A fever in adults starts at 100.4°F (38°C) when measured with a standard thermometer. That’s the threshold used by most hospitals, the CDC, and pediatric guidelines alike. But the number you see on your thermometer depends on where you measure, what time of day it is, and what “normal” actually looks like for your body.
What Counts as a Normal Body Temperature
The textbook number of 98.6°F (37°C) has been the standard since a German physician established it in the 1860s, but modern research shows it’s more of an average than a rule. Normal body temperature actually spans a range from about 97°F (36.1°C) to 99°F (37.2°C). Many healthy adults run slightly below 98.6°F as their personal baseline.
Your temperature also shifts throughout the day. It tends to be lowest in the early morning and rises in the late afternoon and evening. This natural fluctuation can span a full degree or more, which means a reading of 99°F at 6 p.m. might be perfectly normal for you, while that same number at 7 a.m. could signal something is off. Age, activity level, hormonal cycles, and even recent meals can all nudge the number up or down.
The Fever Threshold by Measurement Method
Not all thermometer readings are equal. A rectal thermometer sits closest to your core body temperature, while an armpit reading runs cooler. The Mayo Clinic defines fever differently depending on where you take the measurement:
- Rectal, ear, or forehead thermometer: 100.4°F (38°C) or higher
- Oral thermometer: 100°F (37.8°C) or higher
- Armpit thermometer: 99°F (37.2°C) or higher
These differences matter. If you take your temperature under your arm and get 99.2°F, that’s roughly equivalent to an oral reading of about 100°F. Armpit readings are convenient but consistently read lower than other methods, so the fever cutoff is adjusted downward to compensate. For the most reliable reading at home, an oral thermometer under the tongue (with your mouth closed for a minute beforehand) is the simplest option for adults and older children.
What These Ranges Mean in Practice
Doctors generally group fevers by severity, which helps determine how urgently you need to respond:
- Low-grade fever: 99°F to 100.4°F (37.2°C to 38°C). This gray zone often reflects a mild immune response. Many people feel fine or only slightly off.
- Standard fever: 100.4°F to 103°F (38°C to 39.4°C). This is the range most common infections produce. You’ll likely feel chills, body aches, and fatigue.
- High fever: 103°F to 106°F (39.4°C to 41.1°C). This warrants close attention and typically calls for fever-reducing measures.
- Hyperpyrexia: Above 106.7°F (41.5°C). This is a medical emergency. At this level, organs including the brain, heart, kidneys, and liver can start to fail. Permanent brain damage, swelling of the brain, and coma are all possible without rapid treatment.
Why Your Body Produces a Fever
A fever isn’t a malfunction. It’s a deliberate response controlled by a temperature-regulating center deep in your brain. When your immune system detects an infection, it releases signaling molecules that travel to this control center and essentially turn up the thermostat. Your brain then triggers a cascade of changes: blood vessels near the skin constrict (which is why you feel cold and shiver), your metabolic rate increases to generate more heat, and sweating decreases so you retain warmth. All of this works together to push your temperature above its normal set point.
The purpose is defensive. Many bacteria and viruses reproduce less efficiently at higher temperatures, and your immune cells become more active in a warmer environment. A moderate fever is your body’s way of creating a less hospitable environment for whatever is making you sick.
Fever Ranges for Babies and Young Children
The 100.4°F (38°C) threshold applies to children too, but the stakes are higher for very young infants. The American Academy of Pediatrics has specific guidelines for babies between 8 and 60 days old who develop a rectal temperature at or above 100.4°F, because in this age group even a modest fever can signal a serious infection that an older child’s immune system would handle more easily.
For infants under 3 months, a rectal thermometer gives the most accurate reading and is the preferred method. Forehead and ear thermometers can be less reliable at this age. For toddlers and older children, the same general fever categories apply as for adults, though children tend to spike higher temperatures more easily than adults do, even with minor illnesses. A child with 103°F from a common virus may be running around the room, while an adult at the same temperature feels flattened.
Factors That Raise Temperature Without Infection
Not every elevated reading means you’re sick. Heavy exercise can temporarily push your temperature above 100°F. Overdressing, spending time in hot weather, or sitting in a hot car can all raise your reading. Ovulation typically bumps body temperature up by about half a degree. Some medications, particularly certain antibiotics and seizure drugs, can cause drug fevers. And dehydration alone can nudge your temperature higher because your body has less fluid available for cooling through sweat.
If you get a single elevated reading with no symptoms, wait 15 to 20 minutes, drink some water, and check again. A true fever will persist and usually comes with other signs like chills, headache, or muscle aches.