Parents use four types of thermometers: rectal, oral, ear, and armpit. Learn the best way to use each one.

By Donna Christiano
woman looking at thermometer
Credit: Shutterstock

When you report a fever to your doctor, always tell her where you took your child's temperature and which type of thermometer you used. Here's how different methods measure up.


Because it's the most internal and therefore the most accurate measurement, many doctors recommend taking a rectal temperature in babies and children 3 years of age and younger. To use, first dab petroleum jelly on the bulb of the thermometer. Place your baby belly-down on your lap or on a bed or changing table, then gently insert the bulb 1/2 to 1 inch into your child's rectum. Loosely hold the thermometer in place with two fingers until it beeps.

Con: Older kids don't like it

Pro: It's the best choice for kids up to age 3 or whenever an exact reading is critical.

Accuracy: The highest


Placing the thermometer under the tongue in cooperative children over 3 is also accurate, provided the child keeps his mouth closed for two to three minutes and refrains from drinking hot or cold liquids 15 to 20 minutes before the thermometer is inserted. Should you use a digital or a mercury thermometer here? Whichever you prefer. (Some studies say digitals are as on-the-mark as mercury; other studies find them to be slightly less so.)

Con: The reading can be thrown off if your child doesn't keep the thermometer under her tongue until it beeps.

Pro: Most kids 4 and older can use them correctly, and there's no need to remove their clothing.

Accuracy: Good


Tympanic thermometers can also give you a good gauge of a fever in just a few seconds, but they're expensive—most run about $60—and require batteries. If you don't insert them just right into your child's ear canal or there's a buildup of earwax, they can also be inaccurate. For a more precise result, pull your child's ear slightly up and back before inserting.

Con: Doctors say the tip is difficult to insert properly, especially for kids under 1.

Pro: It's fast and convenient.

Accuracy: Varies


This spot gives the least accurate reading. However, using a mercury thermometer under the arm for four to five minutes is still good in a pinch, especially if your child refuses to allow you to insert one anywhere else. Many doctors recommend adding 1F to the reading for greater accuracy.

Con: If the room is chilly, you may get a false low reading.

Pro: It's very easy to use.

Accuracy: Adequate

No matter which type of thermometer you choose, you should consider ditching mercury varieties. Those old-school thermometers contain mercury, a potent toxin that affects the brain, spinal cord, liver, and kidneys, and can cause learning disabilities. If it breaks, you risk exposing your family to mercury's harmful vapors. Still have one lurking in your medicine cabinet? Don't just toss it into the trash. Take it to your pediatrician (she can dispose of it safely), or drop it off at your local hazardous-waste collection site.

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