Normal body temperature for most adults falls between 97°F (36.1°C) and 99°F (37.2°C), not the single number most of us grew up memorizing. The old 98.6°F standard dates back to 1868, and modern research suggests the true average has actually dropped closer to 97.5°F. Your own “normal” depends on the time of day, how you take your temperature, your age, and your biology.
Why 98.6°F Is Outdated
The 98.6°F figure comes from a German physician named Carl Wunderlich, who published a landmark study in 1868 analyzing millions of temperature readings from roughly 25,000 patients. He declared 37°C (98.6°F) the mean of that enormous dataset, and the number stuck for over 150 years.
A 2020 study covering nearly 160 years of temperature data found that average oral temperature has gradually fallen by more than one degree Fahrenheit since Wunderlich’s era. The revised average lands closer to 97.5°F. Researchers aren’t entirely sure why, but reduced rates of chronic infection and changes in living conditions (central heating, air conditioning) are likely contributors. So if your thermometer reads 97.8°F and you feel fine, that’s perfectly healthy.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily cycle. It hits its lowest point in the early morning, typically between 4 a.m. and 6 a.m., about two hours before you wake up. It then gradually climbs, reaching its peak in the early evening. That swing can easily account for a full degree of difference, which means a reading of 97.3°F at 7 a.m. and 98.4°F at 6 p.m. could both be normal for the same person on the same day.
Exercise, hot drinks, and heavy clothing can also push your reading temporarily higher. If you’re trying to get an accurate baseline, take your temperature after sitting quietly for a few minutes, ideally at the same time each day.
How Age Affects Normal Range
Older adults tend to run cooler than younger people. This matters because a temperature that looks “normal” on paper might actually represent a fever in someone over 65. A reading of 99°F in an elderly person can signal infection even though it falls within the textbook range for a younger adult.
Infants and young children, on the other hand, tend to run slightly warmer and can spike fevers more quickly. For babies under 3 months, even a relatively modest temperature elevation is taken seriously by pediatricians because their immune systems are still developing.
Readings Vary by Measurement Site
Where you place the thermometer changes the number you get, sometimes by a meaningful amount. All comparisons below use an oral reading as the baseline:
- Rectal: reads 0.5 to 1°F higher than oral
- Ear (tympanic): reads 0.5 to 1°F higher than oral
- Armpit (axillary): reads 0.5 to 1°F lower than oral
- Forehead (temporal): varies by device quality, generally close to oral but less consistent
This means a rectal temperature of 99.5°F and an armpit temperature of 97.5°F could reflect the exact same core body temperature. If you’re comparing readings over time, use the same method and the same thermometer each time.
Hormonal Shifts and Body Temperature
For people who menstruate, body temperature follows a predictable pattern across the cycle. After ovulation, basal body temperature rises by about 0.3°C (roughly half a degree Fahrenheit) and stays elevated for three or more days. This post-ovulation bump is the basis for fertility tracking methods that use daily morning temperature readings. It also means a slightly higher reading in the second half of your cycle is a normal hormonal effect, not a sign of illness.
When Temperature Signals a Problem
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold is used consistently across most medical settings and travel health screening. A temperature between 99°F and 100.4°F is sometimes called a “low-grade fever,” though it can also just reflect normal daily variation or recent physical activity.
On the other end of the spectrum, a body temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia falls between 90°F and 95°F, and at that stage you’d typically notice shivering, confusion, and difficulty with coordination. Below 90°F, shivering may actually stop as the body loses its ability to generate heat, which is a sign of moderate to severe hypothermia requiring emergency care.
Context matters more than any single number. A temperature of 99.8°F after a long run is meaningless. The same reading in someone who normally runs cool, especially an older adult, paired with fatigue or other symptoms, is worth paying attention to. Knowing your own baseline makes it far easier to spot a temperature that’s genuinely out of range for you.