A fever in adults is generally defined as a body temperature of 100.4°F (38°C) or higher. This is the threshold used by the CDC and most medical institutions, though the number shifts slightly depending on where you measure and how old you are. Understanding these differences matters because a reading that looks normal on one type of thermometer might actually signal a fever on another.
Why 100.4°F Is the Standard Threshold
The 100.4°F mark applies to rectal, ear, and forehead (temporal artery) readings, which tend to run closest to your actual core body temperature. If you’re using a mouth thermometer, the fever threshold drops slightly to 100°F (37.8°C). Armpit readings run even lower, so 99°F (37.2°C) under the arm is considered a fever. These aren’t different definitions of fever. They reflect the fact that different parts of your body sit at slightly different temperatures, and each measurement site has its own cutoff.
The old benchmark of 98.6°F as “normal” body temperature dates back to 1868, when a German physician published over a million temperature readings from about 25,000 patients. That number has stuck around for more than 150 years, but it’s no longer accurate. Research from Stanford Medicine analyzing over 618,000 oral temperature readings found that the average adult body temperature today is closer to 97.9°F. Normal ranges span from about 97.3°F to 98.2°F. The decline, roughly 0.05°F per decade since the 1800s, likely reflects improvements in health and living conditions that have reduced chronic inflammation.
Fever Thresholds for Children
Children follow the same basic thresholds as adults. A rectal, ear, or forehead temperature of 100.4°F or higher counts as a fever. An oral reading of 100°F or higher, or an armpit reading of 99°F or higher, also qualifies. For infants under three months, any fever at all is taken seriously because their immune systems are still developing and infections can escalate quickly. In older children, the number on the thermometer matters less than how the child is acting: a child with 101°F who is playing and drinking fluids is in a very different situation than one with the same temperature who is lethargic and refusing to eat.
Why Older Adults Need a Lower Threshold
If you’re over 65, the standard 100.4°F cutoff can be misleading. Older adults tend to run cooler at baseline and mount weaker fever responses to infection, which means a temperature that looks “normal” could actually represent a significant immune response. A 2025 study in the Journal of the American Medical Directors Association found that for hospitalized adults aged 75 and older, an oral temperature of just 99°F (37.2°C) was a more reliable indicator of infection. For those 85 and older, the useful threshold dropped even further to about 98.4°F (36.9°C).
This is a real clinical problem. The study found that in the 85-and-older group, standard fever detection only caught about 60% of actual infections. Infections in older adults often show up with atypical symptoms like confusion, fatigue, or loss of appetite rather than the classic high temperature, so a modest rise above that person’s usual baseline can be an important early signal.
Your Temperature Changes Throughout the Day
Body temperature isn’t static. It follows a circadian rhythm, rising in the last hours of sleep and peaking in the late afternoon or early evening. Most people also experience a small dip between about 2 p.m. and 4 p.m. This natural fluctuation means a temperature of 99.1°F at 7 a.m. could be more significant than the same reading at 5 p.m., when your body naturally runs warmer.
Your personal baseline also varies based on age, sex, height, weight, and time of day. Stanford researchers found that these factors together account for about 25% of the variability in an individual’s normal temperature. This is why tracking your own typical temperature when you’re healthy can give you a better reference point than relying on 98.6°F as a universal standard.
What Happens in Your Body During a Fever
A fever isn’t a malfunction. It’s your immune system deliberately raising your body’s thermostat. The process starts when your body detects an infection and releases signaling molecules called pyrogens. These reach the hypothalamus, a small region at the base of your brain that acts as your internal thermostat, and reset its target temperature to a higher level.
Once the set point shifts upward, your body treats its current temperature as “too cold” and activates warming mechanisms. Blood vessels near the skin constrict to reduce heat loss, which is why you might feel chilly and look pale even though your temperature is climbing. Your metabolism ramps up to generate more heat. If the gap between your current temperature and the new set point is large enough, you’ll start shivering, sometimes intensely enough to be called “shaking chills.” All of this is your body working exactly as designed, pushing your temperature to a level that helps fight off pathogens.
Fever Severity and When It Gets Dangerous
Not all fevers carry the same weight. A low-grade fever between 100.4°F and 102°F is common with routine viral infections and often resolves on its own within a few days. Moderate fevers between 102°F and 104°F are more uncomfortable but still represent a normal immune response in most cases.
Temperatures above 104°F (40°C) in adults warrant prompt attention. At this level the fever itself can start causing problems, including significant dehydration, confusion, and in some cases seizures in young children. The extreme end of the spectrum is hyperpyrexia, defined as a body temperature above 106.7°F (41.5°C). This is rare and represents a medical emergency. At these temperatures, proteins in the body can begin to break down and organ damage becomes a real risk. Hyperpyrexia typically results from severe infections, drug reactions, or heat-related illness rather than a common cold or flu.
Choosing the Right Thermometer
The most accurate reading for adults comes from an oral thermometer, placed under the tongue with the mouth closed for the recommended time. For infants under six months, rectal thermometers remain the gold standard because they measure closest to core temperature and are least affected by external factors. Forehead and ear thermometers offer convenience and reasonable accuracy for older children and adults, though ear readings can be thrown off by earwax or improper positioning.
Armpit temperatures are the least reliable, consistently reading lower than core body temperature. If you’re using an armpit thermometer, remember that the fever threshold is a full degree and a half lower than a rectal reading. Whatever method you use, consistency matters most. Stick with the same type of thermometer and the same measurement site so you can track meaningful changes over time rather than comparing numbers from different methods.