A fever is confirmed when your body temperature reaches 100.4°F (38°C) or higher, measured orally or rectally. But the number on a thermometer is only part of the picture. Your body sends several reliable signals that your temperature is climbing, and understanding both the physical sensations and the best way to measure can help you figure out what’s going on.
What a Fever Actually Feels Like
The most common early sign is feeling chilled even when the room isn’t cold. This happens because your brain has temporarily raised its internal temperature target. Your body now perceives its current temperature as too low, so it triggers shivering and constricts blood vessels near the skin to generate and conserve heat. That’s why you can feel freezing cold while your forehead is hot to the touch.
As the fever builds, you may notice body aches, a headache, and deep fatigue that feels out of proportion to your activity level. Your skin may look flushed or feel warm and dry. Once your temperature peaks and starts to come down, the process reverses: blood vessels open up, you start sweating, and you may feel suddenly hot. Other signs that often accompany a fever include loss of appetite, general weakness, and irritability, especially in young children who can’t describe how they feel.
Why 98.6°F Isn’t Really “Normal”
The 98.6°F standard comes from data published in 1868. Research from Stanford Medicine, based on more than 618,000 temperature readings from adult patients, found that today’s average body temperature is closer to 97.9°F. Normal readings ranged from 97.3°F to 98.2°F across the group. That old number has dropped roughly 0.05°F per decade since the 19th century, likely because modern populations have less chronic infection and inflammation than people did 150 years ago.
Your personal baseline also shifts throughout the day. Body temperature is lowest in the early morning and peaks around 4 p.m. Women tend to run slightly warmer than men. Temperature increases with body weight and decreases slightly with height and age. So a reading of 99°F at 7 a.m. is more meaningful than the same reading at 4 p.m., when your body is naturally warmer. This is why a single number can’t define “normal” for everyone.
The Best Way to Take Your Temperature
A digital thermometer placed under the tongue is the most practical option for adults and older children. It provides accuracy close to a rectal reading without the discomfort. Wait at least 15 minutes after eating or drinking before taking an oral reading, since hot or cold foods will skew the result. Keep your mouth closed around the thermometer until it beeps.
Rectal thermometers give the most accurate core temperature and are the standard for infants when a precise reading matters. Ear (tympanic) thermometers are fast and convenient but can give inconsistent results if the probe isn’t positioned correctly in the ear canal. Forehead (temporal artery) thermometers are the least invasive option, though testing has shown that some models fall outside their stated accuracy range. A 2021 evaluation by English researchers found that five out of nine common infrared forehead thermometers failed to meet the medical accuracy standard they claimed.
Whichever method you use, stick with the same one each time. Temperatures taken from different body sites don’t translate neatly to one another, and there’s no reliable formula for converting between them. Consistency gives you a better sense of whether your temperature is actually rising.
Can You Detect a Fever by Touch?
Placing your hand on someone’s forehead can tell you their skin feels warmer than usual, but it’s not a reliable way to confirm a fever or gauge how high it is. Your own hand temperature, the room temperature, and whether the person has been active all affect what you feel. Touch is reasonable as a first check, especially with a sleeping child, but a thermometer is the only way to get an actual number. This matters most for infants, where specific temperature thresholds determine whether a doctor visit is needed.
Why Your Body Raises Its Temperature
Fever is a deliberate response, not a malfunction. When your immune system detects an infection, it releases signaling molecules that travel to the brain’s thermoregulatory center in the hypothalamus. These signals cause the hypothalamus to raise its temperature set point, essentially turning up your internal thermostat. Your body then works to reach that new target by increasing your metabolic rate, reducing blood flow to your skin, and triggering shivering. The higher temperature makes your body a less hospitable environment for many bacteria and viruses, and it speeds up certain immune responses.
This reset is also why fevers break the way they do. Once your immune system gains the upper hand, the set point drops back to normal. Your body is now “too hot” relative to its target, so it rapidly cools itself through sweating and flushing.
Fever Thresholds That Matter
For adults, a temperature of 100.4°F (38°C) or above is a fever. A reading of 103°F (39.4°C) or higher warrants a call to your doctor. Seek immediate care if a high fever comes with a stiff neck, rash, sensitivity to bright light, confusion, difficulty breathing, chest pain, persistent vomiting, or seizures.
For children, a fever lasting longer than three days needs medical attention regardless of the temperature. A child who is listless, making poor eye contact, vomiting repeatedly, or complaining of a severe headache should be evaluated sooner. Febrile seizures, while frightening, are usually brief. Call 911 if a seizure lasts more than five minutes or the child doesn’t recover quickly afterward.
Infants have the strictest thresholds. Any baby under 3 months old with a rectal temperature of 100.4°F or higher needs prompt medical evaluation, even if they seem otherwise fine. Between 3 and 6 months, the concern level rises at 102°F (38.9°C), or at lower temperatures if the baby seems unusually irritable or sluggish. For babies 7 to 24 months, a temperature above 102°F that persists beyond one day without other symptoms also warrants a call.
Armpit Readings Run Lower
If you’re using an armpit (axillary) measurement, the fever threshold is lower: 99°F (37.2°C) or above. Armpit readings consistently run about a degree below oral readings because the thermometer isn’t measuring core body temperature. This method is less precise, but it can be useful for a quick, noninvasive check, particularly in young children who won’t hold a thermometer under their tongue. Just keep the arm pressed snugly against the body for the full measurement time.