For adults, a fever is a body temperature at or above 100.4°F (38°C). That’s the standard threshold used across medicine. Normal body temperature averages around 98.6°F (37°C), though healthy adults can naturally range from 97°F to 99°F throughout the day.
Why 100.4°F Is the Cutoff
Your brain has a built-in thermostat that normally keeps your body temperature hovering close to 98.6°F. When your immune system detects an infection, it releases chemical signals that tell this thermostat to raise the set point. Your body then works to reach that new, higher temperature through shivering, constricting blood vessels near the skin, and increasing your metabolic rate. The result is a fever, which is your body’s way of creating a less hospitable environment for invading bacteria or viruses.
The 100.4°F line separates normal daily fluctuation from a genuine immune response. Body temperature naturally rises and falls over the course of a day. It tends to be lowest in the early morning and highest in the late afternoon, sometimes varying by a full degree. A reading of 99.5°F after exercise or in the evening doesn’t necessarily signal illness. But once you cross 100.4°F, something is actively driving your temperature up.
Fever Severity by Temperature
Not all fevers carry the same level of concern. A temperature between 100.4°F and 102°F is generally considered a low-grade fever. It’s common with routine viral infections like colds and flu, and it often resolves on its own within a few days. You may feel achy, tired, or chilled, but your body is functioning within a manageable range.
Temperatures between 102°F and 104°F are more significant and typically produce noticeable symptoms: headache, muscle pain, sweating, and fatigue. At 104°F (40°C) or above, you should contact a doctor. Above 106.7°F (41.5°C) is a condition called hyperpyrexia, which is rare and considered a medical emergency. At that level, the fever itself can cause organ damage.
Your Thermometer Type Matters
The number on your thermometer depends partly on where you’re measuring. Rectal readings are the most accurate but least practical for most adults. Oral thermometers provide similar accuracy and are the most commonly used at home. Armpit (axillary) readings tend to run about a degree lower than oral readings and are the least reliable. Ear thermometers can be thrown off by earwax or the shape of the ear canal. Forehead (temporal) thermometers are convenient but can give inaccurate results if your skin is sweaty or if you’ve been in direct sunlight or cold air.
Because each method produces slightly different numbers, there’s no reliable formula for converting between them. The best approach is to use the same method consistently so you can track changes accurately. If you’re using an armpit thermometer, keep in mind that 99°F from the armpit roughly corresponds to the 100.4°F fever threshold measured orally.
Older Adults and Lower Baselines
If you’re over 65, your normal resting temperature may be lower than the textbook 98.6°F. As people age, they lose insulating fat under the skin and their metabolism slows, both of which lower baseline body temperature. This means an older adult could have a meaningful fever at a temperature that would look normal in a younger person. A reading of 99°F or 100°F in someone who normally runs at 97.5°F represents a significant jump, even though it falls below the standard 100.4°F cutoff. Paying attention to how you feel, not just the number, becomes especially important.
When a Fever Needs Medical Attention
A fever on its own, especially a low-grade one lasting a day or two, is rarely dangerous for otherwise healthy adults. It’s doing its job. But certain combinations of fever plus other symptoms signal something more serious. Seek immediate medical help if a fever comes with any of the following:
- Confusion or loss of consciousness
- Seizure
- Stiff neck
- Trouble breathing
- Severe pain anywhere in the body
- Swelling or inflammation in any body part
- Pain during urination or foul-smelling urine
Any fever above 104°F (40°C) warrants a call to your doctor, even without additional symptoms. A fever that persists for more than three days or keeps returning after it seemed to break also deserves a professional evaluation, since lingering fevers can point to infections that need targeted treatment.