The regular temperature for a human body is about 97.9°F (36.6°C), which is slightly lower than the 98.6°F (37°C) number most people grew up hearing. That classic figure dates back to the 1800s. A large systematic review of 36 modern studies found the overall average to be 36.59°C, roughly a third of a degree Fahrenheit cooler than older references suggest. The difference is small enough that it doesn’t change how your body functions, but it does mean a “normal” reading can sit comfortably anywhere from about 97°F to 99°F depending on the time of day, where you measure, and who you are.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician named Carl Wunderlich, who collected over a million temperature readings in the 1860s and declared that number the human norm. His work was groundbreaking for the time, but modern instruments are more precise, and human bodies may have genuinely cooled since then. When researchers pooled data across dozens of contemporary studies, the average landed at 36.59°C (97.9°F), noticeably below the 36.8°C (98.2°F) figure still printed in major medical textbooks like Harrison’s Principles of Internal Medicine. Wunderlich himself noted that a healthy person’s temperature rarely sways more than 0.9°F from its baseline, and that observation still holds. The baseline itself just appears to be a bit lower than he thought.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. It tends to be lowest in the early morning, roughly between 8 and 10 a.m., and peaks in the evening, typically between 8 p.m. and 3 a.m. This swing is driven by your internal clock and happens even if you stay in bed all day. If you take your temperature first thing in the morning and again after dinner, you can easily see a difference of half a degree or more, and both readings are perfectly normal.
Factors That Shift Your Baseline
Menstrual Cycle
For people who menstruate, body temperature rises 0.3°C to 0.7°C (roughly 0.5°F to 1.3°F) after ovulation and stays elevated through the luteal phase. Progesterone drives this shift. The temperature climbs about 24 hours after progesterone levels rise, plateaus within 48 hours, and drops back down as menstruation approaches. In cycles where ovulation doesn’t occur, this temperature bump doesn’t happen, which is why basal body temperature tracking is used as a fertility indicator.
Exercise
Physical activity generates heat as a byproduct of increased metabolism. If your body produces heat faster than it can shed it through sweating and blood flow to the skin, core temperature climbs. This is temporary and expected. Sweating is the only cooling mechanism available once the air around you is warmer than your skin, which is why exercising in hot, humid conditions pushes core temperature higher than the same workout in a cool room.
Age
Older adults tend to run cooler than younger adults, partly because metabolic rate declines with age and the body’s temperature regulation becomes less efficient. Infants and young children, on the other hand, can run slightly warmer. These differences matter most when deciding whether a reading counts as a fever, which is why fever thresholds are adjusted by age group.
Where You Measure Matters
Not all thermometer placements give the same number. Rectal readings run about 0.4°C (0.7°F) higher than armpit (axillary) readings, while oral readings fall roughly in between, about 0.25°C (0.5°F) above the armpit. So a “normal” armpit reading might look like 97.5°F, while a rectal reading on the same person at the same moment could show 98.3°F. Neither is wrong. They’re just measuring slightly different spots with different proximity to your core.
If you’re comparing readings over time, consistency matters more than the specific site. Pick one method and stick with it so the numbers are comparable.
Thermometer Accuracy Varies
In a clinical comparison of several consumer thermometers against a hospital-grade device, the ear (tympanic) thermometer had the best agreement, with an average error of just 0.04°C. Digital oral thermometers and infrared forehead thermometers were reasonably close but tended to slightly underestimate temperature. Temporal artery thermometers (the kind you swipe across the forehead) overestimated temperature by about 0.4°C on average, which could make a normal reading look borderline feverish.
Thermal imaging cameras, the type used for screening at airports and building entrances, were the least accurate, off by about half a degree Celsius with wide variability between readings. For reliable home use, an ear thermometer or a standard digital oral thermometer will get you closest to your true temperature.
When a Reading Counts as a Fever
The widely accepted medical threshold for fever is a core body temperature above 100.4°F (38.0°C). For young children, the cutoffs are lower because their normal baselines differ:
- Infants 3 months and younger: above 99.4°F (37.4°C) is considered a fever
- Children 3 months to 3 years: above 99.6°F (37.6°C) is a fever, and above 101.3°F (38.5°C) is a high fever
- Children over 3 years and adults: above 99.9°F (37.7°C) is a fever, and above 103.0°F (39.4°C) is a high fever
On the other end of the spectrum, hypothermia begins when core temperature drops below 95°F (35°C). Mild hypothermia ranges from 90 to 95°F, moderate from 82 to 90°F, and severe hypothermia is anything below 82°F. Severe hypothermia is a medical emergency.
What a “Normal” Reading Looks Like in Practice
Given all these variables, a healthy oral temperature for most adults falls somewhere between 97.0°F and 99.0°F (36.1°C to 37.2°C). If your thermometer consistently reads 97.4°F, that’s your normal. If it consistently reads 98.8°F, that’s also normal. What matters clinically is a significant departure from your own baseline, not whether you match a single textbook number. Tracking a few readings at different times of day when you’re feeling well gives you a personal reference point, making it much easier to spot when something is actually off.