Thermometer, phone, water and notebook on a table for tracking a sick baby
Illnesses/First Aid

How to Take a Baby’s Temperature (and Which Thermometers to Trust)

June 20, 2026

Key takeaways: For babies under 3 months, use a rectal digital thermometer. A fever is 100.4°F (38°C) or higher. Forehead and ear thermometers are less accurate in young babies; confirm a fever rectally. Avoid glass mercury thermometers.

Knowing how to take your baby’s temperature, and which reading to trust, helps you make good decisions about when to call the doctor.

What Counts as a Fever?

A fever is a temperature of 100.4°F (38.0°C) or higher.

  • Under 3 months: a rectal temperature of 100.4°F or higher needs prompt medical evaluation, day or night.
  • 3 to 6 months: call for a temperature of 102°F (38.9°C) or higher, or any fever with signs of illness.
  • Over 6 months: how your child looks and acts often matters more than the number. Call for a fever that lasts more than 2 to 3 days, is 104°F (40°C) or higher, or comes with concerning symptoms.

Which Thermometer to Use

Digital thermometer (rectal): the most accurate method for babies under 3 months and a good choice through about age 3.

Temporal artery (forehead) thermometer: convenient and reasonably accurate in older babies when used correctly. In young infants, confirm a fever with a rectal reading.

Ear (tympanic) thermometer: not reliable for babies under 6 months because their ear canals are small. Can be used in older babies.

Axillary (armpit): easy but less accurate. Useful for screening; confirm with a rectal reading if it shows a fever.

Avoid: glass mercury thermometers (they can break and expose your child to mercury), pacifier thermometers, fever strips, and phone apps, which aren’t accurate enough.

From the author

From First Breath to First Birthday is my month-by-month guide to your baby’s first year, written as a NICU and postpartum nurse. My book covers common first-year illnesses and the warning signs that mean it’s time to call the doctor, explained the way I’d explain them at the bedside.

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How to Take a Rectal Temperature

  1. Clean a digital thermometer with soap and water or rubbing alcohol, then rinse.
  2. Put a small amount of petroleum jelly on the tip.
  3. Place your baby on their back with knees bent toward the chest, or tummy-down across your lap.
  4. Gently insert the tip about half an inch to one inch. Stop if you feel resistance.
  5. Hold the thermometer and your baby still until it beeps.
  6. Remove it and read the number.
  7. Clean the thermometer. Label it “rectal only.”

How to Take an Armpit Temperature

  1. Place the tip in the center of the armpit, against bare skin.
  2. Hold your baby’s arm snugly against their side until it beeps.

How to Use a Forehead Thermometer

Follow the manufacturer’s instructions exactly. Usually, you place the probe on the center of the forehead and slide it across to the hairline. Sweat, hair, or recent bathing can affect the reading.

Tips

  • Don’t take a temperature right after a bath or if your baby has been bundled; wait 15 to 30 minutes.
  • Write down the reading, the time, and the method.
  • Don’t add or subtract degrees for different methods; just tell your doctor which method you used.

Treating Fever

Fever is a sign the body is fighting infection, not an illness in itself. Treat discomfort, not the number.

  • Under 12 weeks: don’t give fever medicine before talking to a doctor.
  • Acetaminophen can be used for babies 2 months and older (ask your doctor first for babies under 3 months), dosed by weight.
  • Ibuprofen can be used for babies 6 months and older, dosed by weight.
  • Never give aspirin to children.
  • Dress your baby lightly and offer fluids.

Call 911 if your baby has trouble breathing, blue or gray lips, a seizure, is limp, or can’t be woken. Call your baby’s doctor promptly for any fever in a baby under 3 months, a fever with a rash that doesn’t fade when pressed, signs of dehydration, or if your baby seems very ill.

The Bottom Line

Use a digital rectal thermometer for young babies, know that 100.4°F is the fever threshold, and call right away for any fever in a baby under 3 months.

Want more support for your baby’s first year? My book, From First Breath to First Birthday, is available on Amazon.

Related Articles

Sources

  1. Sullivan JE, Farrar HC; Section on Clinical Pharmacology and Therapeutics; Committee on Drugs. Fever and antipyretic use in children. Pediatrics. 2011;127(3):580-587.
  2. Pantell RH, Roberts KB, Adams WG, et al; Subcommittee on Febrile Infants. Evaluation and management of well-appearing febrile infants 8 to 60 days old. Pediatrics. 2021;148(2):e2021052228.
  3. Niven DJ, Gaudet JE, Laupland KB, Mrklas KJ, Roberts DJ, Stelfox HT. Accuracy of peripheral thermometers for estimating temperature: a systematic review and meta-analysis. Ann Intern Med. 2015;163(10):768-777.
  4. American Academy of Pediatrics. How to take a child’s temperature. HealthyChildren.org. https://www.healthychildren.org
  5. Goldman RD; Committee on Environmental Health. Mercury in the environment: implications for pediatricians. Pediatrics. 2001;108(1):197-205.

This article is for general education and is not a substitute for individualized medical advice. Always talk with your baby’s health care provider or your own provider about your specific situation. If you think your baby or you may have a medical emergency, call 911.

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