A good body temperature for most adults falls around 97.5°F to 98.9°F (36.4°C to 37.2°C), which is lower than the classic 98.6°F number many of us grew up hearing. That old standard dates back to the 1850s and no longer reflects what researchers see in modern populations. Your temperature also shifts throughout the day, so a single reading is really just a snapshot.
Why 98.6°F Is Outdated
The 98.6°F benchmark comes from a German physician named Carl Wunderlich, who took millions of temperature readings from 25,000 patients in 1851. At the time, average life expectancy was about 38 years, and chronic infections like tuberculosis and syphilis were widespread. Those ongoing infections likely pushed body temperatures higher across the population, skewing what counted as “normal.”
Modern data tells a different story. An analysis of over 35,000 British patients found the average oral temperature to be 97.9°F (36.6°C). A large U.S. study published in eLife found that average body temperature has been dropping by about 0.05°F per decade since the 1800s, adding up to roughly a 1°F decline overall. The likely explanation: fewer chronic infections, better living conditions, and lower levels of background inflammation in the body.
What Counts as Normal
Rather than a single number, normal body temperature sits within a range. For a healthy adult, oral readings between roughly 97°F and 99°F (36.1°C to 37.2°C) are typical. Several things shift your reading within that range on any given day:
- Time of day. Body temperature is lowest in the early morning and peaks in the late afternoon or evening. The difference can be a full degree or more.
- Menstrual cycle. After ovulation, basal body temperature rises by 0.4°F to 1°F and stays elevated through the second half of the cycle.
- Age. Older adults tend to run cooler at baseline, which means a lower number can still be perfectly healthy, and a fever may show up at a lower threshold than you’d expect.
- Activity and environment. Exercise, stress, alcohol, sleep deprivation, and even being awake and moving around for a few minutes before checking can bump your reading up.
Where You Measure Matters
The number on your thermometer depends on where you take the reading. Rectal temperatures run about 0.5°F to 1°F higher than oral readings. Armpit (axillary) temperatures run about 1°F lower than oral and are the least accurate of the common methods. Ear and forehead thermometers fall somewhere in between and can vary depending on technique.
For infants and young children, a rectal thermometer is the most reliable option. If you use a forehead or ear thermometer on a small child and the reading seems off, a rectal check is the best way to confirm. For most adults, an oral thermometer gives a dependable reading as long as you haven’t just had something hot or cold to drink.
When a Temperature Becomes a Fever
The threshold depends on how you’re measuring and who you’re measuring:
- Oral: 100°F (37.8°C) or higher is generally considered a fever in adults and children.
- Rectal, ear, or forehead: 100.4°F (38°C) or higher.
- Armpit: 99°F (37.2°C) or higher, though this should be confirmed with another method.
For adults, a temperature of 103°F (39.4°C) or higher warrants a call to your doctor. For babies under 3 months, a rectal temperature of 100.4°F or higher is a medical concern regardless of how the baby looks or acts. Infants between 3 and 6 months need attention for a rectal temperature above 102°F (38.9°C), or for a lower fever combined with unusual irritability or sluggishness. Children between 7 and 24 months with a rectal reading above 102°F that lasts more than a day also need to be seen.
When a Temperature Is Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. Even a mild drop into the 90°F to 95°F range triggers noticeable effects: shivering, poor judgment, clumsiness, and a spike in heart rate and blood pressure as your body fights to generate heat. Below roughly 82°F (28°C), the situation becomes severe. Shivering stops, consciousness fades, and dangerous heart rhythms can develop.
Hypothermia isn’t only a cold-weather problem. Older adults with lower baseline temperatures, people on certain medications, and anyone with prolonged exposure to even moderately cool environments can be vulnerable. If someone feels cold to the touch and seems confused or unusually drowsy, checking their temperature is a smart first step.
Getting an Accurate Reading
For the most reliable number at home, take your temperature at the same time of day when possible, ideally in the afternoon if you want to catch your daily peak, or first thing in the morning for your baseline low. Avoid eating, drinking, or exercising for at least 15 minutes before an oral reading. If you’re tracking basal body temperature for fertility or cycle awareness, measure before getting out of bed or even talking, since even a few minutes of activity can shift results.
If your normal resting temperature tends to run at 97.4°F rather than 98.6°F, that doesn’t mean something is wrong. It means your personal baseline is simply lower than the old textbook number, which is completely common. What matters more than hitting one specific number is knowing your own pattern and recognizing when a reading is meaningfully higher or lower than your usual.