Normal body temperature for most people is around 36.6°C (97.9°F), not the 37°C (98.6°F) you probably learned growing up. That old number dates back to the 1800s and has been replaced by better data. Your temperature also shifts throughout the day, varies by age, and reads differently depending on where you measure it.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician named Carl Wunderlich, who published it in 1851 based on millions of readings from thousands of patients. For over a century, that number went unquestioned. But when researchers reexamined it with modern thermometers and methods, they found the true average oral temperature was closer to 36.8°C (98.2°F), and the upper limit of normal was 37.7°C (99.9°F) rather than 38°C (100.4°F).
A large systematic review pooling data from 36 studies and over 7,600 people found an average body temperature of 36.59°C (97.9°F). That’s a full degree Fahrenheit below the old textbook number. Wunderlich’s thermometers were likely less precise, and the populations he studied may have had higher baseline temperatures due to chronic infections that were common in that era.
The Normal Range
There’s no single “normal” temperature. Instead, healthy body temperature falls within a range. For oral readings in adults, that range runs from roughly 36.0°C to 37.7°C (96.8°F to 99.9°F). Anything within that window is considered normal, and small fluctuations from day to day or hour to hour are completely expected.
For practical purposes, researchers suggest treating 37.2°C (98.9°F) as the upper limit of normal in the early morning, and 37.7°C (99.9°F) as the upper limit at any point during the day. If your reading is above those thresholds, it’s worth paying attention to how you feel and checking again.
Your Temperature Changes Throughout the Day
Body temperature follows a daily cycle tied to your internal clock. It tends to be lowest in the early morning, typically between 8 and 10 a.m., and peaks in the evening. For working-age adults, the peak comes around 7 p.m. For older adults, it peaks closer to 7:30 p.m. In children, the cycle is even more pronounced, with temperatures peaking later, around 2:45 a.m.
This daily swing matters more than most people realize. A reading of 99.5°F at 6 a.m. is more significant than the same reading at 6 p.m., because your body is naturally cooler in the morning. If you’re monitoring your temperature for any reason, try to measure it at the same time each day so you’re comparing apples to apples.
How Age Affects Temperature
Young children tend to run warmer than adults, and their temperatures fluctuate more dramatically over the course of the day. This is partly because children have a faster metabolism relative to their body size and their temperature-regulation systems are still maturing.
Older adults trend in the opposite direction. Their baseline temperatures tend to be lower, and the daily swing between their low and high points is smaller. This can mask fevers in elderly people. A temperature that looks “normal” on a thermometer might actually represent a significant rise from their personal baseline. If you’re monitoring temperature in someone over 65, keep in mind that even a modest elevation could signal infection.
Hormonal Shifts in Women
In women who menstruate, body temperature rises by 0.3°C to 0.7°C (roughly 0.5°F to 1.3°F) after ovulation. This bump is driven by progesterone and lasts through the second half of the cycle until a period begins. It’s the basis of the basal body temperature method some people use to track fertility. A sustained rise in morning temperature, measured before getting out of bed, signals that ovulation has occurred.
Where You Measure Matters
Different parts of the body give different readings, and the differences are consistent enough that you can roughly convert between them. Rectal temperatures run about 0.4°C (0.7°F) higher than armpit readings. Oral temperatures fall in between, about 0.25°C (0.5°F) higher than armpit readings.
Here’s what that means in practice:
- Armpit (axillary): Reads lowest. Convenient and noninvasive, but less precise. An armpit reading of 37.5°C (99.5°F) corresponds roughly to an oral fever of 38°C (100.4°F).
- Oral: The most common method for adults. Place the thermometer under your tongue with your mouth closed for the recommended time.
- Rectal: Reads highest and is considered the most accurate, especially in infants and young children who can’t hold a thermometer under their tongue reliably.
- Ear (tympanic): Quick and easy, but readings can be affected by earwax or improper positioning.
- Forehead (temporal): Noninvasive and fast, though less reliable in cold or windy environments.
If you’re comparing your reading to a fever threshold, make sure you know which method the threshold assumes. Most published fever cutoffs, like the commonly cited 38°C (100.4°F), refer to oral or rectal readings.
What Counts as a Fever
A fever generally starts at an oral temperature of 38°C (100.4°F), though the updated research suggests anything above 37.7°C (99.9°F) is above the normal range. Temperatures between 37.7°C and 38°C are sometimes called a low-grade fever, a gray zone where you may or may not feel unwell.
Context matters as much as the number. A temperature of 99.5°F after a hard workout or on a hot day is not the same as 99.5°F first thing in the morning with body aches. Your body raises its thermostat during infection as part of the immune response, but physical activity, dehydration, heavy clothing, and even a recent hot meal can temporarily bump your reading. If you suspect a fever, wait 15 minutes after eating or drinking, sit still, and measure again.
Other Factors That Shift Your Baseline
Beyond time of day, age, and hormones, several other things influence what’s “normal” for you specifically. Physical fitness plays a role: people who exercise regularly often have slightly lower resting temperatures. Body composition matters too, since fat tissue insulates and can affect surface readings. Even the ambient temperature in your home can nudge your readings up or down slightly.
Some medications alter body temperature as well. Anti-inflammatory drugs can bring a true fever down, which is useful but can also obscure what’s happening. Thyroid conditions can shift your baseline in either direction: an overactive thyroid tends to raise it, while an underactive thyroid lowers it. If your temperature consistently falls outside the typical range and you feel fine, it may simply be your personal normal. Getting a few baseline readings when you’re healthy gives you a reference point that’s more useful than any population average.