A fever is defined as a body temperature at or above 38°C (100.4°F). This is the threshold used across most medical guidelines for both adults and children, though the exact number on your thermometer depends on where you measure it.
The 38°C Threshold
The standard cutoff of 38°C applies to rectal, ear, and forehead (temporal artery) readings. If you’re taking a temperature by mouth, the fever threshold is slightly lower at 37.8°C, because oral readings tend to run a bit cooler than core body temperature. Armpit readings are lower still, with 37.2°C considered a fever when measured there.
These differences matter. A reading of 37.5°C under the arm means something different than 37.5°C rectally. For the most accurate result, rectal thermometers are the gold standard, especially for infants and young children. Whichever method you use, stick with it consistently so you can track changes reliably.
Normal Body Temperature Is Lower Than You Think
The classic “normal” of 37°C (98.6°F) dates back to the 1800s and is now outdated. A Stanford Medicine analysis of more than 618,000 temperature readings found that the average adult body temperature today is closer to 36.6°C (97.9°F), with a normal range spanning roughly 36.3°C to 36.8°C. The average has dropped by about 0.03°C per decade since the 19th century, likely because modern populations have less chronic inflammation and infection than people did 150 years ago.
Your personal baseline also shifts throughout the day. Body temperature is lowest in the early morning and peaks in the late afternoon. Age, sex, height, and weight all influence it too. So a reading of 37.4°C in the evening might be perfectly normal for you, while the same number at 6 a.m. could signal something is off.
Why Your Body Raises Its Temperature
Fever isn’t a malfunction. It’s a deliberate response controlled by a small region in the brain that acts as your body’s thermostat. When your immune system detects an infection, it releases signaling molecules that effectively turn the thermostat dial up. Your body then works to reach this new, higher set point: blood vessels near the skin constrict to trap heat, and you may start shivering to generate more warmth. That’s why you can feel freezing cold even though your temperature is climbing.
Once the infection is under control, the set point drops back to normal. Blood vessels dilate, you start sweating, and the fever breaks.
Fever Severity Levels in Celsius
Not all fevers carry the same urgency. Here’s a general framework for adults:
- Low-grade fever: 37.8°C to 38.5°C. Common with mild infections like colds. Often resolves on its own with rest and fluids.
- Moderate fever: 38.5°C to 39.5°C. Typical of flu, bacterial infections, or more active viral illnesses. Usually manageable at home but worth monitoring.
- High fever: 39.5°C to 40°C. Your body is mounting a strong immune response. Pay attention to how you feel overall, not just the number.
- Above 40°C: This warrants a call to your doctor. At this level the risk of complications rises.
- Above 41.5°C (hyperpyrexia): A medical emergency. At this extreme temperature, organs including the brain, heart, kidneys, and liver can begin to fail. Permanent brain damage, coma, and death are possible without rapid treatment.
Children, especially infants under three months, are a different situation. Any rectal temperature of 38°C or higher in a newborn needs prompt medical evaluation, regardless of how the baby appears.
How Measurement Method Affects the Number
The same child or adult can show different readings depending on where the thermometer goes. Here’s what each method considers a fever:
- Rectal, ear, or forehead: 38°C or higher
- Oral (under the tongue): 37.8°C or higher
- Armpit (axillary): 37.2°C or higher
Rectal temperatures read closest to true core body temperature. Armpit readings are the least accurate and can underestimate a fever by 0.5°C to 1°C. If you’re using an armpit thermometer and the reading seems borderline, consider switching to an oral or ear thermometer for confirmation.
Symptoms That Matter More Than the Number
A fever of 39°C in someone who is alert, drinking fluids, and resting comfortably is very different from a fever of 38.5°C in someone who is confused or struggling to breathe. The temperature itself is just one data point. Symptoms that signal a more serious situation at any fever level include seizures, loss of consciousness, confusion, a stiff neck, difficulty breathing, and severe pain. These combinations call for immediate medical attention regardless of what the thermometer reads.