Your normal body temperature is probably closer to 97.9°F (36.6°C), not the 98.6°F you’ve always heard. That familiar number dates back to 1868, and researchers at Stanford Medicine have found that average body temperature in the U.S. has been dropping by about 0.05°F every decade since then. Most healthy adults today fall somewhere between 97.3°F and 98.2°F.
But “normal” is also personal. Your baseline depends on your age, the time of day, your hormonal cycle, how you take the reading, and whether you just went for a run. Understanding that range matters more than memorizing a single number.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician who published temperature data over 150 years ago. It stuck in medical textbooks and popular knowledge, but large modern datasets tell a different story. Stanford researchers analyzed hundreds of thousands of temperature measurements and landed on 97.9°F as a more accurate average for today’s adults. The gradual decline over the past century likely reflects lower rates of chronic infection and inflammation in modern populations, meaning our bodies simply run a little cooler than they used to.
This doesn’t mean 98.6°F is abnormal for you. It means a reading of 97.5°F is also perfectly healthy. The key is knowing your own baseline so you can spot meaningful changes.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm tied to your internal clock. It’s lowest in the early morning hours, then starts rising during the last stretch of sleep just before you wake up. It peaks in the late afternoon or early evening, then drops again as your body prepares for sleep. Many people also experience a small dip between 2 p.m. and 4 p.m., which partly explains that familiar afternoon slump.
These fluctuations can easily account for a full degree of difference between morning and evening readings. If you’re trying to establish your personal baseline, take your temperature at the same time each day, ideally in the morning before eating or drinking.
Factors That Shift Your Baseline
Several things push your temperature above or below your personal average on any given day.
Hormonal cycles. After ovulation, basal body temperature rises by 0.4°F to 1°F and stays elevated through the second half of the menstrual cycle. This shift is reliable enough that some people use daily temperature tracking as a fertility awareness tool.
Exercise. Intense physical activity can raise your core temperature well above 100°F. Trained athletes exercising in heat have recorded core temperatures as high as 106.7°F (41.5°C) without any harmful effects, though this is an extreme case. Even moderate exercise will temporarily bump your reading by a degree or more.
Age. Older adults tend to run cooler than younger people, which can make fevers harder to detect. A reading of 99°F in someone over 65 may be more significant than the same reading in a 30-year-old. Infants and young children, on the other hand, tend to have slightly higher baseline temperatures than adults.
Time of day, hot drinks, warm clothing, and stress can all nudge readings up. Cold environments and dehydration can push them down. None of these shifts are cause for concern on their own.
Where You Measure Matters
Not all thermometer placements give the same number, and the differences are consistent enough to predict. Compared to an oral (mouth) reading:
- Rectal readings run 0.5°F to 1°F higher
- Ear (tympanic) readings run 0.5°F to 1°F higher
- Armpit (axillary) readings run 0.5°F to 1°F lower
So if your armpit thermometer says 97.2°F, that’s roughly equivalent to an oral reading of 97.7°F to 98.2°F. Rectal measurement is considered the most accurate for infants and young children. For adults, oral or forehead (temporal artery) readings are the most practical and reliable for everyday use.
When a Temperature Becomes a Fever
A fever generally starts at 100.4°F (38°C) for both adults and children when measured rectally, by ear, or with a forehead thermometer. For oral readings, the same 100.4°F threshold applies. Armpit readings hit fever territory at a lower number, 99°F (37.2°C), because armpit measurements naturally read lower.
A fever isn’t a disease. It’s your immune system deliberately raising the thermostat to make your body less hospitable to viruses and bacteria. Low-grade fevers (under 102°F) in otherwise healthy adults are generally your body doing its job. Adults with temperatures of 103°F or higher typically look and feel noticeably sick.
Temperature Extremes and Safety
At either end of the spectrum, body temperature becomes a medical concern.
Hypothermia begins when your core drops below 95°F (35°C). Mild hypothermia, between 95°F and 89.6°F, causes shivering, confusion, and clumsiness. Moderate hypothermia, between 89.6°F and 82.4°F, brings drowsiness, slurred speech, and a paradoxical feeling of warmth. Severe hypothermia, below 82.4°F, is life-threatening and can cause the heart to stop.
On the high end, sustained temperatures above 104°F (40°C) from environmental heat exposure (as opposed to a fever from illness) signal heat stroke, which requires immediate cooling and emergency care. The body’s own fever response rarely pushes temperature this high on its own.
Finding Your Personal Normal
The most useful thing you can do is figure out your own baseline. Take your temperature a few mornings in a row when you’re feeling well, using the same thermometer and the same method each time. Average those readings. That’s your normal. From there, you’ll know whether a reading of 99.5°F is a meaningful change for your body or just where you land on a warm afternoon.