A normal body temperature for most adults falls between 97.3°F and 98.2°F, with an average around 97.9°F. That’s noticeably lower than the 98.6°F number you probably grew up hearing, which dates back to a study from 1868 and no longer holds up under modern measurement.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician named Carl Wunderlich, who published his findings over 150 years ago. His thermometers were bulky, took 15 to 20 minutes to settle, and he measured temperature under the armpit rather than in the mouth. Modern thermometers are faster, more accurate, and typically used orally or in the ear.
Researchers at Stanford Medicine have found that the average body temperature in the U.S. has dropped by about 0.05°F per decade since the 1800s. The likely explanation: better overall health, improved living conditions, and lower levels of chronic inflammation across the population. When Stanford’s team analyzed current data, 98.6°F wasn’t the mean, the median, or even the most frequently recorded temperature. It didn’t fall within the statistical range of what “normal” looks like today.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It rises and falls over the course of 24 hours, typically reaching its lowest point in the early morning and its highest in the late afternoon. In one large study, the 99th percentile for oral temperature at 6 AM was 98.9°F, while at 4 PM it reached 99.9°F. That means a reading of 99.5°F at 4 PM could be perfectly normal, while the same number at 6 AM would be unusual.
Where You Measure Matters
Different parts of the body give different readings, and the gap is consistent enough that you can roughly translate between them:
- Rectal and ear (tympanic): 0.5 to 1°F higher than an oral reading
- Armpit (axillary): 0.5 to 1°F lower than oral
- Forehead (temporal scanner): 0.5 to 1°F lower than oral
So if your forehead thermometer reads 98.0°F, your oral temperature would likely be somewhere around 98.5 to 99.0°F. Keep this in mind when comparing readings taken at different sites, especially if you’re trying to determine whether someone has a fever.
What Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This is the threshold most clinicians use for adults and children alike. The American Academy of Pediatrics uses the same 100.4°F cutoff when evaluating infants as young as 8 days old.
A reading between 99°F and 100.3°F is sometimes called a “low-grade fever,” but it can also just reflect normal afternoon variation, recent physical activity, or a warm environment. Context matters more than the number alone.
Age, Hormones, and Other Factors
Your baseline temperature is personal, and several things shift it. Age is one of the biggest. As you get older, changes in body fat distribution, muscle mass, skin thickness, and sweat gland function all affect how your body regulates heat. Older adults tend to run cooler, which also means a fever can be harder to detect in someone over 65.
Hormonal cycles create measurable shifts, too. Before ovulation, most people’s basal temperature (the reading you get first thing in the morning, before getting out of bed) sits between 96°F and 98°F. After ovulation, it rises by 0.4 to 1.0°F and stays elevated until the next period begins. This shift is small but reliable enough that some people use it to track fertility.
Exercise, hot drinks, heavy clothing, and even the time of day you last ate can all bump your reading up temporarily. If you want the most accurate baseline, measure at the same time each day, ideally in the morning before you’ve been active.
When a Temperature Is Dangerously Low
Hypothermia begins when your core temperature drops below 95°F (35°C). It’s classified in three stages:
- Mild (90°F to 95°F): shivering, confusion, poor coordination
- Moderate (82.4°F to 90°F): shivering may stop, drowsiness, slurred speech
- Severe (below 82.4°F): loss of consciousness, dangerously slow heart rate
Hypothermia doesn’t only happen in extreme cold. Older adults in poorly heated homes and people exposed to cool, wet conditions for extended periods are at risk even in moderate climates.
When a Temperature Is Dangerously High
A fever above 103°F in an adult warrants attention, but the true emergency threshold is 106.7°F (41.5°C). At that point, a condition called hyperpyrexia sets in, and the body’s own heat can damage organs. This is a life-threatening situation that requires immediate emergency care. It’s rare with a typical infection and more commonly seen with heatstroke, certain drug reactions, or conditions affecting the brain’s temperature-control center.
For infants under 3 months old, any fever of 100.4°F or higher needs prompt medical evaluation, regardless of how the baby appears. Young infants can deteriorate quickly, and fever at that age is treated with more urgency than in older children or adults.