A fever in Celsius is 38°C (100.4°F) or higher. This is the standard threshold used across most medical guidelines for both adults and children, though the exact number shifts slightly depending on where on the body you take the temperature.
Normal body temperature sits around 37°C (98.6°F), but it naturally fluctuates throughout the day. It tends to be lowest in the early morning and highest in the late afternoon. A reading between 37.3°C and 38°C is often called a low-grade fever, meaning your body temperature is elevated but hasn’t crossed into true fever territory yet.
Fever Thresholds by Measurement Site
Not all thermometer placements give the same reading. Rectal and ear thermometers measure closer to your core body temperature, while armpit readings run lower. That means the Celsius number that counts as a fever depends on how you’re taking it:
- Rectal, ear, or forehead (temporal artery): 38°C or higher
- Oral (under the tongue): 37.8°C or higher
- Armpit (axillary): 37.2°C or higher
Armpit readings are the least accurate of the group. If you get a borderline armpit reading, confirming with an oral or ear thermometer gives a more reliable picture. For infants and young children, rectal thermometers remain the most accurate option.
Fever Ranges in Adults
Once you’ve confirmed a fever, the number itself tells you something about severity. A low-grade fever, between 37.3°C and 38°C, is common with mild infections and doesn’t usually need treatment beyond rest and fluids. A standard fever, from 38°C to 39.4°C, is your immune system actively fighting off an infection. Most fevers in this range resolve on their own within a few days.
A high fever starts at 39.4°C (103°F). At this point, it’s worth contacting a healthcare provider for guidance. A fever above 40°C (104°F) warrants a phone call regardless of other symptoms. And anything above 41.5°C (106.7°F) is classified as hyperpyrexia, a medical emergency that requires immediate attention. Untreated fevers above 41°C can become dangerous on their own, independent of whatever illness is causing them.
Fever Thresholds for Children
The same 38°C threshold applies to children, but age changes how urgently a fever needs attention. Infants under 3 months old with any fever of 38°C or higher need emergency care right away, even if they seem otherwise fine. Young babies can’t regulate temperature or fight infections the way older children can, so any fever at that age is treated seriously.
For older children, a fever below 40°C is typically manageable at home with fluids, rest, and age-appropriate fever reducers. A fever above 40°C, or one that persists for more than five days, is a reason to call your child’s provider.
Why Your Body Raises Its Temperature
A fever isn’t a malfunction. It’s a deliberate response controlled by a small region at the base of your brain that acts as your body’s thermostat. When your immune system detects an infection, it releases signaling molecules that raise the thermostat’s set point. Your body then works to reach that new, higher target: blood vessels near the skin constrict to trap heat, and in some cases you start shivering to generate more warmth. That’s why you can feel freezing cold even though your temperature is climbing.
This elevated temperature makes it harder for many bacteria and viruses to replicate, and it speeds up some immune responses. The process reverses once the infection is under control. Your thermostat resets to its normal range, blood vessels dilate, and you start sweating to shed the extra heat. That “fever breaking” moment, when you suddenly feel warm and sweaty, is your body cooling itself back down to baseline.
How to Get an Accurate Reading
Timing matters. Your temperature naturally peaks in the late afternoon and early evening, so a borderline reading at 4 p.m. may look normal by morning. If you’re tracking a fever, try to take your temperature at consistent times.
Drinking hot or cold liquids can throw off an oral reading for up to 15 minutes, so wait before putting a thermometer under your tongue. For armpit readings, the thermometer needs to sit snugly against dry skin with your arm pressed against your body for the full measurement time. Rushing it tends to produce artificially low numbers.
Digital thermometers are reliable for home use across all measurement sites. Glass mercury thermometers are no longer recommended due to the risk of breakage and mercury exposure. Forehead strip thermometers give a rough estimate but aren’t precise enough to confirm whether you’ve crossed the 38°C threshold.