A fever in children is generally defined as a temperature of 100.4°F (38°C) or higher when measured rectally. For an oral thermometer reading, 100°F (37.8°C) or higher is considered a fever. These numbers apply broadly, but the age of your child changes how urgently that fever needs attention.
Fever Thresholds by Measurement Method
The number that counts as a fever depends on where you take the temperature. A rectal thermometer gives the most accurate core body temperature and is the recommended method for children under 3 years old. With a rectal reading, 100.4°F (38°C) is the standard fever threshold used by both the American Academy of Pediatrics and most emergency departments.
An oral thermometer (under the tongue) reads slightly lower, so the fever cutoff drops to 100°F (37.8°C). Armpit (axillary) and ear (tympanic) thermometers are less reliable overall. If you’re using an armpit thermometer and get a borderline reading, it’s worth confirming with a rectal or oral measurement before deciding your child doesn’t have a fever.
Why Age Changes Everything
A fever of 100.4°F means something very different in a 2-month-old than in a 4-year-old. The younger the child, the more seriously any fever should be taken, because young infants have immature immune systems and fewer ways to show you they’re seriously ill.
Here’s how the age brackets break down:
- Under 3 months: Any rectal temperature of 100.4°F or higher warrants an immediate call to your pediatrician, even if your baby seems fine otherwise. The AAP has detailed clinical guidelines specifically for infants 8 to 60 days old with fevers at this threshold, because infections in this age group can escalate quickly with few visible warning signs.
- 3 to 6 months: A fever becomes more concerning when the rectal temperature rises above 102°F (38.9°C), or when your baby seems unusually irritable or sluggish.
- 7 to 24 months: The same 102°F threshold applies. A fever that lasts longer than a day at this age is worth a call to your pediatrician.
- 2 years and older: Fever alone is less of a red flag. The bigger concern is how your child is acting and how long the fever lasts.
How Long Is Too Long
For children under 2, a fever that persists beyond 24 hours is a reason to contact your pediatrician, even if no other symptoms are present. For children 2 and older, the window extends to 72 hours (3 days). These timelines assume the fever stays in a moderate range and your child is still drinking fluids, responsive, and generally acting like themselves between temperature spikes.
Keep in mind that many childhood fevers, particularly those caused by common viruses, will peak in the evening and dip by morning. This cycling pattern is normal and doesn’t reset the clock. If Monday evening was the first fever, Wednesday evening marks the 48-hour point regardless of what happened Tuesday morning.
Signs That Need Immediate Attention
The temperature number matters less than how your child looks and behaves. A child with a 101°F fever who is playing and drinking normally is in a very different situation than a child with the same temperature who can’t stay awake. Certain symptoms alongside a fever signal a potential emergency regardless of the reading on the thermometer:
- Extreme drowsiness or difficulty waking up
- Stiff neck
- A new rash, especially one that doesn’t fade when you press on it
- Trouble breathing or rapid breathing
- Inconsolable crying
- Excessive drooling or difficulty swallowing
- Abdominal pain or tenderness
- Blue or gray color on the lips, tongue, or nails
- Confused speech or trouble walking
Any of these paired with a fever is a reason to head to the emergency department or call 911.
Febrile Seizures
Up to 5% of young children will experience a febrile seizure at some point, most commonly between 14 and 18 months of age. These seizures can happen in children from 6 months to 5 years old and are triggered by the rapid rise in temperature rather than how high the fever gets. A child can seize at 101°F if the temperature climbed quickly.
Febrile seizures are frightening to witness but are typically brief and do not cause lasting harm. They don’t mean your child has epilepsy. If your child has one, lay them on their side on a flat surface, don’t put anything in their mouth, and time the seizure. If it lasts longer than 5 minutes, call 911.
Choosing the Right Thermometer
For babies and toddlers under 3, a digital rectal thermometer is the most accurate option. Lubricate the tip, insert it about half an inch, and hold it in place until it beeps. It’s not the most pleasant experience for anyone involved, but it gives the reading your pediatrician will trust most.
Once your child is around 4 or older and can hold a thermometer under their tongue without biting or talking, an oral reading works well. Temporal artery (forehead) thermometers are convenient for a quick screening, especially for a sleeping child, but they can read lower than the true core temperature. Ear thermometers are fast but can give inaccurate results if the ear canal is small or has wax buildup.
Whichever method you use, know what the fever threshold is for that method. A rectal 100.4°F and an oral 100°F represent roughly the same internal temperature. If you tell your pediatrician your child has a fever, they’ll want to know both the number and where you measured it.
What a Fever Actually Does
Fever is not an illness. It’s your child’s immune system raising the body’s thermostat to create a less hospitable environment for viruses and bacteria. This is why mild fevers in older children who are otherwise comfortable don’t always need to be treated with medication. The goal of fever reducers is to help your child feel better, not to eliminate the fever entirely.
Normal body temperature in children fluctuates throughout the day, running lower in the morning and higher in the late afternoon. A reading of 99°F to 99.9°F in the evening may simply reflect this natural variation rather than an emerging illness. That’s why the formal threshold is set at 100.4°F rectally, giving a clear margin above the normal daily range.