To take an oral temperature, place the tip of a digital thermometer into the pocket under your tongue near the back of your mouth, close your lips around it, and breathe through your nose for about three minutes or until the thermometer beeps. A normal oral reading falls between 97°F and 99°F, and 100.4°F or higher indicates a fever.
Choose the Right Thermometer
A standard digital contact thermometer is the best option for oral readings. These are inexpensive, widely available, and give results quickly. Older glass thermometers filled with mercury are no longer recommended and have been banned in many parts of the United States because mercury is toxic and broken glass poses an injury risk. If you still have one, replace it with a digital version.
Wait Before You Measure
Anything you recently put in your mouth can throw off your reading. Wait 20 to 30 minutes after eating, drinking a hot or cold beverage, or smoking before taking your temperature. Hot coffee or ice water shifts the temperature of the tissue inside your mouth, and it can take up to 15 minutes for your oral cavity to return to its true baseline after exposure to something cold. Skipping the wait is the most common reason for an inaccurate result.
Where to Place the Thermometer
The exact spot under your tongue matters more than most people realize. You’re aiming for the sublingual pocket, which is the deepest point underneath your tongue toward the back of your mouth, close to where the base of the tongue meets the floor of the mouth. This area stays closest to your core body temperature. The front of your mouth and the surfaces near your teeth are more sensitive to outside air and recover more slowly after any thermal change, making them less reliable.
Slide the thermometer tip as far back into that pocket as is comfortable, and press it gently against the tissue underneath your tongue so the sensor stays in full contact with the lining.
Step-by-Step Process
- Clean the thermometer. Wipe the probe with rubbing alcohol on a cotton pad, or wash it with soap and water and rinse with cool water.
- Turn it on. Press the power button and wait for the ready signal if your model has one.
- Position the probe. Place the tip deep into the sublingual pocket under your tongue, as far back as comfortable.
- Close your mouth. Seal your lips tightly around the thermometer shaft. Do not bite down on it.
- Breathe through your nose. Mouth breathing lets outside air flow over the sensor and lowers the reading. Keep your lips closed the entire time.
- Wait for the beep. Most digital thermometers signal when the reading is stable. If yours doesn’t beep, leave it in place for at least three minutes.
- Read and record. Note the number on the display along with the time of day, since body temperature naturally fluctuates throughout the day.
- Clean the thermometer again. Wipe it down with rubbing alcohol or soap and water before storing it.
Why Mouth Breathing Ruins the Reading
Keeping your mouth closed is not optional. Research on thermal behavior in the oral cavity shows that the total duration of mouth breathing significantly affects oral temperature, pulling the reading downward the longer air passes over the sensor. Interestingly, how fast you breathe matters less than whether your mouth is open at all. If you have nasal congestion that forces you to breathe through your mouth, an oral reading will be unreliable. In that case, consider an armpit or forehead reading instead.
What Your Reading Means
The old standard of 98.6°F is an average, not a rule. Typical body temperature ranges from about 97°F to 99°F depending on the person, the time of day, and recent activity. Your temperature is usually lowest in the early morning and peaks in the late afternoon.
An oral temperature of 100.4°F or higher is the widely accepted threshold for a fever in both adults and children old enough to hold a thermometer under their tongue. A reading between 99°F and 100.3°F sometimes gets called a “low-grade fever,” though it can also reflect normal variation after exercise, stress, or hormonal shifts.
Tips for Taking a Child’s Temperature by Mouth
Most children can use an oral thermometer reliably by age four or five, once they can keep their mouth closed and resist biting the probe. For younger children, a rectal or forehead thermometer is more practical. When taking a child’s oral temperature, the same rules apply: wait after food or drinks, place the probe in the back pocket under the tongue, and remind them to breathe through their nose. Sitting still for the full measurement is the hardest part for kids, so using a thermometer with a fast read time helps.
Cleaning and Storage
Clean the thermometer both before and after every use. Rubbing alcohol on a cotton pad is the quickest method. Soap and cool water works just as well. Avoid hot water, which can damage the sensor. Store the thermometer in its protective case to keep the tip from getting bent or contaminated. If multiple family members share a thermometer, cleaning between users is especially important.