Using a thermometer correctly comes down to choosing the right method for the person’s age, placing the sensor in the right spot, and waiting long enough for an accurate reading. A normal body temperature hovers around 98.6°F (37°C), and an oral reading of 100°F (37.8°C) or higher generally counts as a fever. The steps vary depending on whether you’re taking an oral, rectal, ear, forehead, or armpit temperature, so here’s how each method works.
Oral Temperature (Mouth)
Oral readings are the most common method for adults and older children. Place the thermometer tip under your tongue, toward the back of your mouth, and close your lips around it. Breathe through your nose and keep your mouth shut until the thermometer beeps. Most digital thermometers give a reading in 30 to 60 seconds.
If you’ve been eating or drinking, wait 30 minutes before taking your temperature. Hot coffee or ice water can throw off the reading significantly. The same goes for smoking or exercising, both of which temporarily change the temperature inside your mouth.
Rectal Temperature (Infants)
For babies under three months, a rectal thermometer is the most accurate option and the one pediatricians rely on. Lay your baby on their back and gently lift their thighs, or place them belly-down on your lap with one hand against their lower back to hold them steady. Apply a small amount of petroleum jelly or water-based lubricant to the tip, then insert it half an inch to one inch (1.3 to 2.5 centimeters) into the rectum. Never force it. Hold the thermometer in place until it beeps, then slide it out gently.
A rectal temperature of 100.4°F (38°C) or higher in a baby younger than three months is serious enough to call your pediatrician right away, even if the baby seems fine otherwise. For babies between 3 and 24 months, the threshold for concern is higher, around 102°F (38.9°C).
Ear Temperature (Tympanic)
Ear thermometers use infrared light to measure heat from the eardrum. They’re fast, often reading in about one second, but you need a clear, straight path to the eardrum for an accurate result. That means gently tugging the ear to straighten the ear canal before inserting the probe tip.
The direction you pull matters. For babies under one year old, pull the ear straight back. For children over one and adults, pull the ear backward and upward. This small difference in angle accounts for how the ear canal changes shape as a child grows. Fit the probe snugly into the ear canal opening, press the button, and wait for the beep. Earwax buildup or an ear infection can skew the reading, so switch to another method if either is an issue.
Forehead Temperature (Temporal)
Forehead thermometers come in two styles: contact models that you slide across the skin, and non-contact models that you hold a short distance away.
For contact temporal thermometers, press the sensor flat against the center of the forehead and slide it across toward the hairline, following the path described in your thermometer’s instructions. Keep the sensor flush against the skin the entire time. For non-contact (infrared) models, hold the sensor perpendicular to the forehead and keep the person still during the reading. The FDA notes that the correct distance between the device and the forehead varies by manufacturer, so check your specific model’s instructions. Typically it’s somewhere between one and six centimeters.
Sweat on the forehead, direct sunlight, or a hat worn recently can all affect accuracy. Wipe the forehead dry and let the person sit indoors for a few minutes before scanning.
Armpit Temperature (Axillary)
Armpit readings are the least accurate method, but they’re useful as a quick, non-invasive check, especially for young children who won’t tolerate an oral thermometer. Place the tip in the center of the armpit, fold the arm down snugly against the body, and hold it there until the thermometer beeps.
Armpit temperatures run 0.5 to 1°F (0.3 to 0.6°C) lower than oral temperatures. So if an armpit reading shows 99°F, the actual oral equivalent is closer to 99.5 to 100°F. Keep that offset in mind when deciding whether someone has a fever.
Which Method to Use by Age
- Newborns to 3 months: Rectal. It’s the gold standard at this age, and the stakes are highest because fever in very young infants requires prompt medical attention.
- 3 months to 3 years: Rectal remains the most accurate, though ear or forehead thermometers are reasonable alternatives for a quick check.
- Ages 4 and up: Oral, ear, or forehead. Most children this age can hold a thermometer under their tongue without issue.
- Adults: Oral is the default. Ear and forehead are convenient alternatives when speed matters.
Fever Thresholds Worth Knowing
An oral temperature of 100°F (37.8°C) or higher is a fever. For adults, a reading of 103°F (39.4°C) or above warrants a call to your doctor. For infants, the cutoffs are lower and more urgent: 100.4°F (38°C) rectally in a baby under three months, and 102°F (38.9°C) rectally in babies between three and 24 months.
Keep in mind that body temperature naturally fluctuates throughout the day. It tends to be lowest in the early morning and highest in the late afternoon. A reading of 99°F at 4 p.m. may be perfectly normal, while the same reading at 6 a.m. could signal the beginning of a fever.
Cleaning Your Thermometer
Wipe the probe with 70% isopropyl alcohol (rubbing alcohol) before and after each use. The 70% concentration is actually more effective at killing germs than 99%, because the water content helps the alcohol penetrate bacterial cells. You can use an alcohol-soaked cotton ball, a rubbing alcohol wipe, or rinse the tip with cool, soapy water and then swab it with alcohol. Let it air dry before storing.
If you use the same thermometer rectally and orally (not recommended, but it happens), label it clearly and dedicate one to each purpose. Cross-contamination is a real concern.
Skip the Mercury Thermometer
If you still have a glass mercury thermometer in your medicine cabinet, replace it. The EPA and Poison Control both recommend mercury-free alternatives. Mercury is a neurotoxin, and a broken thermometer creates a hazardous spill that you should never vacuum up, as that disperses mercury vapor into the air.
To dispose of a mercury thermometer, place it in a sealed container and bring it to a household hazardous waste collection site. Your local health department can point you to the nearest one. Don’t toss it in the trash. Digital thermometers are inexpensive, faster, and just as accurate, so there’s no reason to keep mercury around.