A temperature of 100.4°F (38°C) or higher, measured rectally, is considered a fever in children. That single number is the standard threshold used by pediatricians and emergency departments alike. But the exact cutoff shifts slightly depending on how 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.
Fever Thresholds by Measurement Method
Not all thermometers read the same, because not all body sites run at the same temperature. A rectal thermometer measures core body temperature directly, which is why it’s the gold standard for young children. An armpit reading, by contrast, measures skin temperature and runs lower. Here are the cutoffs:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
If you’re using an armpit thermometer and want a rough idea of core temperature, add half a degree to a full degree Fahrenheit to the reading. That’s a useful rule of thumb, but it’s not precise enough for medical decisions in very young infants.
Which Thermometer to Use at Each Age
For children 3 and under, a rectal thermometer gives the most accurate reading with a standard digital thermometer. It’s not the most pleasant option, but it matters most in the age group where fever accuracy is most critical. For babies under 3 months, rectal is strongly preferred because even small measurement errors can change how a fever is evaluated.
Ear thermometers are less invasive and work well for older kids, but earwax buildup or poor placement in the ear canal can throw off the reading. Forehead (temporal) thermometers are convenient and can be used on children 3 months and older, though sweat or recent time outdoors can affect accuracy. Armpit readings are the least reliable of all, since the thermometer isn’t measuring internal body temperature.
Why Age Matters More Than the Number
A 102°F fever in a 3-year-old is a very different situation than a 100.4°F fever in a 3-week-old. For infants 8 to 60 days old, any rectal temperature at or above 100.4°F triggers a specific medical evaluation that can include blood work, urine testing, and sometimes a spinal fluid check. That’s not because the fever itself is dangerous. It’s because newborns have immature immune systems, and a fever at that age can be the only visible sign of a serious bacterial infection.
For babies under 8 weeks old, a fever of 100.4°F or higher is treated as urgent regardless of how the baby looks or acts. Between 2 and 6 months, fever still warrants a call to a pediatrician, but the evaluation is less intensive if the baby appears well. After 6 months, the child’s behavior, hydration, and other symptoms generally matter more than the number on the thermometer.
Fever Is a Defense, Not a Disease
Fever gets a bad reputation it doesn’t entirely deserve. When your child’s temperature rises, their body is actively mounting an immune response. The heat itself triggers several protective mechanisms: the body produces more infection-fighting white blood cells, ramps up production of antibodies that will recognize the same invader next time, and creates specialized cells that essentially engulf and destroy bacteria and viruses.
Most fevers are not dangerous and are, in fact, a beneficial response to infection. Yet “fever phobia” is widespread. One study published in the Canadian Journal of Emergency Medicine found that most parents believed fever could cause seizures, dehydration, and brain damage. Surprisingly, 40% of family physicians in that same study also expressed concerns about brain damage from fever, and 34% worried about death. Neither concern is supported by evidence for typical childhood fevers. Brain damage from temperature alone requires sustained temperatures well above what a common infection produces, typically above 107°F.
Febrile seizures, which occur in about 2 to 5% of children between 6 months and 5 years, are frightening to witness but are brief, do not cause brain injury, and do not lead to epilepsy. They’re triggered by the rapid rise in temperature, not by how high the fever gets, which means aggressively treating a fever doesn’t reliably prevent them.
When a Fever Needs Immediate Attention
The number on the thermometer is only one piece of the picture. Certain symptoms alongside a fever signal a true emergency, regardless of how high the temperature reads:
- Lethargy: Your child stares into space, won’t smile, barely responds to you, is too weak to cry, or is very hard to wake up.
- Stiff neck: Your child can’t touch their chin to their chest. This can be an early sign of meningitis.
- Trouble breathing: Rapid breathing, grunting sounds, bluish lips, or skin pulling in between the ribs with each breath.
- Dehydration: No urination for 8 hours, crying without tears, a dry mouth, or a sunken soft spot on an infant’s head.
- Purple or blood-red spots: Spots on the skin that don’t fade when you press on them can indicate a serious bloodstream infection.
- Confusion: Your child is awake but says strange things, sees things that aren’t there, or doesn’t recognize you.
- Inconsolable crying: Constant crying that can’t be soothed, suggesting severe pain.
- Bulging soft spot: In babies, a tense, bulging fontanelle suggests pressure on the brain.
Managing a Fever at Home
For children older than 6 months who are otherwise acting reasonably normal, the goal of treating a fever isn’t to eliminate it. It’s to keep your child comfortable enough to rest and drink fluids. If your child is playing, drinking, and sleeping fairly well, you may not need to treat the fever at all.
When your child is uncomfortable, acetaminophen and ibuprofen are the two standard options. Acetaminophen can be given every 4 to 6 hours, up to 5 times in 24 hours. Ibuprofen can be given every 6 to 8 hours, up to 4 times in 24 hours. Ibuprofen is not recommended for babies under 6 months. Dosing for both medications is based on your child’s weight, not age, so check the packaging or ask your pharmacist if you’re unsure. Never give aspirin to children, as it’s linked to a rare but serious condition called Reye’s syndrome.
Beyond medication, hydration is the single most important thing you can do. Small, frequent sips of fluid or even popsicles help replace what the body loses through sweating and faster breathing. Dress your child in light, breathable clothing and offer a blanket if they have chills. A lukewarm bath can provide some comfort, but avoid cold water or ice baths. Cold causes shivering, which actually drives body temperature higher.
How Long Fevers Typically Last
Most childhood fevers are caused by common viral infections and resolve within 3 to 5 days. The fever may spike higher in the evening and dip in the morning, which is a normal pattern tied to the body’s circadian rhythm, not a sign that the illness is getting worse. If a fever persists beyond 5 days, or if your child develops new symptoms like a rash, ear pain, or persistent vomiting, that’s a reason to check in with a pediatrician. A fever that goes away for 24 hours and then returns can sometimes indicate a secondary bacterial infection, like an ear infection following a cold.