A temperature of 98.6°F is normal, but it’s not the precise benchmark it was once thought to be. The true average for a healthy adult today is closer to 97.5°F to 97.9°F, and your own normal can fall anywhere from about 97.2°F to 99.5°F depending on the time of day, your age, your sex, and how you measure it.
Where 98.6°F Came From
The 98.6°F standard dates back to a German physician named Carl Wunderlich, who published a landmark study in 1868 analyzing over one million armpit temperature readings from roughly 25,000 patients. He identified 37.0°C (98.6°F) as the average temperature of a healthy adult. He also noted a normal range stretching from 97.2°F to 99.5°F and documented that body temperature dips between 2 and 8 AM and peaks between 4 and 9 PM. Despite all those caveats, the single number 98.6 became lodged in popular understanding as “the” normal temperature.
The problem is that Wunderlich’s subjects lived in 19th-century Germany, where chronic infections like tuberculosis, syphilis, and gum disease were rampant and largely untreated. Persistent inflammation raises body temperature. So the population he studied genuinely did run warmer than people living today.
Why Average Body Temperature Has Dropped
A large study using temperature records spanning from the Civil War era to 2017 found that men born in the early 1800s had body temperatures about 1.06°F higher 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 since the 1890s. That decline hasn’t leveled off; it appears to still be happening.
Two main factors explain the shift. First, modern medicine has dramatically reduced the burden of chronic infection and inflammation, which means less baseline immune activation generating heat. Second, resting metabolic rate in the U.S. population has fallen by an estimated 6%, partly because physical activity has dropped by roughly 27 minutes per day since 1820. Lower metabolic rates produce less heat, which translates directly to lower resting body temperatures.
What “Normal” Actually Looks Like
Your body temperature is not a fixed number. It’s a range that shifts predictably throughout the day. The single biggest factor is time of day: temperatures are coolest in the early morning and warmest around 4 PM. That swing from trough to peak can be anywhere from 0.5°F to 1.9°F in a healthy person.
Beyond the clock, several personal factors shape your baseline. Women tend to run slightly warmer than men. Heavier individuals tend to have slightly higher temperatures, while taller people skew slightly lower. So a reading of 97.4°F at 7 AM and 98.8°F at 5 PM could both be perfectly normal for the same person.
How Age Changes the Picture
Older adults typically run cooler than younger people, sometimes significantly so. As you age, you lose insulating fat under the skin of your hands and feet, your skin becomes drier (which increases heat loss), and your metabolism slows. Certain common medications, including beta blockers and some psychiatric drugs, can lower temperature further. An underactive thyroid has the same effect.
This lower baseline matters because it can mask a fever. An older person with an infection might register 99.5°F and actually be running a meaningful fever relative to their personal normal of 97.0°F. It also means older adults are more vulnerable to hypothermia, which is clinically defined as a body temperature below 95°F.
Where You Measure Matters
Not all thermometer placements give the same reading. Oral (under the tongue) is the most common reference point, and the other methods differ from it in predictable ways:
- Rectal: 0.5 to 1.0°F higher than oral
- Ear (tympanic): 0.5 to 1.0°F higher than oral
- Armpit (axillary): 0.5 to 1.0°F lower than oral
- Forehead (temporal): 0.5 to 1.0°F lower than oral
This means an armpit reading of 97.6°F and an ear reading of 99.0°F could reflect the exact same core temperature. If you’re comparing a reading to any threshold, knowing which method you used is essential.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold applies regardless of measurement site, though you should mentally adjust for the offsets above. A rectal reading of 100.4°F represents a lower core temperature than an oral reading of 100.4°F.
Most fevers in adults are not dangerous on their own. The temperature at which fever becomes a more serious concern is 103°F (39.4°C). At that level, or at any fever accompanied by symptoms like a stiff neck, confusion, difficulty breathing, seizures, sensitivity to light, a rash with small bleeding spots under the skin, or persistent vomiting, the situation warrants emergency care. A low-grade fever of 99 to 100°F, on the other hand, is often just your immune system doing its job.
Finding Your Personal Normal
Rather than comparing yourself to 98.6°F, it helps to know your own baseline. Take your temperature a few times over several days when you’re feeling well, using the same method and the same thermometer each time. Note the time of day. After a handful of readings, you’ll have a personal range to compare against when you feel sick. For most healthy adults, that range will land somewhere between 97.2°F and 98.6°F for an oral reading, with afternoon values on the higher end.