A normal body temperature falls in the range of 97°F to 99°F (36.1°C to 37.2°C) for most adults, not the single number you probably learned in school. The old standard of 98.6°F dates back to 1851 and turns out to be slightly higher than what modern humans actually run. Your personal normal depends on the time of day, how you took your temperature, your age, and even the decade you were born in.
Why 98.6°F Is Outdated
The 98.6°F benchmark comes from a German physician who averaged thousands of readings in the mid-1800s. For over 150 years, that number stuck. But a large-scale study published in eLife, analyzing body temperature data spanning nearly two centuries of Americans, found that average human body temperature has been dropping steadily, about 0.05°F per decade of birth. Men born in the early 1800s ran roughly 1°F warmer than men today. Women have seen a similar decline of about 0.6°F since the 1890s.
The likely explanation involves changes in our environment and health. Lower rates of chronic infection, reduced inflammation, climate-controlled living spaces, and less physically demanding daily life all contribute to cooler baseline temperatures. Today, a healthy adult’s oral temperature averages closer to 97.9°F to 98.3°F than the textbook 98.6°F.
Normal Ranges by Age
Infants and young children run warmer than adults. Their metabolisms are higher relative to body size, and their temperature regulation systems are still maturing. A rectal reading of 100.4°F (38°C) or higher in a baby under 3 months is considered a fever and needs prompt medical attention, even if the baby seems fine otherwise. For babies 3 to 24 months old, the fever concern rises at 102°F (38.9°C) rectally.
Older adults tend to run cooler. A baseline of 96.8°F or 97°F is not unusual for someone over 65. This matters because it means a reading of 99°F in an elderly person could represent a meaningful temperature spike, even though it would be unremarkable in a younger adult. If you’re caring for someone older, knowing their personal baseline helps you catch fevers earlier.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily cycle. It bottoms out between 6:00 and 8:00 in the morning, often dipping below 97.5°F, and peaks between 6:00 and 8:00 in the evening, sometimes climbing above 99°F. That’s a swing of more than a full degree over the course of a normal day, with no illness involved. A reading of 99°F at 7 p.m. is perfectly ordinary, while the same number at 7 a.m. would be more noteworthy.
Exercise also raises your core temperature significantly. During intense physical activity, your metabolic rate spikes and your body generates heat faster than it can shed it, especially in the first several minutes before sweating fully kicks in. If you’re dehydrated, the rise is even steeper because your body becomes less efficient at cooling itself through sweat and blood flow to the skin. Taking your temperature right after a workout or a hot shower will give you an artificially high reading.
Hormonal cycles play a role too. In women, body temperature typically rises by about 0.5°F to 1°F after ovulation and stays elevated until the next period begins. This is the basis for the basal body temperature method of fertility tracking.
Where You Measure Matters
Not all thermometer placements give the same number. Rectal readings run closest to true core body temperature and are considered the most accurate, especially for infants. Oral readings tend to be about 1°F lower than rectal, though the gap can vary widely from person to person, anywhere from nearly identical to almost 3°F lower. Ear (tympanic) readings average close to rectal, but they’re inconsistent. A single ear reading can be off by as much as 2°F in either direction.
Forehead and armpit (axillary) readings are the most convenient but least precise. Armpit temperatures typically run 0.5°F to 1°F lower than oral. If you’re using a forehead scanner, keep in mind that ambient temperature, sweat, and even a breeze can throw off the result. For the most reliable at-home reading, an oral thermometer under the tongue with the mouth closed for a full minute remains the practical standard for older children and adults.
Fever Thresholds to Know
An oral temperature of 100°F (37.8°C) or higher generally counts as a fever. A rectal temperature of 100.4°F (38°C) is the threshold most commonly used for infants. For adults, a reading of 103°F (39.4°C) or higher warrants a call to your doctor regardless of other symptoms.
Low-grade fevers, those hovering between 99°F and 100.4°F, are common with minor viral infections and usually resolve on their own. They represent your immune system working as designed. What matters more than the exact number is the pattern: how long the fever lasts, whether it responds to rest and fluids, and what other symptoms come with it. A persistent low-grade fever lasting more than a few days deserves more attention than a brief spike to 101°F during a cold.
When a Temperature Is Too Low
The conversation about “normal” usually focuses on fever, but a temperature that’s too low is its own concern. Mild hypothermia begins at 95°F (35°C), and even readings in the 95°F to 96°F range in someone who hasn’t been in a cold environment can signal an underlying problem, particularly in older adults. Moderate hypothermia (82°F to 90°F) and severe hypothermia (below 82°F) are medical emergencies involving confusion, slowed heart rate, and loss of consciousness.
Outside of cold exposure, a consistently low body temperature can sometimes reflect an underactive thyroid, severe infection (paradoxically, some serious infections cause temperature to drop rather than rise), or simply individual variation. If your temperature regularly sits below 97°F and you feel fine, it’s likely just your normal. If it’s paired with fatigue, cold intolerance, or other symptoms, that’s worth investigating.
Finding Your Personal Baseline
The most useful thing you can do is figure out what’s normal for you specifically. Take your temperature a few times over the course of a week when you’re feeling well, using the same thermometer and the same method each time. Note the time of day. After several readings, you’ll have a personal baseline that makes it much easier to tell when something is off. A jump of 1.5°F to 2°F above your own average is more meaningful than any single number on a chart.