Normal body temperature for a healthy adult is around 36.6°C to 37.0°C when measured orally, not the 37°C (98.6°F) figure most of us grew up with. That long-standing number dates back to the 1860s and turns out to be slightly too high by modern standards. Your actual temperature at any given moment depends on the time of day, where on your body you measure, your age, and your hormonal status.
Where 37°C Came From
The 37°C standard traces back to a German physician named Carl Wunderlich, who published a massive study in 1868 analyzing over one million armpit temperature readings from roughly 25,000 patients. It was an impressive effort for the era, but his tools were far from precise. The thermometers were bulky, had to be read while still in the armpit, and required 15 to 20 minutes to reach a stable reading. Those limitations likely introduced errors that nudged the average upward.
More recent data paints a different picture. A large analysis of over 35,000 British patients found the mean oral temperature to be 36.6°C. The most likely explanation for this downward shift isn’t better thermometers alone. Researchers point to a genuine decline in average human body temperature since the Industrial Revolution, driven by reduced rates of chronic infection, better sanitation, improved dental hygiene, and the widespread use of antibiotics. In other words, because modern populations carry less background inflammation, our bodies actually run a bit cooler than they did 150 years ago.
How Temperature Changes Throughout the Day
Your body temperature isn’t a fixed number. It follows a predictable daily cycle, dipping to its lowest point in the early morning (typically between 4 a.m. and 6 a.m.) and peaking in the early evening around 8 p.m. The total swing can be about 0.5°C or more between your coolest and warmest readings. This means a reading of 36.2°C at 6 a.m. and 36.9°C at 8 p.m. could both be perfectly normal for the same person on the same day.
Measurement Site Matters
The number you see on the thermometer depends heavily on where you take it. Rectal and ear readings run closer to true core temperature, while armpit readings tend to be lower. Oral readings fall somewhere in between. Here’s what to expect from each site:
- Rectal, ear, or forehead: normal range roughly 36.6°C to 37.2°C
- Oral (under the tongue): roughly 36.2°C to 37.0°C
- Armpit: roughly 35.9°C to 36.7°C
These aren’t rigid cutoffs, but they give you a sense of how much variation the measurement site alone introduces. If you’re comparing readings over time, use the same method each time.
Differences by Age and Sex
Infants and young children tend to run warmer than adults, partly because their metabolic rate is higher relative to their body size. Older adults often trend slightly cooler, and their daily temperature rhythm can shift, with the lowest point of the day occurring later in the morning rather than at dawn.
For women of reproductive age, body temperature also tracks the menstrual cycle. After ovulation, basal temperature rises by 0.2°C to 0.6°C and stays elevated until the next period begins. This shift is small but consistent enough that some women use daily temperature tracking as a fertility awareness tool. If you’re charting temperatures for this purpose, the reading that matters is your basal body temperature, taken first thing in the morning before getting out of bed.
When Temperature Signals a Problem
A fever is generally defined as a core body temperature at or above 38.0°C (100.4°F) when measured rectally, by ear, or on the forehead. For oral readings, the threshold is slightly lower at 37.8°C. For armpit readings, 37.2°C or above is considered a fever. These thresholds apply to both children and adults, though in infants under three months even a low-grade fever warrants prompt medical attention because their immune systems are still developing.
On the other end of the spectrum, a core temperature below 35°C is classified as hypothermia. This is a medical emergency that can develop from prolonged cold exposure, certain medications, or underlying conditions that impair the body’s ability to generate heat. Older adults are especially vulnerable because they often have a lower baseline temperature and may not shiver as effectively.
What a “Normal” Reading Looks Like in Practice
Rather than fixating on 37°C as a magic number, it helps to think of normal body temperature as a range. For most healthy adults taking an oral reading, anything between about 36.1°C and 37.2°C is unremarkable. Your personal baseline might sit at the lower or higher end of that window, and it will shift predictably with your sleep-wake cycle, physical activity, meals, and hormonal fluctuations.
If you want to know your own normal, take your temperature at the same time of day, using the same method, for several days when you’re feeling well. That gives you a personal baseline, which is far more useful than any single universal number when you’re trying to figure out whether a future reading is actually elevated.