A temperature of 100.4°F (38°C) or higher, measured rectally, is considered a fever in children. That’s the standard threshold used by pediatricians, but the exact number depends on where you take the temperature. An oral reading of 100°F (37.8°C) or an armpit reading of 99°F (37.2°C) also qualifies as a fever. These differences matter because each measurement site runs slightly cooler or warmer than the others.
Fever Thresholds by Measurement Site
The number on the thermometer only means something in context. A reading of 99.5°F could be a fever if you took it under the arm, but not if you took it rectally. Here’s how the thresholds break down:
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral (under the tongue): 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
Rectal temperatures are the most accurate, especially for babies and toddlers. Armpit readings are the least reliable of the three main methods. If an armpit or ear reading seems off, confirming with a rectal temperature gives you the most trustworthy number.
Which Thermometer to Use at Each Age
For babies from birth to 3 months, a standard digital thermometer used rectally is the gold standard. Forehead (temporal artery) thermometers may also give accurate readings in newborns, but rectal remains the go-to when precision matters most.
From 3 months to 4 years, you can use a rectal, armpit, or forehead thermometer. Ear thermometers become an option once a child reaches 6 months old. Before that age, earwax and small ear canals can throw off the reading.
By age 4, most children can hold a thermometer under their tongue long enough to get an oral reading. At this point, oral, ear, forehead, and armpit thermometers are all reasonable options.
Why Fever in Babies Under 3 Months Is Different
A rectal temperature of 100.4°F or higher in a baby younger than 3 months requires immediate medical attention. At this age, a young immune system can’t reliably fight off serious infections, and a fever may be the only visible sign that something significant is happening. Even if the baby seems otherwise fine, this is a situation that needs prompt evaluation.
One important note: do not give any fever-reducing medication to a baby under 8 weeks old before they’ve been seen by a provider. The fever itself is a useful diagnostic signal, and treating it before evaluation can mask what’s going on.
Can a High Fever Cause Brain Damage?
This is one of the most common fears parents have, and the answer is reassuring. Fevers caused by infections do not cause brain damage. Only body temperatures above 108°F can damage the brain, and infections essentially never push a child’s temperature that high. Temperatures in that range are associated with environmental causes like heatstroke, not with illness.
A fever of 104°F is uncomfortable and worth treating, but the number itself isn’t a sign of brain injury. What matters more than the number is how your child looks and acts.
Febrile Seizures
Some children have seizures triggered by fever, most commonly between 6 months and 5 years old, with the highest risk between ages 1 and 3. These episodes are frightening to watch but are generally harmless, even when they last several minutes. They don’t cause lasting neurological damage in the vast majority of cases.
Children are more likely to have febrile seizures if they have a family history of them, if their first seizure happened before 18 months, or if the fever was actually relatively low when the seizure occurred. That last point surprises many parents: it’s often the rapid rise in temperature, not the peak number, that triggers a seizure. Prolonged febrile seizures lasting more than 30 minutes carry a small increased risk of epilepsy later, but this is rare.
When a Fever Needs Medical Attention
Beyond the under-3-months rule, certain signs mean a child needs to be seen right away, regardless of the temperature reading:
- Stiff neck that limits normal movement
- Difficulty waking up or acting confused and “out of it” when awake
- Severe breathing difficulty, struggling for each breath
- Seizure during the fever
- Purple or blood-colored spots on the skin
- Bulging soft spot on the head in babies under 1 year
- Signs of dehydration: no urine for more than 8 hours, very dry mouth, no tears when crying
- Nonstop crying or crying when touched or moved
A fever above 104°F (40°C) also warrants a call to your pediatrician, as does shaking chills lasting more than 30 minutes. If a fever persists for 5 days or more without improvement, that’s another reason to check in, since most viral fevers resolve before then.
Managing Fever at Home
The two common over-the-counter options for bringing down a child’s fever are acetaminophen and ibuprofen. They work differently but both reduce fever effectively.
Acetaminophen can be given to children 8 weeks and older. Doses are based on weight, not age, and can be repeated every 4 to 6 hours as needed, up to 5 times in 24 hours. Ibuprofen is not recommended for babies under 6 months old. It can be given every 6 to 8 hours, up to 4 times per day. Both medications include weight-based dosing charts on the packaging.
Keep in mind that the goal of treating a fever isn’t necessarily to bring the temperature back to normal. Fever is part of the body’s immune response. The real purpose of medication is comfort: helping your child rest, drink fluids, and feel less miserable while their body does its work. If your child has a low-grade fever but is playing and drinking normally, medication may not be necessary at all.