A normal body temperature for most people falls between 97°F and 99°F (36.1°C to 37.2°C), not the 98.6°F you probably grew up hearing. That famous number dates back to 1868, when a German physician published findings from millions of temperature readings taken from roughly 25,000 patients. More recent research tells a different story: a large-scale analysis of over 35,000 patients found the modern average oral temperature is closer to 97.9°F (36.6°C).
Why 98.6°F Is Outdated
Human body temperature has genuinely dropped since the 1800s. The decline isn’t a measurement error. Researchers attribute it to a broad reduction in chronic inflammation across the population, driven by improved sanitation, better dental hygiene, the near-elimination of diseases like tuberculosis and malaria, and widespread antibiotic use. In other words, people in the 19th century ran slightly warmer because their immune systems were fighting off more infections on a daily basis.
This means a reading of 97.5°F or 98.0°F is perfectly healthy. If you’ve ever taken your temperature and worried because it wasn’t exactly 98.6°F, there was nothing wrong.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a predictable daily cycle, hitting its lowest point in the early morning (around 4 to 6 a.m., typically about two hours before you wake up) and peaking in the early evening around 8 p.m. The swing from low to high can range from 0.5°F to nearly 2°F. So a reading of 97.4°F at 7 a.m. and 99°F at dinnertime could both be completely normal for the same person on the same day.
How Age Affects Normal Temperature
Children tend to run warmer than adults, partly because their metabolisms are faster. Older adults, on the other hand, generally have lower baseline temperatures. This matters because an elderly person with a serious infection might register a temperature that looks unremarkable on paper but actually represents a significant spike for them. A reading of 99°F in someone whose baseline sits around 97°F is worth paying attention to, even though it wouldn’t technically qualify as a fever by standard definitions.
Where You Measure Makes a Difference
The number on your thermometer depends on where you place it. Oral readings (under the tongue) are the most common reference point, but other methods read consistently higher or lower:
- Rectal: 0.5°F to 1°F higher than oral
- Ear (tympanic): 0.5°F to 1°F higher than oral
- Armpit (axillary): 0.5°F to 1°F lower than oral
If you take your temperature under your arm and get 97.6°F, that’s roughly equivalent to an oral reading of 98.1°F to 98.2°F. For infants, rectal thermometers give the most accurate core temperature reading.
Other Factors That Shift Your Baseline
Physical activity raises your temperature, sometimes substantially, depending on intensity and duration. Hormonal cycles play a role too. After ovulation, basal body temperature rises by about 0.3°C (roughly half a degree Fahrenheit) and stays elevated through the second half of the menstrual cycle. This small but consistent shift is reliable enough that some people use daily temperature tracking as a method of natural family planning.
Hot and humid environments also push your temperature up. Your body sheds heat through sweating and by directing blood flow toward the skin, but when the air is both hot and humid, sweat can’t evaporate efficiently. At a certain threshold of heat and humidity, your cooling system simply can’t keep pace. This is how heat stroke develops: core temperature climbs high enough to damage organs, particularly in people with existing heart or kidney conditions.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold is used consistently across most medical settings for both adults and children. Temperatures between 99°F and 100.3°F occupy a gray zone sometimes called a “low-grade fever,” though many clinicians consider readings in this range a normal variation rather than a true fever.
For infants, the stakes are higher. Any baby under 3 months old with a rectal temperature of 100.4°F or above needs immediate medical evaluation. At that age, fever can signal a serious infection that progresses quickly. For children 2 and older, a fever of 100.4°F that persists beyond three days warrants a call to their doctor, even if the child seems otherwise okay.
When a Temperature Is Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. It doesn’t take extreme cold to get there. Prolonged exposure to even mildly cool conditions, wet clothing, or certain medications can gradually pull your core temperature down.
- Mild hypothermia (95°F to 89.6°F): shivering, confusion, clumsiness
- Moderate hypothermia (89.6°F to 82.4°F): shivering may stop, drowsiness, slurred speech
- Severe hypothermia (below 82.4°F): loss of consciousness, dangerously slow heart rate
The fact that shivering stops during moderate hypothermia is counterintuitive and dangerous. It can make a person feel like they’re improving when the opposite is true.
Finding Your Own Normal
Given how much normal temperature varies by person, time of day, age, and measurement method, the most useful thing you can do is learn your own baseline. Take your temperature a few times over several days when you feel healthy, using the same thermometer and the same method each time. Most people will land somewhere between 97°F and 99°F. Once you know your personal average, you’ll have a much clearer sense of what counts as elevated for you, rather than relying on a number from 1868.