A fever in adults is generally defined as a body temperature at or above 100.4°F (38°C), measured orally. Some providers use 100.0°F (37.8°C) as the cutoff, but 100.4°F is the most widely accepted threshold in clinical practice.
Fever Ranges by Severity
Not all fevers carry the same weight. Harvard Health Publishing breaks adult fevers into three tiers:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate-grade: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
A low-grade fever often resolves on its own and may not need any treatment. Moderate fevers are more likely to cause noticeable discomfort like chills, body aches, and fatigue. High-grade fevers deserve close attention and usually warrant fever-reducing treatment.
Above 106.7°F (41.5°C), the situation becomes a medical emergency called hyperpyrexia. At that level, the body’s organs, including the brain, heart, kidneys, and liver, can start to fail. Hyperpyrexia can cause permanent brain damage or death if the temperature isn’t brought down quickly.
Why Your Reading Depends on Where You Measure
The number on your thermometer shifts depending on where you take the reading. Compared to an oral temperature, a rectal reading runs about 0.5 to 1°F higher, while an armpit or forehead reading runs about 0.5 to 1°F lower. Ear thermometers tend to read 0.5 to 1°F higher than oral as well.
This matters because a forehead scan of 99.5°F could reflect an actual core temperature closer to 100°F or above. Rectal thermometers are considered the most accurate, which is why they’re the standard for infants. If you’re using a forehead or armpit thermometer, keep these offsets in mind before deciding whether your reading qualifies as a true fever.
Fever Thresholds for Babies and Children
The same 100.4°F threshold applies to infants and children, but the response should be very different depending on age. For a baby 28 days old or younger, a rectal temperature of 100.4°F or higher is an emergency room situation. For babies between 1 and 3 months, a rectal reading of 100.4°F or higher warrants an immediate call to the pediatrician, and a trip to the ER if you can’t be seen right away.
For children older than 6 months, a fever alone isn’t necessarily cause for alarm. If your child is drinking fluids, sleeping reasonably well, and still playing, the fever is generally doing its job (fighting off infection) and doesn’t always need to be treated. The American Academy of Pediatrics recommends against giving fever-reducing medication to babies under 3 months without first being seen by a healthcare provider.
A temperature of 105°F (40.5°C) or higher in a child is a red flag, especially if it doesn’t drop by 1 to 2 degrees after fever-reducing medication. The same applies if a child is difficult to wake, inconsolable, or refusing to drink.
What’s Actually Happening in Your Body
A fever isn’t a malfunction. It’s a deliberate response orchestrated 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 chemical signals called pyrogens. These signals essentially turn up the thermostat’s set point, so your body starts generating more heat through shivering, increased metabolism, and blood vessel constriction. The higher temperature makes it harder for many viruses and bacteria to reproduce, giving your immune system an edge.
This is why you feel cold and shivery at the start of a fever, even though your temperature is rising. Your brain has set a new target, and your body is working to reach it. When the fever breaks, the set point drops back to normal, and you suddenly feel hot and sweaty as your body dumps the excess heat.
Febrile Seizures in Young Children
One concern parents often have is febrile seizures, which are convulsions triggered by a rapid rise in body temperature. These occur in children between 6 months and 5 years old, most commonly around 12 to 18 months. They’re most frequent with fevers of 102°F (38.9°C) or above, though they can happen with lower fevers too. The speed of the temperature spike matters more than the peak number.
Febrile seizures look frightening but are usually brief and don’t cause lasting harm. They don’t mean a child has epilepsy. Most children who experience one never have another.
Normal Body Temperature Isn’t Always 98.6°F
The classic 98.6°F (37°C) “normal” comes from a study conducted in the 1800s, and more recent data suggests the average baseline runs slightly lower, closer to 97.5 to 97.9°F for most adults. Body temperature also fluctuates throughout the day, running lowest in the early morning and peaking in the late afternoon. Women’s temperatures shift with their menstrual cycle as well, rising slightly after ovulation.
This natural variation is one reason why a reading of 99.5°F might feel feverish in someone whose baseline runs low, even though it technically falls below the 100.4°F cutoff. Knowing your own typical temperature gives you a better reference point than relying on a universal number alone.