The forehead scanner says 99.7°F, the armpit reads 98.8°F, and the ear thermometer shows 100.4°F. Differences between methods are common, so it helps to know which method to trust and which number to use when you call the doctor.

Why the Numbers Differ

Temperature is most reliable when measured where it reflects the body’s core. The AAP’s HealthyChildren guidance explains that rectal readings are the most accurate, forehead (temporal artery) readings come next, and armpit readings are the least accurate. The armpit sits close to the skin surface and is affected by room temperature, clothing, and sweat. Even so, the armpit method can be used as a quick screen at any age.

AAP News’s “Thermometer use 101” adds that for children younger than 3 years, a rectal reading gives the best result. A difference of a few tenths of a degree between devices usually reflects the method and the device rather than a real change, so repeating the same method over time often tells you more than any single number.

How It Differs by Age

  • Under 3 years: a digital rectal thermometer is generally considered the most accurate.
  • About 3 months to 4 years: rectal or forehead readings are described as reasonably accurate. Ear thermometers are used from about 6 months and are not meant for young infants.
  • Age 4 and older: HealthyChildren advises oral readings only once a child is 4 or older and can cooperate by holding the thermometer under the tongue with the mouth closed.
  • Forehead (temporal artery) thermometers can be used across ages, but each model works a bit differently, so the manual is worth reading.

Mercury thermometers are not recommended because they are hazardous if broken; a basic digital thermometer is enough.

Care at Home

For a rectal reading, put a small amount of lubricant on the tip, hold the baby securely on the stomach or with the legs raised, and insert only the tip gently. Keep your child still until the thermometer signals. For the armpit, dry the skin, place the tip deep in the armpit, and hold the arm snug against the body. Readings can be off right after a bath, when bundled in thick clothes, or after crying or running, so waiting 10-15 minutes and measuring again gives a steadier number.

Many sources use 100.4°F (38.0°C) or higher by rectal, forehead, or ear as the fever line. When you call the doctor, mention which thermometer and body site you used.

Common Misunderstandings

First, that only an exact number tells you how sick a child is. Behavior, drinking, and wet diapers matter as well. Second, that a hot-feeling forehead means a confirmed fever. Touch is unreliable, so use a thermometer. Third, that a higher number always means a worse illness. Age and how the child looks and acts matter more than the height of the fever. Fourth, that a normal armpit reading rules out fever. Armpit readings tend to run lower, so if your child looks unwell, you can recheck with another method.

Signs That Need Emergency Care Right Away

HealthyChildren advises calling the pediatrician right away if a baby younger than 3 months (12 weeks) has a rectal temperature of 100.4°F or higher. At that age, a fever is worth checking at the doctor’s office or emergency room at any hour. At any age, go to the emergency room right away for trouble breathing, being hard to wake or very limp, a seizure, a stiff neck, or a spreading rash with purple spots. If it is not an emergency but the fever lasts several days, or your child will not drink and urinates less, contact your pediatrician. Consider talking with a pediatrician if you are unsure. This information reflects general guidance from the United States; if you live elsewhere, check with a local pediatrician. This information isn’t a diagnostic tool and doesn’t replace advice from a medical professional.