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 or forehead thermometer. An armpit reading of 99°F (37.2°C) or higher is also considered a fever, since armpit measurements run lower than other methods. These thresholds apply to adults and children alike, though what counts as urgent differs significantly by age.
Low-Grade vs. High Fever
Not all fevers carry the same weight. Many healthcare providers consider a temperature between 99.5°F (37.5°C) and 100.3°F (37.9°C) a low-grade fever. This range sits above normal but below the standard fever threshold, and it often shows up during mild infections, after vigorous exercise, or in the days following a vaccination. Low-grade fevers rarely need treatment on their own.
Once your temperature crosses 100.4°F, you’re in standard fever territory. Most fevers from common infections like colds, flu, or urinary tract infections fall somewhere between 100.4°F and 103°F. A temperature above 103°F in an adult is considered a high fever and warrants closer attention.
At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At that level, the heat itself starts damaging organs. The brain, heart, kidneys, and liver can all be affected, and the risk of brain swelling, permanent brain damage, or coma rises sharply without rapid cooling.
Why Your Reading Depends on Where You Measure
The number on your thermometer can shift by nearly a full degree depending on the measurement site. Rectal temperatures run about 0.5 to 1°F higher than oral readings. Armpit temperatures run 0.5 to 1°F lower than oral readings. So if you take your temperature under your arm and get 99°F, that’s roughly equivalent to an oral reading of 99.5 to 100°F.
This is why fever thresholds differ by method. The 100.4°F cutoff applies to oral, rectal, ear, and forehead thermometers. For armpit readings, the threshold drops to 99°F because the measurement itself runs cooler. If you’re comparing readings over time, try to use the same method each time.
Normal Body Temperature Fluctuates
The old “98.6°F is normal” figure is an average, not a fixed number. Your body temperature shifts throughout the day in a predictable pattern tied to your internal clock. It tends to be lowest in the early morning and rises during the day, typically peaking in the late afternoon or early evening. There’s also a small dip for most people between 2 p.m. and 4 p.m. These swings mean a reading of 99°F in the evening could be perfectly normal, while that same number first thing in the morning might suggest something is brewing.
Other factors that push your baseline up include physical activity, heavy clothing, hot weather, a recent meal, and the menstrual cycle. Knowing your own typical temperature at different times of day makes it easier to spot a true fever early.
What a Fever Actually Does
A fever is not a malfunction. It’s your immune system deliberately raising your body’s thermostat. When your body detects an infection, immune cells release signaling molecules that act on the temperature-control center in your brain. That center then resets your target temperature higher, triggering the shivering, chills, and blood vessel constriction that generate and trap heat. The elevated temperature makes it harder for many bacteria and viruses to reproduce and helps certain immune cells work more efficiently.
This is why a low or moderate fever during an ordinary illness isn’t necessarily something to suppress. The discomfort is real, but the fever itself is doing useful work. Treating a fever with over-the-counter medications is about comfort, not about making the infection worse or better in most cases.
Fever Thresholds for Infants and Young Children
Age changes everything when it comes to fevers in children. The younger the child, the more seriously any fever should be taken.
- Under 3 months: Any temperature of 100.4°F (38°C) or higher requires immediate medical attention. Newborns and very young infants can’t localize infections the way older children can, so even a modest fever could signal something serious.
- 3 to 6 months: A temperature up to 100.4°F paired with signs of illness, or any reading above 100.4°F, warrants a call to your pediatrician.
- 6 to 24 months: A fever above 100.4°F that persists for more than one day needs medical evaluation.
- Under 2 years: A temperature of 102.5°F (39.2°C) or higher is an emergency-level concern according to the American Red Cross.
In young children, the fever number matters, but so do the accompanying symptoms. Trouble breathing, a seizure, neck pain, rash, decreased urination, poor feeding, or a significant drop in energy or activity level alongside a fever all point toward needing urgent care, regardless of what the thermometer says.
Febrile Seizures in Children
Some children between the ages of about 6 months and 5 years experience seizures triggered by a rapid spike in body temperature. These febrile seizures are frightening to witness but are usually brief and don’t cause lasting harm. A first-time febrile seizure, one that lasts longer than five minutes, one that repeats, or one followed by a quick further rise in temperature all warrant a 911 call.
When a Fever Is Concerning in Adults
For otherwise healthy adults, a fever under 103°F that lasts a day or two and responds to rest and fluids is typically part of the body fighting off an infection. The concern level rises with the temperature and the duration. A fever above 103°F, one that persists beyond three days, or one accompanied by severe headache, stiff neck, confusion, persistent vomiting, difficulty breathing, or a rash shifts the situation from routine to something that needs professional evaluation.
People with weakened immune systems, chronic illnesses, or those taking immunosuppressive medications operate under tighter rules. Even a modest fever can be significant when the immune system is compromised, because the body may not mount the same robust response that a healthy person would.