Taking a child’s temperature: a digital thermometer that fits their age, and 100.4°F as the fever line

Why now

• Flu season. CDC says flu season in the U.S. usually runs through fall and winter, and flu activity most often peaks between December and February. [S5]
• Not just flu. Colds, COVID-19 and RSV spread during flu season too. CDC calls RSV the most common cause of severe respiratory illness in young children. [S5]
• Before school or sports. AAP says it’s a good idea to check a child you think may be sick for fever before school, child care or sports. [S1]
• A hand isn’t a thermometer. Feeling the forehead is quick but not very exact, and AAP says it’s less accurate still if a baby has chills. [S1] [S2]

What counts as a fever

AAP says a fever is usually a temperature of 100.4°F (38°C) or higher. “Normal” varies with a child’s age, how active they’ve been and the time of day. FDA puts a typical normal temperature at 98.6°F (37.0°C) and says studies show normal can range from 97°F (36.1°C) to 99°F (37.2°C). [S1] [S4]

For babies, AAP treats a rectal reading of 100.4°F or higher as a fever, and says to use that same 100.4°F line for readings by mouth, ear or forehead. Armpit readings are the exception: their fever cutoff may be lower. [S2]

A fever itself is a sign that the body is fighting an illness. An AAP pediatrician writes that it’s the immune system doing its job, and that a fever doesn’t hurt a child. How your child looks and acts matters as much as the number. [S1] [S3]

Thermometer types

AAP says to use a digital thermometer. Don’t use a mercury thermometer: the thin glass can break and release toxic mercury fumes, so if you still have one at home, remove it. [S1]

AAP ranks the methods this way: rectal readings are the most accurate, forehead readings are next, readings by mouth and ear are accurate if done properly, and armpit readings are the least accurate, though they can be used to screen a child of any age. [S1]

• Digital multi-use. Reads at the tip, in the bottom, the mouth or the armpit. AAP doesn’t recommend use by mouth until age 4, and says to label the thermometer “oral” or “rectal” so one thermometer isn’t used in both places. [S1]
• Forehead (temporal artery). Reads infrared heat from the blood vessel just under the skin of the forehead, and AAP says it can be used at any age. Contact and non-contact models read in different spots, so check the instructions for how close to hold it. Non-contact models may help reduce the risk of passing germs. Direct sunlight and coming in from the cold can throw off the reading. [S1]
• Ear (tympanic). Reads infrared heat from the eardrum. AAP says it works from 6 months up, not for younger babies, whose ear canals are too narrow. It has to sit correctly in the ear canal, and cold from being outside can affect the reading. [S1] [S2]
• Quick screens. Armpit readings, pacifier thermometers (for children 1 year and younger) and fever strips on the forehead are less accurate. AAP says parents can use them for a quick check, but they don’t work as well as a rectal, forehead, ear or mouth reading with a digital thermometer. It calls fever strips not reliable for babies. [S1] [S2]

By age

• 3 months or younger. AAP says a rectal reading gives the best reading, especially for babies under 3 months, and it’s the reading AAP’s newborn fever rule is written for. A forehead thermometer can be used at any age. No ear and no mouth readings yet. [S1] [S2]
• Over 3 months to 6 months. Rectal or forehead. Still no ear thermometer: AAP says to avoid it under 6 months. [S1] [S2]
• Over 6 months to 3 years. Rectal, forehead or ear. Readings by mouth wait until age 4. [S1]
• 4 years and up. By mouth is now an option, along with forehead and ear. [S1]
• Any age. An armpit reading can screen a child of any age, but it’s the least accurate. [S1]

How to take it

AAP’s first rule for every method: follow the instructions that came with the thermometer, and clean it the way the package says before you use it. [S1]

