Normal body temperature for adults averages about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us grew up hearing. That familiar figure dates back to 1868, and modern research shows human body temperature has been steadily declining since then. Your own “normal” falls somewhere in a range, influenced by the time of day, where you measure, your age, and other biological factors.
Why 98.6°F Is Outdated
The 98.6°F standard comes from German physician Carl Wunderlich, who published over a million temperature readings from roughly 25,000 patients in 1868. He identified 98.6°F as the average for healthy adults, and the number became medical gospel. But his methods were imprecise by today’s standards. His thermometers were bulky, had to be read while still in contact with the body, and required 15 to 20 minutes under the armpit to produce a reading.
Researchers at Stanford Medicine analyzed more than 618,000 oral temperature readings from adult outpatients seen between 2008 and 2017. They found that normal adult temperatures range from 97.3°F to 98.2°F, with an overall average of 97.9°F. Separate work from the same team found that average U.S. body temperature has dropped by about 0.05°F per decade since the 19th century, likely due to reduced rates of chronic infection and inflammation in modern populations. So if your thermometer reads 97.5°F and you feel fine, that’s completely normal.
The Normal Range for Adults
There is no single number that qualifies as “normal.” Healthy adults generally fall between 97°F and 99°F (36.1°C to 37.2°C) when measured orally. That’s a two-degree window, which means your baseline temperature could be meaningfully different from someone else’s. What matters most is knowing your own typical range so you can recognize when something is off.
A temperature of 100.4°F (38°C) or higher is the standard clinical threshold for fever, the cutoff used by the CDC and most hospitals. Between 99°F and 100.4°F is often considered a low-grade fever, though definitions vary. Anything above 100.4°F signals that your immune system is actively fighting something.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. It hits its lowest point just before you wake up, often dipping into the mid-96°F range, and gradually climbs throughout the day. It peaks about an hour before bedtime, typically in the late afternoon or early evening. Wunderlich himself noted this pattern back in 1868, describing a daily swing of about 0.9°F between the morning low (around 2 to 8 AM) and the evening high (around 4 to 9 PM).
This means a reading of 99°F at 6 PM is less concerning than the same reading at 7 AM. If you’re checking your temperature because you feel unwell, keep the time of day in mind before interpreting the number.
Fever Thresholds for Children
Children run slightly warmer than adults, and their fever thresholds depend on how you take the temperature. A child has a fever if they reach:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
Rectal readings are considered the gold standard for infants because they’re closest to core body temperature. Armpit readings are convenient but consistently run lower, which is why the fever threshold for that method is set lower too.
How Measurement Site Affects the Reading
Not all thermometer placements give you the same number. Compared to an oral reading, rectal and ear temperatures typically read 0.5 to 1°F higher, while armpit temperatures run 0.5 to 1°F lower. So a rectal temperature of 99.5°F and an oral temperature of 99°F and an armpit temperature of 98.5°F could all reflect the same core body temperature.
Forehead (temporal artery) thermometers are popular for their speed and ease, but they’re the least consistent of the bunch. Direct sunlight, cold air, and sweat on the forehead can all throw off the reading. Oral digital thermometers offer a good balance of accuracy and convenience for most people. If you’re comparing readings over time, use the same method and the same thermometer each time.
Factors That Shift Your Baseline
Several biological factors push your temperature up or down independently of illness. Age is one of the biggest. Older adults tend to run cooler, which means a reading of 99°F in someone over 65 could represent a more significant immune response than it would in a younger person.
For people who menstruate, body temperature rises after ovulation and stays elevated through the second half of the cycle. This shift is small, typically between 0.4°F and 1°F, but it’s consistent enough that tracking basal body temperature (your temperature first thing in the morning, before getting out of bed) is used as a fertility awareness tool. Physical exercise, hot drinks, and heavy clothing can also temporarily raise your reading, so it’s best to wait at least 15 minutes after any of these before taking your temperature.
When Temperature Drops Too Low
While most people focus on fever, dangerously low temperatures are equally important to recognize. Hypothermia is classified in three stages based on core body temperature:
- Mild hypothermia: 90°F to 95°F (32.2°C to 35°C), causing shivering, confusion, and poor coordination
- Moderate hypothermia: 82.4°F to 90°F (28°C to 32.2°C), where shivering may actually stop and drowsiness sets in
- Severe hypothermia: below 82.4°F (28°C), a life-threatening emergency
Hypothermia doesn’t require extreme cold. Older adults, very young children, and people with certain chronic conditions can develop it in mildly cool environments, especially if they’re wet or inactive for extended periods. A home thermometer that reads below 95°F warrants immediate attention.