A fever starts at 100.4°F (38°C) for both adults and children, measured orally or rectally. That single number is the most widely accepted threshold in medicine. But fever exists on a spectrum, and the number on your thermometer means different things depending on where you measure, how old the person is, and what other symptoms are present.
What Counts as Normal Body Temperature
The textbook number of 98.6°F (37°C) is still used as a baseline, but normal body temperature actually spans a range from about 97°F to 99°F (36.1°C to 37.2°C). Your temperature fluctuates throughout the day, running lower in the morning and peaking in the late afternoon. Exercise, hormonal cycles, heavy clothing, and even a hot meal can nudge it higher without meaning anything is wrong.
This natural variation is why a reading of 99°F doesn’t automatically signal a problem. It might just be your normal at 4 p.m. on a warm day.
Fever Ranges: Low, Moderate, and Dangerous
Harvard Health defines a low-grade fever as 99.1°F to 100.4°F (37.3°C to 38°C). This is the gray zone. You might feel slightly off, a little warm, or not notice it at all. Many doctors don’t consider this a true fever, but it can be an early sign that your immune system is responding to something.
Once you cross 100.4°F, you’re in clear fever territory. Here’s how the severity generally breaks down for adults:
- Low-grade fever: 99.1°F to 100.4°F (37.3°C to 38°C)
- Moderate fever: 100.4°F to 103°F (38°C to 39.4°C)
- High fever: 103°F (39.4°C) and above, which warrants a call to your doctor
- Hyperpyrexia: Above 106.7°F (41.5°C), a medical emergency that can damage the brain, heart, lungs, liver, and kidneys
Hyperpyrexia is rare but life-threatening. At that extreme, the body’s own heat starts destroying organs. It can cause brain swelling, permanent brain damage, and coma if the temperature isn’t brought down quickly.
Why Where You Measure Matters
The same person, measured at the same moment, will get different readings depending on the thermometer’s location. Rectal and ear (tympanic) readings run about 0.5°F to 1°F higher than oral readings. Armpit and forehead readings run about 0.5°F to 1°F lower than oral.
This means a forehead thermometer showing 99.5°F could represent the same internal temperature as an oral reading of 100.4°F. If you’re using an armpit or forehead thermometer and getting a borderline number, the actual core temperature is likely a bit higher than what you see. Rectal thermometers are the most accurate, which is why they’re the standard for infants.
Different Thresholds for Babies and Children
The 100.4°F threshold applies to children too, but the urgency is very different depending on age. For a baby under 3 months old, a rectal temperature of 100.4°F or higher needs immediate medical attention. Infants that young can’t fight infections the way older children can, and a fever may be the only visible sign of something serious.
For babies between 3 and 6 months, a rectal temperature above 102°F (38.9°C) is the flag, or any fever combined with unusual irritability or sluggishness. Between 7 and 24 months, the same 102°F threshold applies if the fever lasts more than a day with no other symptoms. For older children, a fever lasting longer than three days, or one accompanied by confusion, repeated vomiting, severe headache, or a seizure, calls for medical evaluation.
What’s Actually Happening in Your Body
Fever isn’t a malfunction. It’s your immune system deliberately turning up the thermostat. When your body detects an infection, immune cells release signaling molecules that trigger the production of a chemical messenger called prostaglandin E2. This messenger travels to the hypothalamus, the brain’s temperature control center, and raises its set point.
Once the set point goes up, your body treats its current normal temperature as “too cold.” That’s why you feel chilled and start shivering at the beginning of a fever, even though your temperature is actually rising. The shivering generates heat through rapid muscle contractions. Blood vessels near your skin also constrict, reducing heat loss and making you look pale. All of this works together to push your core temperature up to the new, higher target.
The higher temperature actually helps fight infection. Many bacteria and viruses reproduce less efficiently at elevated temperatures, and certain immune responses work better when the body is warmer. This is also why fever-reducing medications like ibuprofen and acetaminophen work the way they do: they block the production of that same chemical messenger, which lowers the hypothalamus set point back toward normal.
When a Fever Becomes Concerning in Adults
Most fevers in adults are caused by common viral infections and resolve within a few days without complications. A moderate fever of 101°F or 102°F, while uncomfortable, is generally your body doing its job. The Mayo Clinic recommends contacting a healthcare provider if your temperature reaches 103°F (39.4°C) or higher.
The temperature number alone isn’t the whole picture, though. Certain accompanying symptoms signal something more serious regardless of how high the fever is:
- Stiff neck with pain when bending your head forward (a hallmark of meningitis)
- Severe headache with unusual sensitivity to bright light
- Mental confusion, altered speech, or strange behavior
- Rash appearing alongside the fever
- Difficulty breathing or chest pain
- Persistent vomiting
- Seizures or convulsions
Any of these combinations warrants immediate medical attention, even if the thermometer shows only 101°F. A fever following exposure to extreme heat, such as being in a hot car, is also an emergency because it may indicate heatstroke rather than an immune response, and the body’s cooling mechanisms may have failed entirely.