A fever starts at 100.4°F (38°C) for both adults and children. That’s the threshold used by most major medical organizations, including the American Academy of Pediatrics. But not all fevers are equal: a temperature of 101°F calls for a different response than one of 104°F, and the numbers shift depending on where you measure and how old the person is.
Normal Body Temperature
The textbook number is 98.6°F (37°C), but normal body temperature actually spans a range. Healthy people routinely measure anywhere from 97°F (36.1°C) to 99°F (37.2°C) depending on the time of day, physical activity, and individual variation. Body temperature tends to be lowest in the early morning and highest in the late afternoon.
This range matters because a reading of 99.2°F doesn’t necessarily mean you have a fever. It might just be your normal baseline at 4 p.m. after a workout. The 100.4°F threshold exists precisely because it sits above the upper end of normal variation for nearly everyone.
Fever Grades in Adults
Fever isn’t a single category. It breaks down into distinct ranges, each with different implications:
- Low-grade fever: 99.1°F to 100.4°F (37.3°C to 38°C). This sits in the gray zone between normal variation and true fever. It often accompanies mild infections, ovulation, or the early stages of an illness.
- Moderate fever: 100.6°F to 102.2°F (38.1°C to 39°C). This is the most common range during infections like the flu or a urinary tract infection. It’s uncomfortable but generally manageable at home with rest and fluids.
- High-grade fever: 102.4°F to 105.8°F (39.1°C to 41°C). This range signals a more aggressive immune response. Adults with a temperature at or above 103°F (39.4°C) should contact a healthcare provider.
- Hyperpyrexia: Above 106°F (41°C). This is a medical emergency. At this level, the body’s temperature regulation has essentially broken down, and elevated brain temperature can cause seizures, brain swelling, and organ damage.
Hyperpyrexia is different from a regular fever in an important way. During a normal fever, your brain deliberately raises its temperature target and carefully regulates heat production to match. During hyperpyrexia, that control system fails, and the body overheats beyond its ability to cool itself. This can happen with severe infections or conditions like heatstroke.
How Fever Works in the Body
Your brain has a built-in thermostat located in a region called the hypothalamus. Under normal conditions, it keeps your core temperature hovering around 98.6°F. When your immune system detects an invading pathogen, immune cells release chemical signals that travel through the bloodstream to the brain. These signals trigger the production of a specific molecule (prostaglandin E2) that essentially turns up the thermostat’s set point.
Once the set point rises, your brain treats your current normal temperature as “too cold.” That’s why you shiver and feel chilled at the start of a fever, even though your temperature is actually climbing. Your body constricts blood vessels near the skin to conserve heat and generates more heat through shivering until it reaches the new, higher target. When the infection clears and those chemical signals drop off, the set point returns to normal, and you suddenly feel hot and start sweating as your body dumps the excess heat.
Fever Thresholds for Children
The 100.4°F threshold applies to children of all ages, but the urgency of a fever depends heavily on how old the child is. Younger immune systems are less predictable, and very young infants can develop serious infections with relatively modest temperature elevations.
For babies under 3 months old, any rectal temperature of 100.4°F or higher warrants immediate medical evaluation, even if the baby looks fine. The immune system at this age is still immature, and fever can be the only visible sign of a serious bacterial infection. The AAP provides specific evaluation guidelines for infants aged 8 to 21 days, 22 to 28 days, and 29 to 60 days, reflecting how quickly risk profiles change in the first weeks of life.
For babies between 3 and 6 months, the concern level rises at 102°F (38.9°C), or at lower temperatures if the baby seems unusually irritable, sluggish, or uncomfortable. For children 7 to 24 months old, a rectal temperature above 102°F that persists for more than a day without other symptoms is worth a call to your pediatrician. For older children (above age 3), a high fever is generally considered anything above 103°F (39.4°C).
Why Measurement Location Matters
The number on your thermometer depends on where you take the reading. Rectal temperatures run highest, oral readings fall slightly lower, and armpit (axillary) measurements come in lowest. The differences are consistent enough to follow rough rules of thumb.
Armpit temperatures average about 0.5°F (0.25°C) lower than oral readings and nearly 0.8°F (0.43°C) lower than rectal readings. So an armpit temperature of 99.5°F roughly corresponds to an oral reading of 100°F or a rectal reading of 100.4°F. If you’re using an armpit thermometer, an axillary reading of 99.5°F (37.5°C) is the best cutoff for detecting a true oral fever of 100.4°F.
Rectal thermometers are considered the gold standard for infants and young children because they’re closest to core body temperature. Oral thermometers work well for older children and adults, though drinking hot or cold liquids beforehand can throw off the reading. Forehead and ear thermometers are convenient but tend to be less precise, so if you get a borderline reading, it’s worth confirming with an oral or rectal measurement.
When a Fever Becomes Urgent
For adults, a temperature of 103°F or higher on its own merits a call to your doctor. But certain accompanying symptoms turn any fever into something more urgent, regardless of the number. These include a stiff neck with pain when bending the head forward, unusual sensitivity to bright light, confusion or altered speech, persistent vomiting, difficulty breathing, chest pain, rash, or seizures. That cluster of symptoms, especially stiff neck, light sensitivity, and high fever together, can point to meningitis, which requires emergency care.
For children, a fever lasting longer than three days deserves medical attention. A fever-related seizure, while terrifying to witness, is usually brief and not harmful on its own, but any seizure lasting longer than five minutes or followed by slow recovery is a 911 situation. And any child who develops a fever after being left in a hot car needs emergency care immediately, since that represents heat illness rather than an infection-driven fever.
One important detail: the height of a fever doesn’t always correlate with how serious the illness is. A child with a 104°F fever from a common virus may recover in a day or two, while a vulnerable infant with a 100.5°F reading could have a bacterial infection that needs urgent treatment. The overall picture, including age, behavior, other symptoms, and how long the fever has lasted, matters more than the number alone.