How long does a thermometer need to be in your mouth?

If you think you or your child may have a fever, you reach for a thermometer to check. However, getting an accurate reading means knowing the right way to use the type of thermometer you have. Several different ones are available, from temporal to oral, rectal to axillary, and it's easier than you may think to get it wrong.

Types of Thermometers

You have the option of digital or manual (mercury) thermometers for taking a temperature in three ways:

• Oral
• Rectal
• Axillary (armpit)

Two other types of digital-only thermometers are available:

• Tympanic (ear)
• Temporal (forehead)

The American Academy of Pediatricians recommends digital thermometers for taking a child's temperature because they're fast and accurate. The specific type of digital thermometer they suggest varies by age.

*Discard old rectal thermometer and buy a new one for oral use.

Oral Thermometer Use

Oral thermometers are not the best option for young children, who may not be able to keep their mouths closed long enough to get a good reading.

To use an oral thermometer:

1. Wash your hands before handling the thermometer.
2. Place it under the tongue.
3. Make sure the mouth stays closed the entire time.
4. Wait approximately five minutes (manual thermometer) or for the beep (digital thermometer).

Don't take an oral temperature right after eating or drinking something; it will affect the results.

Axillary Thermometer Use

While this is the least accurate way to get a child's temperature, it's often used in schools and daycare to avoid spreading germs.

To use an axillary thermometer:

1. Place the thermometer under the arm with the tip in the deepest crease of the armpit.
2. Wait approximately five minutes (manual thermometer) or for the beep (digital thermometer).

Rectal Thermometer Use

Rectal thermometers are specifically designed with short tips that allow them to get a proper reading without going too far into the body. This method should be used for infants or those whose temperature cannot be taken any other way.

To use a rectal thermometer:

1. Use lubrication, such as petroleum jelly, to ease insertion.
2. Place the thermometer's tip in the rectum.
3. Wait approximately five minutes (manual thermometer) or for the beep (digital thermometer).

Cleaning Your Thermometer

Wash your thermometer before and after use with cold water, then rubbing alcohol. Rinse thoroughly to remove the alcohol.

Tympanic Thermometer Use

These in-the-ear thermometers are very popular, especially among parents of small children, since they're faster than regular digital thermometers and are easy to use. However, tympanic thermometers can be difficult to use on babies and are often inaccurate because their ear canals are so small. 

To use a tympanic thermometer:

• Pull the top of earlobe up and back
• Place the tip of the thermometer (covered with probe cover) in the ear-canal opening. (Be sure you are pointing the probe into the ear canal opening and not at the wall of the ear.)
• Press the button until it beeps.

Make sure excess earwax isn't built up before using this method, as it can cause less accurate results.

Temporal Thermometer Use

The newest and most expensive thermometer on the market, temporal thermometers read heat coming from the temporal artery, which is right under the skin of your forehead. They are the fastest and probably the easiest thermometers to use. However, they may read too low at times.

Different models may have different instructions for use. Generally, to use a temporal thermometer:

• Press the button down.
• Sweep the probe across the forehead and release the button when done.

Note: Some models require a swipe across the forehead and on the neck below the ear.

This is fairly new technology, but research suggests that it's at least as accurate as tympanic devices.

Mercury Thermometers

Mercury thermometers are no longer sold in the United States. They pose a danger if they break and release the mercury, which is toxic.

If you have an old mercury thermometer that you decide to use, shake it to get the mercury down to below 96 degrees F. Then hold it in place for about five minutes to get an accurate reading.

Temperature Ranges

The "normal" body temperature is usually stated as 98.6 degrees F. However, there's actually a range of body temperatures that's influenced by all kinds of factors, including age, height, weight, sex, ethnicity, and even time of day and activity level.

Interestingly, what's average appears to have dropped over time. A 2017 study found that the average body temperature is closer to 97.88 degrees F. This is relatively new information, though, and it hasn't yet impacted what the medical community considers normal and abnormal.

When to Call the Doctor

Not all fevers need treatment. If a fever is making you uncomfortable, you can take over-the-counter fever reducers such as aspirin (adults only), Motrin or Advil (ibuprofen), or Aleve (naproxen).

Certain temperatures or symptoms do warrant medical attention, though.

When it comes to your child, you should call the doctor when:

• A baby 3 months old or younger has a temperature of 100.4 degrees F
• A child of any age has repeated fevers over 104 degrees F
• A child under 2 has a fever of 100.4 that lasts for more than 24 hours
• A child 2 or older has a fever of 100.4 lasting more than 72 hours
• Your baby cries or fusses and can't be soothed

For an adult, you should call the doctor if you have a fever:

• Over 103 degrees F that doesn't drop within two hours of taking a fever reducer
• That lasts longer than two days
• That's in the high range and accompanied by a rash
• That's accompanied by a stiff neck and confusion or irritability, sensitivity to light (photophobia), dehydration, or seizure

Any fever over 105 degrees F is a life-threatening emergency. Call 911 or have someone take you to the emergency room right away.

Frequently Asked Questions

• Is a forehead thermometer reading accurate?

  Yes. However, the reading is 0.5 to 1 degree F lower than an oral reading, so an average body temperature with a forehead thermometer might read 97.6 to 98.1 instead of the normal 98.6.

• How can I check my child’s temperature to screen for COVID-19 symptoms?

  Contactless forehead thermometers are a good choice for screening for COVID. Anything over 100.5 F could be a sign of an infection, which could be COVID or another illness. Keep in mind: Using a forehead thermometer in direct sunlight and testing a child who has been running around or is overheated could give you an inaccurately high temperature.

• Can you tell someone is feverish by touching their forehead?

  Not definitively. Researchers have studied whether mothers could accurately determine if a child has a fever by touching them. Mothers correctly identified a child as having a fever 79% of the time, and as non-feverish 99% of the time. The bottom line: Using touch is a good first check, but it’s best to confirm a temperature with a thermometer when possible.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. Sulaski Wyckoff A. Thermometer use 101. AAP News. 2009;30(11);29. doi:10.1542/aapnews.20093011-29a

2. El-Radhi AS. Determining fever in children: the search for an ideal thermometer. Br J Nurs. 2014;23(2):91-94. doi:10.12968/bjon.2014.23.2.91

3. Geijer H, Udumyan R, Lohse G, Nilsagård Y. Temperature measurements with a temporal scanner: systematic review and meta-analysis. BMJ Open. 2016;6(3):e009509. doi:10.1136/bmjopen-2015-009509

4. U.S. Environmental Protection Agency. Mercury thermometers.

5. Stanford Children's Health, Lucile Packard Children's Hospital. Fever in children.

6. Mott Children’s Hospital. Fever Temperatures: Accuracy and Comparison.

7. AAP News. Thermometers 101: How to check temperature during COVID-19 pandemic.

8. Teng CL, Ng CJ, Nik-Sherina H, Zailinawati AH, Tong SF. The accuracy of mother’s touch to detect fever in children: a systematic review. Journal of Tropical Pediatrics. 2007;54(1):70-73. doi:10.1093/tropej/fmm077

Additional Reading

• American Academy of Pediatrics: healthychildren.org. How to take a child's temperature.

• Obermeyer Z, Samra JK, Mullainathan S. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ. 2017;359:j5468. doi:10.1136/bmj.j5468

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.  

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