A fever in Celsius is any body temperature at or above 38°C (100.4°F). This is the standard threshold used across medical guidelines, regardless of age. Normal body temperature averages around 37°C when measured orally, though it naturally fluctuates throughout the day, running slightly lower in the morning and higher in the late afternoon.
Fever Grades in Celsius
Not all fevers carry the same level of concern. They’re generally grouped into ranges:
- Low-grade fever: 38°C to 39°C. This is the most common range during minor infections. In adults, low-grade fevers rarely require treatment beyond rest and fluids.
- Moderate fever: 39.1°C to 40°C. You’ll likely feel noticeably unwell at this point, with chills, body aches, or fatigue.
- High-grade fever: 40°C to 41°C. At this level, it’s worth contacting a healthcare provider, especially if the fever persists or is accompanied by other symptoms.
- Hyperpyrexia: Above 41.5°C. This is a medical emergency. At sustained temperatures this high, organs can begin to malfunction and eventually fail.
For adults, fevers below 39.4°C are generally not dangerous on their own. The threshold for concern is lower in children. A child with a fever above 40°C should be evaluated promptly.
Where You Measure Matters
The number on your thermometer depends heavily on where you take the reading. Each measurement site produces a slightly different result, and knowing the offset helps you interpret your reading accurately.
Oral temperature (under the tongue) is the baseline most guidelines reference, with a normal average of 37°C. Rectal and ear (tympanic) thermometers read 0.3 to 0.6°C higher than oral. So a rectal reading of 38°C is roughly equivalent to an oral temperature of 37.4 to 37.7°C. Armpit (axillary) and forehead (temporal) thermometers run in the opposite direction, reading 0.3 to 0.6°C lower than oral. An armpit reading of 37.5°C, for example, could reflect an actual core temperature closer to 38°C.
Rectal thermometers are considered the most accurate, which is why they’re preferred for infants and young children. Forehead and armpit readings are convenient for screening but can underestimate a true fever. If you get a borderline result from a forehead or armpit thermometer, it’s worth rechecking with an oral or ear thermometer for a more reliable number.
Why 37°C Isn’t Always “Normal”
The 37°C benchmark dates back to the 1800s, and more recent data suggests the true average has shifted slightly lower over time for many populations. Individual variation is also significant. Some people run a baseline closer to 36.5°C, while others sit naturally at 37.2°C. Older adults tend to have lower baseline temperatures, which means a reading of 37.8°C could represent a meaningful fever in someone whose normal is 36.4°C, even though it falls below the official 38°C cutoff.
Time of day matters too. Body temperature can swing by about 0.5°C over 24 hours, bottoming out in the early morning and peaking in the late afternoon or evening. A temperature of 37.7°C at 6 a.m. is more notable than the same reading at 5 p.m.
What a Fever Actually Does
Fever is not an illness itself. It’s your immune system deliberately raising your body’s thermostat to create an environment less hospitable to viruses and bacteria. The elevated temperature also speeds up certain immune responses, helping your body fight infection more efficiently. This is why low-grade fevers often don’t need to be treated with medication. Letting a mild fever run its course can actually support recovery.
That said, fevers above 40°C can cause significant discomfort, dehydration, and in rare cases, confusion or delirium. Fevers that persist beyond three days, keep climbing despite treatment, or accompany symptoms like a stiff neck, difficulty breathing, or a rash warrant medical evaluation regardless of the exact number on the thermometer.