• Rectal. Put a little lubricant, such as petroleum jelly, on the tip and on your child’s bottom. Lay your child belly down across your lap or on a firm surface, or on their back with the legs pulled up to the chest. Turn the thermometer on and slide it in gently: 1/2 inch for babies under 6 months, up to 1 inch otherwise. There should be no resistance; if there is, stop. Hold your child still until it beeps. Disinfect it before and after, with soap and water or an alcohol swab. [S1]
• By mouth (age 4 and up). Wait 30 minutes after a hot or cold drink. Put the tip under one side of the tongue, toward the back. Your child holds it with their lips and fingers, not their teeth, and keeps their lips sealed until it beeps. [S1]
• Forehead. Follow the package for how and where to slide or aim the sensor across the forehead, then read the screen. [S1]
• Ear (6 months and up). Pull the ear back, and back and up if your child is over 1 year old. Aim the tip between the opposite eye and ear. After time outdoors on a cold day, wait 15 minutes indoors first. [S1]

For a non-contact forehead thermometer, FDA lists the conditions its instructions typically ask for. [S4]

• The room. Draft-free, out of direct sun and away from radiant heat. The room should usually be between 60.8°F and 104°F (16°C to 40°C), with humidity below 85%. [S4]
• The thermometer. Let it sit in the room for 10 to 30 minutes first so it adjusts. Don’t touch the sensor, keep it clean and dry, and don’t dunk it in water; FDA says most should never be immersed. [S4]
• The forehead. Clean, dry and uncovered. Head covers such as headbands or bandanas, heavy clothing, or face cleansing products such as cosmetic wipes can push the reading up or down. [S4]
• The reading. Hold it straight on to the forehead, at the distance in the manufacturer’s instructions, and ask your child to stay still. [S4]

Check a reading

Pick your child’s age and how you took the temperature, then type the number. The check puts the AAP lines from this page in one place. It doesn’t replace a call to your pediatrician. [S1] [S2] [S3]

The check only compares numbers with AAP’s published lines. It can’t see your child, so how they look and act still comes first, along with the warning signs below.

When to call the pediatrician

AAP’s pages list these reasons to call your pediatrician right away or get medical care. [S1] [S2] [S3]

• A young baby with any fever. 3 months or younger with a rectal reading of 100.4°F or higher. AAP says to call immediately, even if the baby seems fine and shows no other signs of illness. [S2] [S3]
• Higher readings. AAP says you may need to call at 101°F (38.3°C) or more for a baby between 3 and 6 months, and at 103°F (39.4°C) or more for a child older than 6 months. Call right away if a fever rises above 104°F (40°C) repeatedly, at any age. [S1] [S2]
• A fever that hangs on. Under 2 years old, a fever lasting more than 24 hours. Age 2 and up, more than 3 days (72 hours). [S3]
• How your child seems. Looks very ill, is unusually drowsy or very fussy, is very sleepy or weak, pees less than usual, shows signs of dehydration or has trouble breathing. [S1] [S3]
• Other symptoms. A stiff neck, a severe headache, a severe sore throat, severe ear pain, an unexplained rash, or repeated vomiting or diarrhea. [S1]
• Other risks. Immune system problems such as sickle cell disease or cancer, taking steroids, a seizure, or time somewhere very hot, such as an overheated car. [S1]

Comfort at home

An AAP pediatrician writes that most viral illnesses that cause fever in children clear up within a week, and that a child with a fever who is still playful will likely do okay at home. [S3]

• Fluids. Plenty of fluids and rest. Small, frequent sips or ice pops help. [S3]
• Clothes. Comfortable, breathable clothes, and a blanket if your child has chills. For a baby in a crib, our cold-night baby sleep guide covers wearable blankets and a bare crib. [S3]
• Baths. A lukewarm bath may feel nice. Skip ice baths and cold-water baths: they cause shivering, which makes more heat. [S3]
• Fever medicine. It’s for comfort only, and it won’t cure the illness or make it shorter. Call your pediatrician before giving any if your baby is under 2 months old. Ibuprofen generally isn’t recommended under 6 months. Read the label: doses go by weight and age. Store brands have the same active ingredients as name brands. [S3]
• Combination medicines. Children’s cough-and-cold products that also contain a fever reducer aren’t typically recommended, because the fever-reducing part can end up underdosed. [S3]

This guide doesn’t give doses. Use the label and your pediatrician’s advice.

Before the next sick day

Quick answers