The standard answer is 98.6°F (37°C), but that number is outdated. It comes from a massive study published in 1868, and modern research shows the average human body temperature has dropped since then. Today, a normal reading for most adults falls somewhere between 97°F and 99°F (36.1°C to 37.2°C), depending on the time of day, where on the body you measure, your age, and your hormones.
Where 98.6°F Came From
The 98.6°F benchmark traces back to a German physician named Carl Reinhold August Wunderlich, who analyzed several million temperature readings from an estimated 25,000 patients and published his findings in 1868. He calculated 37°C (98.6°F) as the mean of that enormous dataset, and the number stuck for over 150 years. It became the textbook answer taught in schools and printed on thermometer packaging.
But human bodies appear to have changed. Research from Harvard Health confirms a gradual decline in average body temperature over many decades. One study tracking 96 adults over two weeks found individual averages ranging from 95.4°F (35.2°C) to 99.3°F (37.4°C). The reasons for this cooling trend aren’t entirely clear, though reduced rates of chronic infection and changes in metabolic activity are leading explanations. The bottom line: 98.6°F is a useful reference point, not a precise target your body is supposed to hit.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily cycle tied to your internal clock. It bottoms out around 6 a.m. in healthy people and peaks around 8 p.m., regardless of age or sex. The swing from lowest to highest point in a given day typically ranges from 0.5°F to 1.9°F in healthy adults. So a reading of 97.3°F first thing in the morning and 98.8°F in the evening can both be perfectly normal for the same person.
This daily rhythm matters when you’re checking for a fever. A temperature of 99°F at 7 a.m. is more noteworthy than the same reading at 8 p.m., because your body is normally cooler in the early morning hours.
Where You Measure Makes a Difference
Not all thermometer readings are equal. The number you get depends on which part of the body you measure, and the offsets are consistent enough to predict.
- Oral (under the tongue): The standard reference point for most adults.
- Rectal: Reads 0.5°F to 1°F higher than oral. Considered the most accurate for infants and young children.
- Ear (tympanic): Reads 0.5°F to 1°F higher than oral.
- Armpit (axillary): Reads 0.5°F to 1°F lower than oral.
- Forehead (temporal): Reads 0.5°F to 1°F lower than oral.
This means a forehead reading of 97.8°F and a rectal reading of 99.4°F could reflect the exact same core temperature. If you’re comparing readings over time, use the same method and the same site each time.
Age and Normal Temperature
Babies and young children tend to run slightly warmer than adults, partly because their metabolisms are higher relative to their body size. Older adults often run cooler, sometimes registering well below 98°F as a baseline. This is important because an elderly person with a serious infection may never reach the classic 100.4°F fever threshold, even when their body is mounting a significant immune response. A reading that’s a full degree above their personal baseline can be just as meaningful.
For adults of any age, 100.4°F (38°C) is the most widely used cutoff for defining a fever.
Hormonal Shifts and Ovulation
In people who menstruate, body temperature shifts predictably across the menstrual cycle. After ovulation, basal body temperature (your temperature right when you wake up, before getting out of bed) rises by roughly 0.4°F to 1°F and stays elevated until the next period begins. This post-ovulation warming is driven by progesterone and is reliable enough that some people use daily temperature tracking as a method of fertility awareness. The shift is small, though, so it requires a thermometer that reads to the tenth of a degree and consistent morning measurements to detect.
When Body Temperature Gets Dangerous
The body works hard to stay within its narrow safe range, but extreme environments, illness, or prolonged exertion can push it outside those limits.
Hypothermia
When core temperature drops below about 95°F (35°C), the body enters hypothermia. Mild hypothermia (above 89.6°F or 32°C) causes intense shivering, confusion, and poor coordination. Moderate hypothermia (82.4°F to 89.6°F, or 28°C to 32°C) brings slurred speech, drowsiness, and a dangerous loss of the shivering response. Severe hypothermia (below 82.4°F or 28°C) can cause unconsciousness and cardiac arrest. The heart becomes increasingly unstable as core temperature falls.
Hyperthermia and Heatstroke
On the high end, a core temperature above 104°F (40°C) signals heatstroke, a medical emergency. The body’s cooling systems have been overwhelmed, and organ damage can begin rapidly. Unlike a fever, where the brain deliberately raises your set point to fight infection, hyperthermia means the body has lost control of temperature regulation entirely. Hot environments, strenuous exercise, and dehydration are the most common triggers.
What “Normal” Really Means for You
Your personal normal temperature is just that: personal. It’s shaped by the time of day, where you place the thermometer, your age, your hormonal status, and even long-term trends in human physiology that researchers are still studying. Rather than fixating on 98.6°F as a target, it’s more useful to know your own baseline. Take a few readings at different times of day when you’re feeling well. That gives you a reference point so that when you do feel off, you can tell whether a number on the thermometer is genuinely elevated for you, or just your body doing what it always does at 8 p.m.