A fever is generally defined as a body temperature at or above 100.4°F (38°C) when measured orally. That said, there’s no single hard cutoff, because normal body temperature varies from person to person, fluctuates throughout the day, and reads differently depending on where you measure it. Most healthcare providers use 100.4°F as the standard threshold, though some consider 100.0°F (37.8°C) sufficient.
Normal Body Temperature Isn’t 98.6°F
The familiar 98.6°F figure dates back to data published in 1868. Research from Stanford Medicine has found that the average body temperature in the U.S. has actually dropped by about 0.05°F per decade since the 19th century. Today, normal body temperature for adults falls between 97.3°F and 98.2°F, with an overall average closer to 97.9°F.
Your temperature also follows a daily rhythm. It’s lowest during the night while you sleep and peaks in the late afternoon or early evening. This natural swing covers roughly 1°F over the course of 24 hours. That means a reading of 99.5°F at 4 p.m. may be perfectly normal, while the same reading at 6 a.m. could signal something is off.
How Your Body Creates a Fever
A fever isn’t your body malfunctioning. It’s a deliberate response. When your immune system detects an invader like a virus or bacteria, immune cells release signaling molecules that travel to a region of the brain responsible for regulating temperature. These signals essentially raise your internal thermostat, pushing the “normal” set point higher. Your body then works to reach that new target by generating more heat (shivering, for example) and conserving it (constricting blood vessels near the skin). The result is that you feel cold even though your temperature is climbing.
Measurement Method Changes the Number
Where you take your temperature matters. Rectal readings run about 0.8°F (0.43°C) higher than armpit readings, while oral temperatures fall roughly in between. That’s why the same person can get three different numbers from three different thermometers at the same time.
Armpit (axillary) measurements are the least reliable for detecting a true fever. Studies show that using the standard 100.4°F cutoff under the arm misses about 40% of fevers that a rectal thermometer would catch. If you’re using an armpit thermometer, a reading of 99.5°F (37.5°C) or higher is a better indicator that a real fever is present.
For infants, rectal thermometers are considered the most accurate. For older children and adults, oral or forehead thermometers are the most practical and reasonably reliable options.
Fever Thresholds for Infants and Children
The stakes are higher for very young babies. The American Academy of Pediatrics considers any rectal temperature of 100.4°F or higher a fever in infants 8 to 60 days old, and it warrants prompt medical evaluation. For babies younger than 3 months, any fever at that level needs immediate attention regardless of how the baby appears.
For older infants and toddlers, the picture becomes more nuanced:
- 3 to 6 months: A rectal temperature above 102°F (38.9°C), or a lower fever paired with unusual irritability or sluggishness, warrants a call to the pediatrician.
- 7 to 24 months: A temperature above 102°F that persists for more than a day without other symptoms should be evaluated.
- Children of any age: Fever lasting more than three days, repeated vomiting, confusion, poor eye contact, or a seizure all call for medical attention. If a seizure lasts more than five minutes, call 911.
Why Older Adults Need a Lower Threshold
People over 65 tend to run cooler at baseline, and their bodies are less effective at mounting a fever response. Between 20% and 30% of older adults with an active infection show up to the emergency department without a significant fever. The current clinical consensus is that for older adults, a temperature of just 99.0°F (37.2°C), or any increase of 1.3°F or more above their personal baseline, should be treated as a potential fever.
When a Fever Becomes Dangerous
For adults, a temperature of 103°F (39.4°C) or higher is worth contacting a healthcare provider about, even if you otherwise feel okay. Certain accompanying symptoms at any fever level signal something more serious: a stiff neck with pain when bending forward, a rash, sensitivity to bright light, confusion or altered speech, difficulty breathing, chest pain, persistent vomiting, or seizures. These combinations can point to conditions like meningitis or sepsis that need urgent care.
At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At that level, the heat itself begins to damage organs. The brain is especially vulnerable, and prolonged temperatures in this range can cause brain swelling, permanent brain damage, or coma. Typical illness-related fevers almost never reach this point on their own. Hyperpyrexia is more commonly associated with heatstroke, certain drug reactions, or severe central nervous system infections.
Managing a Fever at Home
Most fevers in otherwise healthy adults and older children don’t need aggressive treatment. A fever is your immune system doing its job, and bringing the number down doesn’t necessarily speed recovery. The main reason to treat a fever is comfort: if you feel miserable, reducing the temperature can help you rest and stay hydrated.
Acetaminophen and ibuprofen are the two standard options for lowering a fever. For children under 2, neither should be given without guidance from a doctor. For children under 12, liquid acetaminophen (160 mg per 5 mL) can be given every 4 hours, with no more than 5 doses in 24 hours. Staying hydrated matters more than medication in most cases, since fever increases fluid loss through sweating and faster breathing.
Lukewarm baths can provide some relief, but avoid cold water or ice baths, which cause shivering and can actually raise your core temperature. Dress in light clothing and keep the room comfortably cool rather than piling on blankets.