The average normal human body temperature is 98.6°F (37°C), but healthy adults typically fall anywhere between 97°F and 99°F. That 98.6°F number has been the medical standard for over 170 years, though recent evidence suggests the true average has been quietly dropping over time.
Where 98.6°F Came From
The 98.6°F standard dates back to 1851, when German physician Carl Wunderlich measured the armpit temperature of 25,000 patients and calculated the average. That number became the textbook answer taught in medical schools worldwide and has stuck ever since. The figure was likely accurate for the time, but the human body appears to have changed since the mid-1800s.
A large-scale study published in eLife analyzed temperature records spanning nearly two centuries of American data. It found that average body temperature has decreased steadily by about 0.05°F per decade of birth. Men born in the early 19th century ran temperatures roughly 1.06°F higher than men today. Women showed a similar decline of about 0.58°F since the 1890s. The reasons aren’t entirely clear, but reduced rates of chronic infection and inflammation, along with more climate-controlled living environments, are leading explanations. So if your thermometer consistently reads 97.5°F or 98.0°F, that’s well within the modern normal range.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable daily cycle, hitting its lowest point just before you wake up in the morning and peaking about an hour before you go to bed at night. This swing can easily account for a full degree or more of variation within the same day, which is why a reading of 97.3°F in the morning and 98.8°F in the evening can both be perfectly normal for the same person.
Physical activity, eating, stress, and hormonal shifts (including the menstrual cycle) can also push your temperature up temporarily. If you’ve just exercised or had a hot drink before checking, your reading will be artificially high.
How Age Affects Body Temperature
Baseline body temperature doesn’t shift dramatically as you get older. However, aging does make it harder for your body to regulate its temperature. Older adults may not mount as strong a fever when they’re sick, which can mask infections. They’re also more vulnerable to both overheating and hypothermia because the body’s thermostat becomes less responsive. For older adults, even a modest rise to 99°F can sometimes signal an infection that would produce a higher fever in a younger person.
Readings Vary by Thermometer Location
The 97°F to 99°F range assumes an oral (under-the-tongue) reading. Different measurement sites give slightly different numbers, and the offsets are consistent enough to account for:
- Rectal: reads 0.5°F to 1°F higher than oral
- Ear (tympanic): reads 0.5°F to 1°F higher than oral
- Armpit (axillary): reads 0.5°F to 1°F lower than oral
So an armpit reading of 97.0°F and a rectal reading of 99.5°F could both reflect the same actual core temperature. If you’re comparing readings over time, use the same method and the same thermometer each time for consistency.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This is the threshold used in clinical settings, airports during disease screenings, and most medical guidelines. Temperatures between 99°F and 100.3°F fall into a gray zone sometimes called a “low-grade fever,” which may or may not indicate illness depending on the time of day and circumstances.
On the other end of the spectrum, a core temperature below 95°F is classified as hypothermia, a potentially dangerous condition where the body loses heat faster than it can produce it.
Getting an Accurate Reading
For the most reliable oral reading, wait at least 15 minutes after eating, drinking, or exercising. Place the thermometer under your tongue toward the back of your mouth and keep your lips closed around it. Digital thermometers are the standard recommendation now; glass mercury thermometers are no longer sold in most places due to breakage risks.
If you’re tracking your temperature for health reasons, take it at the same time each day. A single reading is just a snapshot. Your personal baseline might sit at 97.8°F or 98.4°F, and knowing your own normal makes it much easier to spot when something is off.