A fever is generally defined as a body temperature at or above 100.4°F (38°C), whether measured orally, rectally, or in the ear. That single number is the most widely used clinical threshold for adults, children, and infants alike. But the full picture is more nuanced: your normal baseline temperature is probably lower than you think, readings vary by measurement site, and your body temperature naturally shifts throughout the day.
The Standard Threshold
The Mayo Clinic defines fever as a rectal, ear, or forehead temperature of 100.4°F (38°C) or higher. Oral readings use the same cutoff. Armpit (axillary) measurements run cooler, so a reading of 99°F (37.2°C) or higher from the armpit is considered a fever. These thresholds apply to both adults and children.
For infants under 60 days old, the American Academy of Pediatrics uses the same 100.4°F cutoff but treats it with far more urgency. A baby that young with a rectal temperature at or above 100.4°F needs prompt medical evaluation, even if the baby appears well otherwise. Rectal thermometers are the preferred method for infants because they give the most accurate core temperature reading.
Why 98.6°F Is Outdated
The idea that normal body temperature is 98.6°F comes from a German study published in 1868. More recent research paints a different picture. A systematic review pooling data from multiple studies found the overall average human body temperature to be about 97.9°F (36.6°C), roughly a full degree lower than the old standard. The normal range, defined as two standard deviations from the mean, spans from about 97.1°F to 98.6°F (36.2°C to 37.0°C).
Age matters too. Adults under 60 tend to run slightly warmer (average 97.9°F) than those over 60 (average 97.7°F). This means an older adult could have a genuine fever at a temperature that looks unremarkable on paper. If your personal baseline runs low, a reading of 99.5°F might represent a meaningful elevation for you, even though it falls below the official fever line.
Readings Vary by Measurement Site
Where you place the thermometer changes the number you get. In order from warmest to coolest, average readings across studies break down like this:
- Rectal: 98.7°F (37.0°C)
- Ear (tympanic): 97.9°F (36.6°C)
- Oral: 97.9°F (36.6°C)
- Armpit (axillary): 96.7°F (36.0°C)
You might expect a simple formula to convert between sites, but research shows the gap between armpit and core temperature varies too much from person to person to be reliable. The best approach is to know which site your thermometer is designed for and compare your reading to the fever threshold for that site: 100.4°F for oral, rectal, or ear, and 99°F for armpit.
Your Temperature Shifts Throughout the Day
Body temperature follows a circadian rhythm, rising and falling in a predictable daily cycle. In humans, the daily swing ranges from about 1.3°F to 2.9°F (0.7°C to 1.6°C). Temperature typically bottoms out in the early morning hours and peaks in the late afternoon or early evening, roughly 7 to 11 hours after waking.
This is why a temperature of 99.5°F at 7 a.m. is more significant than the same reading at 5 p.m. If you’re monitoring a fever, try to take your temperature at the same times each day so you’re comparing like with like.
How Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate response orchestrated by your brain. When your immune system detects an infection, immune cells release signaling molecules into the bloodstream. These signals travel to a temperature-regulating area in the hypothalamus, a small region at the base of the brain that acts as your internal thermostat.
Once these signals arrive, they trigger the production of a chemical messenger (prostaglandin E2) that essentially turns the thermostat up. Your brain now treats your normal temperature as “too cold” and activates warming mechanisms: blood vessels near the skin constrict to trap heat, and your muscles begin to shiver to generate it. That’s why you can feel freezing cold and shaky even as your temperature climbs. Common fever-reducing medications work by blocking the production of that chemical messenger, dialing the thermostat back down.
When a Fever Becomes Dangerous
Most fevers in the 100.4°F to 103°F range are uncomfortable but not inherently dangerous. They’re your body fighting off an infection. Temperatures above 105.8°F (41°C) enter a category called hyperpyrexia, which is rare and serious. At this level, the fever itself can cause harm. Hyperpyrexia can result from severe infections, bleeding in the brain, heat stroke, certain drug reactions, or conditions like malignant hyperthermia. People under 4 and over 65 face higher risk for heat-related illness that can push temperatures this high.
Warning Signs That Accompany a Fever
The temperature number alone doesn’t tell the whole story. What matters just as much is what’s happening alongside the fever. In adults, a fever paired with any of the following warrants immediate medical attention: a stiff neck (especially pain when bending the head forward), confusion or altered speech, persistent vomiting, difficulty breathing, chest pain, a rash, unusual sensitivity to bright light, seizures, or severe headache. Several of these, particularly stiff neck, light sensitivity, and confusion together, can signal meningitis.
In children, watch for repeated vomiting, extreme irritability, severe headache, or a fever that follows being left in a hot car. Febrile seizures, which are convulsions triggered by fever, affect some young children. If a seizure lasts more than five minutes, or if it’s your child’s first febrile seizure, that’s an emergency room visit. Most febrile seizures are brief and don’t cause lasting harm, but they need evaluation.
For infants under 3 months, any fever at or above 100.4°F is treated as urgent regardless of how the baby looks or acts. Their immune systems are too immature for a wait-and-see approach.