How to Use a Thermometer to Check for Fever

Thermometers used to measure body temperature vary by design. Temperatures can be taken by mouth (oral), anus (rectal), forehead (temporal), armpit (axillary), inner ear (tympanic), or remotely (infrared).

To ensure you get an accurate temperature reading, it is important to know which thermometer to use and how to use it correctly.

This article explains how to use a thermometer to check body temperature. It provides step-by-step instructions for how to use each type of thermometer. It also explains how to clean thermometers and how to interpret the results.

Types of Thermometers

Body temperature can be measured in a few different ways. Standard digital thermometers and manual (mercury) thermometers can be used to take a temperature in three ways:

• Oral (mouth)
• Rectal (anus)
• Axillary (armpit)

Three other types of digital-only thermometers are available:

• Tympanic (ear)
• Temporal (forehead)
• Remote infrared thermometers (temple, forehead, or wrist)

Remote infrared thermometers and digital thermometers are generally recommended for children and adults because they're fast and accurate.

For children, the American Academy of Pediatrics (AAP) recommends different types of digital thermometers based on age.

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

Normal Body Temperatures by Location

Where you place the thermometer will make a difference in interpreting the results. Body temperature varies by location. For example, the average normal body temperature is:

• Oral temperature: 98.6 degrees F 
• Rectal temperature: 99.1 to 99.6 degrees F (0.5 to 1.0 degrees higher than oral)
• Forehead temperature: 97.6 to 98.1 degrees F (0.5 to 1.0 degrees lower than oral) 
• Ear temperature: 99.1 to 99.6 degrees F (0.5 to 1.0 degrees higher than oral)
  
• Armpit (axillary) temperature: 97.6 to 98.1 degrees F (0.5 to 1.0 degrees lower than oral) 
• Wrist temperature: 97.1 to 97.6 degrees (1 to 1.5 degrees lower than oral)
  

How to Use a Non-Contact Thermometer

Non-contact thermometers use infrared technology to measure skin temperature. They are held an inch or two away from the skin and can take a temperature from the forehead, temple, or wrist.

Also known as remote infrared or contactless thermometers, non-contact thermometers can be safely used on people of any age, including newborns.

Each device works slightly differently, so be sure to read the package instructions. In general, to take a temperature using a remote infrared thermometer:

• Turn the machine on and let it warm up.
• Hold the thermometer about three fingers width away from the skin (1 to 2 inches) of the forehead or wrist.
• Depending on the model, you may need to press and hold the button, press and release the button, or simply hold it near the skin.
• When ready, the thermometer may beep or simply display the temperature.
• To interpret the results, add 1 degree to a forehead reading and 1.5 degrees to a wrist reading. This will give you the equivalent oral temperature.

While non-contact thermometers can be safely used on newborns, the AAP recommends rectal temperature over remote temperatures for children under 3 months of age due to accuracy.

How to Take an Oral Temperature

Oral thermometers are commonly recommended for adults. However, they are not the best option for younger children.

To get an accurate oral temperature reading, a person needs to keep their mouth closed long enough to get a good reading. Children are usually able to do this around age 4.

To use an oral thermometer:

1. Wash your hands and clean the thermometer. Put a disposable probe cover over the end of the thermometer, if available.
2. Turn the thermometer on (if digital) and place it under the tongue.
3. Close the mouth and make sure it stays closed the entire time.
4. Wait for the beep (digital thermometer) or approximately five minutes if using a manual thermometer.
5. Remove the thermometer and read the results.
6. Wash the thermometer.

How to Take an Armpit Temperature

Armpit—or axillary—temperature is typically measured with a multiuse digital thermometer.

Despite being the least accurate method, schools and daycares often prefer taking axillary temperatures to avoid spreading germs.

To use an axillary thermometer:

1. Wash or disinfect the thermometer and put a disposable probe cover on it if available.
2. Turn the thermometer on and place it against the skin under the arm. The tip of the thermometer should be in the deepest crease of the armpit.
3. Hold the arm closed to ensure an accurate reading.
4. Wait for the beep.
5. To interpret the results, add 1 degree. This gives you the approximate equivalent to an oral temperature.
6. Wash or disinfect the thermometer.

