To use a mouth thermometer, place the tip under your tongue as far back as you can, close your mouth, and wait for the beep. The whole process takes under a minute, but a few small details (where exactly the tip goes, what you did in the last 30 minutes) make the difference between an accurate reading and a misleading one.
Before You Start
If you’ve eaten, had something to drink, or smoked in the last 30 minutes, wait before taking your temperature. Hot coffee or ice water can temporarily change the temperature inside your mouth by a full degree or more, and it takes about half an hour for conditions to normalize. This is the most common reason people get inaccurate oral readings.
Breathing through your mouth also pulls cooler air across the thermometer and lowers the reading. If you’ve been exercising heavily or breathing hard, give yourself a few minutes to recover and breathe normally through your nose before you begin.
Step-by-Step Instructions
Turn on the digital thermometer. Most models have a single power button and will display a ready signal or flashing numbers when they’re set to go.
Lift your tongue and place the thermometer tip into the pocket at the very back of the space underneath your tongue, on either the left or right side. This spot, called the sublingual pocket, sits close to blood vessels that reflect your core body temperature. Placing the tip near the front of your mouth or on top of your tongue gives a reading that skews low.
Close your lips around the thermometer and keep them sealed. Breathe through your nose. Hold the thermometer steady with your fingers or lips so the tip stays pressed against the tissue at the back of that pocket. Try not to bite down on it.
Wait for the beep. Most digital thermometers take somewhere between 20 and 60 seconds. Remove the thermometer and read the number on the display.
What Your Reading Means
Normal oral temperature isn’t one fixed number. It ranges from about 97°F (36.1°C) to 99°F (37.2°C) depending on the time of day, your activity level, and your individual baseline. The old “98.6°F is normal” benchmark is just an average.
An oral reading of 100.4°F (38°C) or higher is generally considered a fever. A reading of 103°F (39.4°C) or above in an adult signals a high fever that typically comes with obvious symptoms like chills, body aches, and fatigue.
If you’re comparing your reading to a number someone took under the arm (axillary), keep in mind that armpit readings run about 1°F lower than oral ones. So an armpit reading of 99.4°F roughly equals an oral reading of 100.4°F.
Getting a Consistent Reading
A few factors trip people up repeatedly:
- Tip placement. The farther back and deeper under the tongue, the more accurate the reading. A shallow placement near the front of the mouth can read a full degree lower than one taken in the correct spot.
- Mouth breathing during the reading. Even a few seconds of open-mouth breathing cools the sensor. Research on thermal behavior in the oral cavity confirms that the total duration of mouth breathing significantly affects the reading, while breathing rate itself matters much less. In other words, one long open-mouth breath hurts accuracy more than several quick ones.
- Taking it too soon after eating or drinking. The 30-minute rule applies to anything you put in your mouth, including gum and mints.
- Moving the thermometer. Shifting the tip mid-reading forces the sensor to re-adjust. Hold still until the beep.
If a reading seems off, wait a minute and try again, making sure the tip is as far back under the tongue as comfortable.
Using a Mouth Thermometer With Children
The American Academy of Pediatrics recommends waiting until a child is at least 4 years old before using an oral thermometer. Younger children have trouble keeping their mouths closed around the probe long enough, and they’re more likely to bite down or move it around. For children under 4, a rectal, forehead, or ear thermometer is more reliable.
For kids 4 and older, the steps are the same as for adults. You may need to hold the thermometer in place yourself and remind them to breathe through their nose and keep their lips sealed.
Cleaning Your Thermometer
Wipe the thermometer tip with a cotton ball or pad soaked in 70% isopropyl alcohol (rubbing alcohol) before and after each use. This concentration kills bacteria and viruses without damaging the sensor. Let the tip air dry or rinse it with cool water before putting it in your mouth. Store the thermometer in its protective case to keep the tip clean between uses.
Avoid using hot water to clean a digital thermometer, as heat can damage the electronic sensor. Never share a thermometer between household members without disinfecting it first.