How to Use the Thermometer | Readings That Actually Count

Using a thermometer correctly depends on matching the right site—oral, rectal, armpit, ear, or forehead—to the right device.

Getting an accurate temperature reading sounds simple, but the method matters more than most people realize. Using a digital oral thermometer under your arm, or taking an ear reading with a device meant for the forehead, gives you a number that means very little. The fix for how to use the thermometer starts with one question: what kind do you have?

Each style has its own approved site, its own timing, and its own pitfalls. The steps below match the major types to their correct use, with timing guidance from MedlinePlus (NIH) and the Cleveland Clinic. Read the manual that came with your device first, though—Mayo Clinic notes that exact placement, timing, and insertion depth vary by model.

Oral, Rectal, Armpit: The Contact Methods

Contact thermometers need direct skin or mucous-membrane contact, and each site follows a different procedure.

Oral (digital): place the tip under the tongue, close your mouth, and wait for the beep. MedlinePlus says to expect roughly 3 minutes, while the Cleveland Clinic notes many digital oral models finish in about 40 seconds. The key qualifier: don’t eat, drink, or smoke for 30 minutes beforehand.

Rectal (digital): this is the standard for infants and small children, or when a doctor specifically instructs it. Lubricate the tip with petroleum jelly, insert gently, and never force it if you feel resistance. MedlinePlus warns that inserting too deeply can injure tissue, and a struggling child can push the thermometer in further. These typically beep in around 30 seconds per the Cleveland Clinic.

Armpit (axillary): place the tip in the armpit and hold your arm snugly against your body until the reading completes. MedlinePlus puts this at about 5 minutes. This method is less precise for body temperature than oral or rectal readings, so treat it as a screening tool rather than a definitive number.

Ear and Forehead: The Non-Contact Options

Tympanic (ear) and temporal (forehead) thermometers read heat rather than direct contact, which changes the technique completely.

For an ear thermometer, position it in the ear canal only as far as the device instructions allow, and hold it perfectly steady until the beep. For a forehead or infrared model, aim it at the temple from the distance the manual specifies—often around 3–5 cm—without touching skin unless the device requires contact. Give infrared units up to 15 minutes to warm up if the manual asks for it, and wipe away perspiration before measuring, since sweat distorts the reading.

Some infrared units have a “body reading” mode for human temperature as opposed to surface temperature—confirm that setting is active before you use it on a person.

Timing, Hygiene, and Consistent Tracking

When you take the reading matters as much as how. Heavy exercise or a hot bath can skew a temperature for up to an hour, so wait that long before measuring.

The hygiene rules are simple and non-negotiable:

• Clean the thermometer with rubbing alcohol or soap and lukewarm water before and after every use.
• Wash your hands before handling the device.
• Never use the same thermometer for both oral and rectal temperatures—label separate devices.

For tracking a fever over several days, take the temperature at the same time each day and use the same method each time. That consistency matters more than the specific site you choose.

If you’re taking temperatures for a child in the tub, a bath thermometer built into the tub itself removes the guesswork of separate probes—see our picks for the best bathtub with thermometer if that fits your routine.

FAQs

What is the most accurate way to take a temperature?

Rectal readings are the most accurate for infants and small children, while oral readings are the standard for older children and adults. Armpit readings are the least precise of the three contact methods. The most accurate method for your situation is whichever one you can perform correctly and consistently.

How long should you leave a thermometer in your mouth?

MedlinePlus advises leaving a digital oral thermometer in place about 3 minutes or until it beeps. The Cleveland Clinic says many digital oral thermometers complete a reading in about 40 seconds. The beep always signals completion, regardless of the exact second count.

Why is my temperature different in each ear?

Slight differences between ears are normal, which is why you should always measure the same ear consistently. Perspiration, earwax, or poor positioning are the usual culprits.