For adults, a fever is generally defined as a body temperature of 100.4°F (38°C) or higher when measured orally, rectally, or with an ear thermometer. The CDC uses this same 100.4°F threshold as its standard definition. If you’re measuring under the armpit, the cutoff is lower: 99°F (37.2°C) counts as a fever.
That single number doesn’t tell the whole story, though. Your normal baseline temperature, where you measure, your age, and even the time of day all affect what “fever” actually means for you.
Why 98.6°F Isn’t Always “Normal”
The 98.6°F figure comes from a 19th-century study and has stuck around as a rough average. In reality, normal body temperature ranges from about 97°F to 99°F depending on the person. Your temperature is naturally lowest in the early morning and rises throughout the day, so a reading of 99°F at 8 p.m. is less meaningful than the same reading at 6 a.m.
This range matters because a fever isn’t just a number on a thermometer. It represents a shift above your personal baseline. If your body normally runs at 97.4°F, hitting 100°F is a bigger jump than it would be for someone who sits at 98.8°F most of the time.
How Your Thermometer Affects the Reading
Not all thermometers give the same number, and there’s no reliable formula for converting between them.
- Oral thermometers are the most commonly used for adults and provide good accuracy. A reading of 100.4°F or higher indicates a fever.
- Rectal thermometers are considered the most accurate but are rarely used in adults. The fever threshold is the same: 100.4°F.
- Ear (tympanic) thermometers are convenient but can be thrown off by earwax buildup or ear infections. The fever cutoff is 100.4°F.
- Forehead (temporal artery) thermometers are the least invasive option but tend to be less accurate, especially in direct sunlight, cold environments, or when the forehead is sweaty. The fever cutoff is also 100.4°F.
- Armpit (axillary) thermometers read lower than internal methods. A fever here starts at 99°F (37.2°C).
If you’re getting a borderline reading from a forehead scan, switching to an oral thermometer will give you a more reliable number.
What Happens in Your Body During a Fever
A fever isn’t a malfunction. It’s a deliberate response controlled by a small region in your brain that acts as your body’s thermostat. When your immune system detects an infection, it sends chemical signals that raise this thermostat’s set point. Your body then works to reach that new, higher target: blood vessels near your skin constrict to trap heat, and you may start shivering to generate warmth. That’s why you can feel freezing cold even though your temperature is climbing. Once the infection clears and the set point drops back to normal, you suddenly feel hot and start sweating to cool down.
Low-Grade vs. High Fever
A temperature between 100.4°F and about 102°F is typically considered a low-grade fever. It’s uncomfortable but usually not dangerous on its own. Most low-grade fevers from common viruses resolve within a few days without treatment beyond rest and fluids.
Once a fever climbs above 103°F (39.4°C), it warrants a call to your doctor. At 104°F (40°C) or higher, medical guidance from Harvard Health is clear: contact a healthcare provider. Fevers rarely reach dangerous levels on their own from typical infections, but a sustained high temperature can signal something that needs attention.
Fever Thresholds Are Lower for Older Adults
Adults over 65 tend to run cooler at baseline than younger adults, which means waiting for 100.4°F to confirm a fever can miss real infections. Research from the University of Maryland found that lowering the fever threshold to 99°F (37.2°C) for older adults increased the ability to detect bacterial infections from 40% to 83%, while still being accurate 89% of the time.
This is a significant gap. An older adult with a temperature of 99.5°F could have a serious infection that would go unnoticed under the standard definition. If you’re monitoring an older family member, treat any temperature at or above 99°F with the same seriousness you’d give 100.4°F in a younger adult.
Symptoms That Make a Fever More Serious
The temperature number alone doesn’t determine how urgent a fever is. What matters just as much is what comes with it. Seek immediate medical care if a fever occurs alongside any of these:
- Stiff neck, especially with pain when bending your head forward
- Severe headache or unusual sensitivity to bright light
- Confusion, altered speech, or strange behavior
- Difficulty breathing or chest pain
- Persistent vomiting
- Seizures
- Rash
- Severe pain anywhere in the body
- Pain when urinating or foul-smelling urine
Several of these combinations, particularly a stiff neck with fever and light sensitivity, can indicate conditions like meningitis that require rapid treatment. A fever of 101°F with a stiff neck is far more concerning than a fever of 103°F with nothing but body aches.
When You Can’t Take a Temperature
Sometimes you don’t have a thermometer handy. The CDC recognizes several alternatives: feeling warm to the touch, visible facial flushing, chills, or a self-reported history of feeling feverish. These aren’t as precise as a measured reading, but they’re valid signals that your body is fighting something. Keep in mind that fever-reducing medications like ibuprofen or acetaminophen can mask a fever, so a normal reading after taking one of these doesn’t necessarily mean the fever is gone.