How to Take a Rectal Temperature

Rectal temperatures are considered the most accurate method for checking a newborn's temperature. The AAP recommends taking rectal temperature for all children under 3 months old and for children up to 4 years old.

Rectal temperatures are taken with a digital multiuse thermometer or manual mercury thermometer. (Be sure to clearly label it for rectal use only.)

To take a rectal temperature:

1. Clean and disinfect the thermometer, then put a disposable probe cover over the end of it, if available.
2. Lubricate the tip of the thermometer with petroleum jelly (or similar lubricant) to ease insertion.
3. For infants and small children, place them face down on your lap or on their back on a changing table. Older children and adults can lie on their stomachs.
4. Turn the thermometer on and gently place the thermometer's tip into the anus. Only put about a half inch of the thermometer in. If you feel any resistance, you have likely gone far enough.
5. Hold the child still until it beeps, then remove the thermometer. (If using a mercury thermometer, you will need to wait about five minutes.)
6. To compare the results to an oral temperature, subtract 1 degree.
7. Wash and disinfect the thermometer when finished.

How to Use an In-Ear Thermometer

Tympanic or in-the-ear thermometers are fairly accurate and used in healthcare offices and by parents. In fact, a 2021 study found tympanic thermometers consistently provide more accurate readings than any other type of thermometer.

However, tympanic thermometers can be difficult to use on babies and may not be accurate because their ear canals are so small. 

To use a tympanic thermometer:

• Pull the top of the earlobe up and back.
• Place the tip of the thermometer (covered with a 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.

How to Use a Forehead Thermometer

Forehead (temporal) thermometers read heat coming from the temporal artery, which is right under the skin of your forehead. These thermometers are fast and easy to use but may read too low at times.

Different varieties of temporal thermometers are available. In its earliest form, forehead thermometers measured temperature through skin contact.

Each model has different instructions for use. Be sure to consult the package instructions. In general, to use a temporal contact thermometer:

• Press the button down.
• Sweep the probe across the forehead and release the button when done. Some models require a swipe across the forehead and on the neck below the ear.
• To compare the results to an oral temperature, add 1 degree.
• Wipe the probe with rubbing alcohol to clean and disinfect it.
  

Mercury Thermometers

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

If you have an old mercury thermometer, here is how to use it:

1. Clean or disinfect the thermometer.
2. Firmly hold the thermometer between your fingers and shake it to get the mercury down to below 96 degrees F.
3. Put the thermometer under the tongue, in the armpit, or in the rectum, as explained above.
4. Hold the thermometer in place for about five minutes to get an accurate reading.
5. Remove the thermometer and closely look at it to read the temperature.
6. Clean or disinfect the thermometer.

Temperature Ranges

The "normal" body temperature is usually stated as 98.6 degrees F. However, there's actually a range of body temperatures that are influenced by a variety 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 than the often-cited 98.6 degrees F.

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 rectal temperature of 100.4 F or higher
• Children
• A baby between 3 and 12 months old has a fever of 102.2 F or higher
• A child under 2 has a fever of 102.2 F or higher for more than 24 hours
• A child 2 or older has a fever of 102.2 F or higher for more than 72 hours
• A child has a fever of 105 F that does not quickly come down with treatment

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

• 105 F or higher that does not quickly come down with treatment 
• 103 F or higher for longer than 48 hours
• That comes and goes for a week or more, even if they are not very high
  

Summary

Knowing how to use a thermometer correctly can help ensure accurate temperature readings. The right thermometer to use depends on age.

Most people can get a fairly accurate reading using a quality non-contact infrared thermometer. However, the American Academy of Pediatrics still recommends using a multi-use digital thermometer to take a rectal temperature in children under 3 months old. Contactless thermometers can read temperatures at the temple, forehead, or wrist.

Body temperature can also be measured using a multiuse thermometer under the tongue or armpit, an in-ear thermometer, or a contact forehead thermometer. If you are unsure if the reading is accurate, try a different thermometer.