A fever in adults and children is generally defined as a body temperature of 100.4°F (38°C) or higher. This is the threshold used by the CDC, the Mayo Clinic, and most healthcare providers. But the number can shift depending on where you take the temperature and the age of the person being measured.
The Standard Fever Threshold
The old textbook number for “normal” body temperature is 98.6°F (37°C), but typical body temperature actually ranges between 97°F (36.1°C) and 99°F (37.2°C). It fluctuates throughout the day, running lower in the morning and higher in the late afternoon. So a single reading of 99°F doesn’t necessarily mean anything is wrong.
A true fever starts at 100.4°F (38°C) when measured orally, rectally, in the ear, or on the forehead. Many healthcare providers also recognize a middle zone: a body temperature between 99.5°F (37.5°C) and 100.3°F (37.9°C) is often considered a low-grade fever. It may signal your immune system is responding to something, but it’s not as clinically significant as a reading above 100.4°F.
How Measurement Location Changes the Number
Not all thermometer readings are equal. The site where you take the temperature affects the number you see, sometimes by a full degree. Here’s how different methods compare to an oral reading:
- Rectal: Reads 0.5°F to 1°F higher than oral
- Ear (tympanic): Reads 0.5°F to 1°F higher than oral
- Armpit (axillary): Reads 0.5°F to 1°F lower than oral
- Forehead (temporal): Reads 0.5°F to 1°F lower than oral
This is why the fever threshold for an armpit reading is lower: 99°F (37.2°C) rather than 100.4°F. If you’re using a forehead scanner and get a reading of 99.5°F, that could correspond to an oral temperature around 100°F to 100.5°F. The differences are rough estimates rather than exact conversions, but they’re useful when you’re trying to decide whether a borderline reading counts.
Fever Thresholds for Babies and Children
For infants under 3 months old, a rectal or forehead temperature of 100.4°F (38°C) or higher is considered a fever and warrants a call to the pediatrician. This is a lower bar than for older children because young infants are more vulnerable to serious infections, and fever at that age can be one of the few visible signs something is wrong.
For children between 3 months and 3 years, the concern level rises at a higher number. A rectal or forehead temperature of 102°F (38.9°C) or above is the typical threshold for contacting a doctor. For any child of any age, a temperature of 104°F (40°C) or higher needs prompt medical attention regardless of other symptoms.
Why Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate response your body mounts against infection. When your immune system detects bacteria or viruses, it releases signaling molecules that trigger a chain reaction. The end result is a chemical called prostaglandin E2, which travels to the temperature-control center in your brain (a small region in the hypothalamus). Prostaglandin E2 essentially resets your internal thermostat to a higher point.
Once that set point rises, your body works to reach the new target. You shiver to generate heat, your blood vessels constrict to retain it, and you feel cold even though your temperature is climbing. This elevated temperature makes it harder for many pathogens to reproduce and helps certain immune cells work more efficiently. When the infection clears, the thermostat resets back to normal, and you sweat to cool down.
Low-Grade vs. High-Grade vs. Dangerous
Fevers exist on a spectrum, and the number matters. A low-grade fever (99.5°F to 100.3°F) is common with mild viral infections and often resolves on its own within a day or two. A standard fever (100.4°F to about 103°F) accompanies most common illnesses like the flu, colds, and ear infections. It’s uncomfortable but typically not dangerous in otherwise healthy adults.
A high fever above 103°F (39.4°C) in adults deserves closer attention, especially if it persists for more than a couple of days or comes with severe symptoms like confusion, chest pain, or difficulty breathing. At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At that level, proteins in the body can begin to break down and organs are at risk of damage. Hyperpyrexia is rare with ordinary infections and more commonly results from conditions like heat stroke or severe drug reactions.
When You Can’t Use a Thermometer
A measured temperature is the most reliable way to confirm a fever, but the CDC acknowledges it’s not always available. Skin that feels warm to the touch, visible flushing, chills, or a person’s own report of feeling feverish all count as reasonable indicators. Keep in mind that fever-reducing medications like ibuprofen or acetaminophen can temporarily mask a fever, so a normal reading right after taking one of those doesn’t necessarily mean the fever has resolved.