physician Carl Wunderlich, who took millions of armpit temperature readings from 25,000 patients in Leipzig in 1851. That number stuck for over a century. But a landmark reanalysis at the University of Maryland found that 98.2°F, not 98.6°F, was the actual mean oral temperature in healthy adults, and that the upper limit of normal was closer to 99.9°F rather than 100.4°F.
More recently, a Stanford study analyzing over 677,000 temperature measurements spanning 157 years found that body temperature has genuinely dropped over time. Men born in the early 1800s ran about 1.06°F (0.59°C) warmer than men today, declining at a steady rate of roughly 0.05°F per decade. Women showed a similar pattern, dropping about 0.58°F (0.32°C) since the 1890s. The likely explanations include lower rates of chronic infection, reduced inflammation, and changes in living environments like widespread climate control.
Normal Ranges by Measurement Site
Where you take your temperature matters. Each method reads differently because it’s sampling a different part of the body. Here are the accepted normal ranges for adults:
- Oral (under the tongue): 96.4°F to 99.1°F (35.8°C to 37.3°C)
- Armpit (axillary): 96.4°F to 97.3°F (34.8°C to 36.3°C)
- Ear (tympanic): 97.0°F to 100.2°F (36.1°C to 37.9°C)
- Rectal: 98.2°F to 100.8°F (36.8°C to 38.2°C)
- Forehead (temporal): 95.4°F to 98.6°F (35.2°C to 37.0°C)
Rectal readings tend to run about 1°F higher than oral readings, while armpit readings tend to run about 1°F lower. Forehead thermometers are convenient but can be affected by sweating or ambient temperature. If you’re comparing a reading to a fever threshold, make sure you know which method was used.
What Counts as a Fever
Most clinicians use 100.4°F (38.0°C) as the standard fever threshold for oral temperature, though some researchers have argued this cutoff should be lower, closer to 99.9°F (37.7°C), based on updated data. In practice, a reading between 99°F and 100.4°F is sometimes called a low-grade fever, though it can also fall within the normal range for some people later in the day.
Wunderlich’s original classifications defined a slight fever as 100.4°F to 101.1°F, a moderate fever as 101.3°F to 102.2°F, and a high fever as 103.1°F to 104.9°F. These categories are still broadly used, with morning thresholds running slightly lower than evening ones.
On the other end, hypothermia begins when core body temperature drops below 95°F (35°C).
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. It bottoms out around 6 a.m. and peaks between 4 and 6 p.m., with an average swing of about 0.9°F (0.5°C) between the low and the high. This means a reading of 99°F at 5 p.m. can be perfectly normal even if the same reading at 6 a.m. would be slightly elevated for you.
Physical activity raises body temperature temporarily. Eating and drinking hot or cold beverages can also shift oral readings by a small amount, which is why most guidelines suggest waiting 15 to 30 minutes after eating or drinking before taking an oral temperature.
How Sex and the Menstrual Cycle Affect Temperature
Women tend to run slightly warmer than men on average. A more pronounced shift happens during the menstrual cycle: after ovulation, during the luteal phase, core body temperature rises by 0.5°F to 1.3°F (0.3°C to 0.7°C) due to increased progesterone. This is the basis for fertility tracking methods that use daily temperature readings. The higher baseline persists until the next period begins.
Body Temperature Drops With Age
Older adults consistently run cooler than younger adults. A study of people with an average age of about 81 found that mean oral temperatures ranged from 97.3°F to 97.8°F depending on time of day. Among nursing home residents, 97% had morning temperatures below 98.6°F, and 94% were still below that mark in the afternoon. Community-dwelling older adults showed a similar pattern, with 90% reading below 98.6°F at midday.
The oldest participants were the coldest and showed almost no rise in temperature over the course of the day, losing the typical daily rhythm seen in younger people. This has a practical consequence: because their baseline runs lower, older adults can have a significant infection without ever reaching the standard 100.4°F fever threshold. A temperature of 99°F in an 85-year-old may represent a more meaningful change than the same reading in a 30-year-old.
Finding Your Own Baseline
Given how much normal temperature varies from person to person, knowing your own baseline is more useful than relying on a single number. You can establish this by taking your temperature at the same time of day, using the same method, over several days when you’re feeling well. Most people will find their personal average falls somewhere between 97.5°F and 98.5°F orally. Once you know your baseline, a jump of 1°F or more above it is a more reliable signal that something is off than simply comparing to 98.6°F.