Normal body temperature for most adults falls in the range of 97°F to 99°F (36.1°C to 37.2°C), with 98.6°F (37°C) still serving as the traditional benchmark. That said, your personal “normal” can sit anywhere within that range depending on your age, the time of day, and how you take the reading.
Why 98.6°F Isn’t Quite Right Anymore
The 98.6°F number dates back to a German study from the 1860s, and for over a century nobody seriously questioned it. But a large-scale analysis published in eLife, drawing on nearly 190,000 temperature measurements spanning from the Civil War era to 2017, found that average human body temperature has been dropping steadily: about 0.05°F per decade of birth. Men born in the early 1800s ran temperatures roughly 1°F higher than men today. Women showed a similar decline of about 0.6°F since the 1890s.
The reasons aren’t entirely settled, but researchers point to lower rates of chronic infection, reduced inflammation, and climate-controlled environments as likely contributors. The practical takeaway: if your thermometer reads 97.5°F on a normal day, that’s perfectly healthy. You don’t need to hit 98.6 to be “normal.”
How Readings Vary by Measurement Method
Where you place the thermometer matters more than most people realize. Oral readings (under the tongue) are the standard reference point for adults. Other methods consistently read higher or lower by a predictable margin:
- 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
So if your armpit reading says 97.8°F, that likely corresponds to an oral temperature somewhere around 98.3°F to 98.8°F. These offsets aren’t exact, and they vary somewhat from person to person, but they give you a useful ballpark for comparison.
Normal Temperature in Children
Kids tend to run slightly warmer than adults, and their temperatures fluctuate more throughout the day. For children, the Mayo Clinic defines a fever as a rectal, ear, or temporal artery reading of 100.4°F (38°C) or higher, an oral reading of 100°F (37.8°C) or higher, or an armpit reading of 99°F (37.2°C) or higher.
Rectal thermometers are considered the most accurate method for infants and toddlers. For older children who can hold a thermometer under their tongue, oral readings work well. Forehead and ear thermometers are convenient but slightly less precise, especially in very young babies.
What Affects Your Temperature Throughout the Day
Your body temperature isn’t static. It follows a daily rhythm, typically hitting its lowest point in the early morning (often around 4 a.m.) and peaking in the late afternoon or early evening. This swing can easily be 1°F or more, which means a reading of 99°F at 5 p.m. might be completely normal for you even though it looks borderline on paper.
Other factors that push your temperature up or down:
- Exercise: Physical activity raises core temperature noticeably, and it can stay elevated for a while after you stop.
- Menstrual cycle: After ovulation, progesterone causes a rise of about 0.4°F to 0.9°F that lasts through the second half of the cycle. This is the basis for fertility tracking methods that use basal body temperature.
- Age: Older adults often run cooler baseline temperatures, which can make fevers harder to detect. A reading that looks “normal” in a younger person might actually represent a significant increase for someone in their 70s or 80s.
- Hot or cold drinks: Drinking something hot or cold right before an oral reading can throw it off. Wait 15 minutes for the most accurate result.
When Temperature Signals a Fever
For adults, a fever is generally defined as an oral temperature of 100.4°F (38°C) or higher. Using other methods, the thresholds shift to match: 100.4°F for rectal or ear readings, and 99°F for armpit readings. These numbers apply to children as well, though the stakes are different. A temperature of 100.5°F in a healthy adult is usually a minor concern, while the same reading in a newborn under three months old is treated much more seriously.
Fevers themselves aren’t an illness. They’re your immune system’s response to infection, inflammation, or other triggers. Your body deliberately raises its thermostat because many viruses and bacteria replicate less efficiently at higher temperatures.
When Temperature Drops Too Low
On the other end of the spectrum, a core body temperature below 95°F (35°C) is classified as hypothermia. This is broken into three stages: mild (90°F to 95°F), moderate (82.4°F to 90°F), and severe (below 82.4°F). Even mild hypothermia causes shivering, confusion, and impaired coordination. It’s most common in cold-weather exposure but can also happen indoors in elderly people with poor heating or certain medical conditions that impair temperature regulation.
Most household thermometers won’t read accurately at very low body temperatures, so if you suspect hypothermia, the reading on a standard thermometer may understate the problem.
Finding Your Personal Baseline
Because the “normal” range spans nearly 2°F, knowing your own baseline is more useful than comparing yourself to a population average. Take your temperature a few times over the course of a week when you’re feeling healthy, using the same method and the same thermometer each time. Note the time of day. After a handful of readings, you’ll have a sense of where you personally sit. That number becomes your reference point, so when you feel sick, you can tell whether a reading of 99.2°F is a meaningful change for you or just your typical afternoon temperature.