Normal body temperature is not actually 98.6°F (37°C), despite what you may have learned growing up. A systematic review of 36 studies found the overall average human body temperature to be 97.9°F (36.6°C), meaningfully lower than the long-accepted standard. Your own “normal” can fall anywhere within a range, and it shifts throughout the day, varies by age, and depends on where you measure it.
Where 98.6°F Came From
The 98.6°F number dates back to 1851, when a German physician named Carl Wunderlich took the temperatures of roughly 25,000 patients in Leipzig and concluded that the average sat at 37°C (98.6°F). That figure stuck for over 150 years and became a medical standard taught in textbooks worldwide.
Modern data tells a different story. Several factors likely explain the drop. People today live in climate-controlled environments, spending most of their time between 68°F and 72°F, which means the body doesn’t work as hard to maintain its temperature. More importantly, the widespread reduction of chronic inflammatory conditions like tuberculosis, malaria, and dental infections has lowered baseline temperatures across populations. Inflammation triggers proteins that raise metabolism and, with it, body heat. As infectious disease burdens have fallen over the past century and a half, so has the average human temperature.
The Actual Normal Range
Rather than a single number, normal body temperature spans a range that depends on where you measure it:
- Oral (mouth): 96.4°F to 99.3°F (35.7°C to 37.4°C)
- Rectal: 97.4°F to 100.0°F (36.3°C to 37.8°C)
- Armpit (axillary): 95.0°F to 98.5°F (35.0°C to 36.9°C)
- Ear (tympanic): 96.4°F to 99.5°F (35.8°C to 37.5°C)
These ranges represent where about 95% of healthy people fall. Rectal readings run the highest because they reflect core body temperature more closely. Armpit readings run the lowest, typically about 0.8°F (0.4°C) below rectal and about 0.5°F (0.25°C) below oral. If you’re comparing a reading to a fever threshold, this offset matters.
Why Your Temperature Changes Throughout the Day
Body temperature follows a circadian rhythm, rising and falling in a predictable pattern over 24 hours. The total swing ranges from about 1.3°F to 2.9°F (0.7°C to 1.6°C) across the day. You’re coolest in the early morning hours, roughly between 4 a.m. and 6 a.m. Your temperature peaks in the late afternoon or early evening, typically 7 to 11 hours after you wake up.
This means a reading of 99°F at 5 p.m. could be completely normal for you, while the same reading at 6 a.m. might signal the beginning of a fever. Context matters more than any single number.
Age, Hormones, and Other Factors
Children tend to run warmer than adults, partly because of their higher metabolic rate relative to body size. Older adults often run cooler, which can make fevers harder to detect. A temperature that looks borderline in a younger person may actually represent a significant immune response in someone over 65.
The menstrual cycle also creates a reliable temperature shift. After ovulation, progesterone released by the ovaries raises basal body temperature by about 0.5°F to 1°F. This increase persists throughout the second half of the cycle (the luteal phase) and is the basis for fertility tracking methods that use a morning thermometer. If you menstruate and notice your temperature seems slightly elevated for a stretch of days each month, that’s the hormonal shift at work.
Thyroid function plays a role as well. Thyroid hormones regulate the rate of energy production in cells throughout the body. An underactive thyroid can lower baseline temperature, while an overactive one can raise it. Body composition, physical activity, and even the climate you live in also influence your baseline. People living in colder regions tend to have slightly higher resting metabolic rates than those in tropical environments.
When Temperature Becomes a Fever
Fever is generally defined as a temperature at or above 100.4°F (38°C) when measured orally, rectally, or by ear. For armpit measurements, the threshold is lower: 99°F (37.2°C) or higher, because armpit readings naturally run cooler.
A reading between your normal baseline and the fever threshold is sometimes called a “low-grade fever,” though this isn’t a formal clinical category. It can reflect mild infection, physical exertion, ovulation, or simply being measured at the warmest part of your day. A single borderline reading with no symptoms is rarely meaningful on its own.
When Temperature Drops Too Low
On the other end of the spectrum, hypothermia begins when core body temperature falls below 95°F (35°C). The stages break down by severity:
- Mild hypothermia: 90°F to 95°F (32.2°C to 35°C), marked by shivering, confusion, and poor coordination
- Moderate hypothermia: 82.4°F to 90°F (28°C to 32.2°C), where shivering may stop and consciousness fades
- Severe hypothermia: below 82.4°F (28°C), a life-threatening emergency
Hypothermia is most associated with cold-weather exposure, but it can also occur indoors in elderly adults or people with metabolic conditions that impair heat production.
Getting an Accurate Reading
If you’re checking your own temperature, timing and method both matter. Measure at the same time of day when comparing readings, ideally in the morning before eating or drinking. Oral thermometers should sit under the tongue with the mouth closed for the time specified by the device. Avoid taking a reading right after drinking hot or cold liquids, as this temporarily skews the result.
For infants and young children, rectal thermometers provide the most reliable reading. Forehead (temporal artery) thermometers are convenient but can be less precise, especially if the skin is sweaty or the device isn’t positioned correctly. Whatever method you use, the key is consistency: track your own baseline over several days so you know what’s normal for you, not just what’s normal on a chart.