A fever in kids is a body temperature at or above 100.4°F (38°C) when measured rectally, which is the most accurate method for young children. That single number is the standard used by the American Academy of Pediatrics and most pediatricians, though the exact reading that qualifies depends on how you take the temperature. A fever itself isn’t a disease. It’s your child’s immune system turning up the heat to fight off an infection.
Fever Thresholds by Measurement Method
The number on the thermometer only means something in context of where you measured it. A rectal, ear, or forehead (temporal artery) reading of 100.4°F or higher counts as a fever. An oral temperature of 100°F or higher qualifies. An armpit reading of 99°F or higher is considered a fever, though armpit measurements run the lowest and are the least precise.
These differences matter because thermometers don’t all measure the same thing. A rectal thermometer reads your child’s core body temperature directly. A forehead thermometer reads blood flow near the skin’s surface, and research shows it tends to run about 0.2°C (roughly 0.4°F) lower than a rectal reading. In one study, forehead thermometers missed about 20% of confirmed fevers in children between 3 and 36 months old. Armpit thermometers perform even worse, detecting only about 12% of fevers that a rectal thermometer would catch.
For babies under 3 months, a rectal thermometer is the only method pediatricians trust for an accurate reading. For older toddlers and school-age kids, oral or forehead thermometers are reasonable for everyday use, but if a reading seems borderline and your child looks unwell, a rectal check gives you the most reliable number.
Why Children Get Fevers
Most childhood fevers are caused by viral infections: colds, flu, stomach bugs, roseola. The body deliberately raises its temperature because many viruses and bacteria replicate less efficiently in warmer conditions, and the immune system’s infection-fighting cells work faster at higher temperatures. This is why a fever after a vaccination is also common and generally harmless.
Less often, bacterial infections like ear infections, strep throat, or urinary tract infections cause fevers. These sometimes need antibiotics, while viral fevers simply run their course. The height of the fever alone doesn’t reliably tell you whether the cause is viral or bacterial. A child with a 103°F fever from a common cold can look and feel worse than a child with a 101°F fever from something that actually needs treatment. How your child is acting matters more than the number.
When a Fever Needs Urgent Attention
Age is the single biggest factor in how seriously to take a fever. A temperature of 100.4°F or higher in a baby younger than 3 months old requires a call to your pediatrician right away, even if the baby seems fine. Young infants don’t have mature immune systems, and a fever can be the only visible sign of a serious infection. The AAP has specific clinical guidelines for managing fever in infants 8 to 60 days old because this age group carries the highest risk.
For older babies and children, the fever itself is less concerning than the symptoms surrounding it. Call your pediatrician if your child:
- Has a fever above 104°F at any age
- Has had a fever for more than three consecutive days
- Is unusually difficult to wake up or extremely lethargic
- Develops a rash, stiff neck, or persistent vomiting alongside the fever
- Seems to be getting worse rather than better, even if the temperature comes down
Febrile Seizures
Between 2% and 5% of children experience a febrile seizure, a convulsion triggered by a rapid rise in body temperature. These typically happen between 6 months and 5 years of age and are terrifying to watch but, in most cases, not dangerous. A child having a febrile seizure may stiffen, shake, roll their eyes back, or become unresponsive for a short period.
Most febrile seizures last under five minutes and stop on their own. If one lasts longer than five minutes, call 911. A seizure lasting longer than 15 minutes is classified as complex and requires emergency treatment. After a simple febrile seizure, your child will likely be sleepy and confused for a short time but should recover fully. Having one febrile seizure does raise the chance of having another during future fevers, but it does not mean your child has epilepsy.
Managing a Fever at Home
The goal of treating a fever isn’t necessarily to bring the number back to normal. It’s to keep your child comfortable enough to rest and drink fluids. If your child has a low-grade fever but is playing, eating, and acting like themselves, medication isn’t always necessary.
When your child is miserable, acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) are the two options. Acetaminophen can be given every 4 to 6 hours, with a maximum of 5 doses in 24 hours. It’s approved for infants 3 months and older (with your pediatrician’s guidance for the youngest babies). Ibuprofen can be given every 6 to 8 hours, up to 4 doses per day, but only for children 6 months and older.
Both medications are dosed by weight, not age. A 24-to-35-pound child, for example, gets 5 mL of children’s liquid acetaminophen (160 mg per 5 mL) or 5 mL of children’s liquid ibuprofen (100 mg per 5 mL). Using the dosing chart on the package or one from your pediatrician’s office is the safest approach. Never give aspirin to children or teenagers, as it’s linked to a rare but serious condition called Reye’s syndrome.
Watching for Dehydration
Fevers increase the rate at which your child loses fluids, and sick kids often don’t feel like drinking. Dehydration is one of the most common complications of a fever, especially when vomiting or diarrhea is also involved.
The clearest early sign in babies is fewer wet diapers. If your infant goes three or more hours without a wet diaper, that’s a red flag. Other signs include a dry mouth, no tears when crying, sunken eyes, skin that stays “tented” when you gently pinch it instead of flattening back immediately, and unusual crankiness or low energy. In toddlers and older kids, watch for dark yellow urine, dry or cracked lips, and dizziness.
Offer small, frequent sips rather than large amounts at once. Water, breast milk, formula, or an oral rehydration solution all work. Popsicles count too. If your child refuses all fluids or shows multiple signs of dehydration, that warrants a call to your pediatrician.
Returning to School or Daycare
CDC guidance recommends that children stay home until they have been fever-free for at least 24 hours without using any fever-reducing medication. This means if you gave your child ibuprofen at bedtime and they wake up cool, the clock doesn’t start until 24 hours after the medication wears off and the fever stays gone on its own. Many schools and daycares follow this exact policy, though some have stricter rules, so it’s worth checking with your child’s specific program.