How to Use a Thermometer Correctly? | Home Guide

Getting a reliable temperature reading matters more than most people realize. A number that reads low when a fever is climbing, or high after a hot drink, can send you down the wrong path entirely. The way you use a thermometer — where you place it, how long you wait, and what you do beforehand — changes the result as much as the device itself does.

Different routes read differently. Rectal temperatures run highest, followed by oral, then underarm and forehead readings. That’s why consistency matters: if you’re tracking a fever over several days, take the temperature the same way each time so you’re comparing like with like.

Choose the Right Thermometer for the Route

Thermometer selection starts with who you’re measuring and where. Digital thermometers work for oral, rectal, and underarm readings, while infrared models read forehead temperatures. Mayo Clinic’s guidance stresses that thermometer choice depends on age and measurement site — a rectal reading is the most accurate for infants, while oral works well for adults and older children who can hold still.

One rule applies across all routes: never use the same thermometer for oral and rectal temperatures. Keep separate devices, label them clearly, and clean each one before and after every use with rubbing alcohol or soap and lukewarm water. Wash your hands before handling the thermometer, and always read the manufacturer’s instructions that came with your specific model.

How to Take an Oral Temperature

Taking an oral temperature correctly starts before the thermometer goes in your mouth — you need to wait 30 minutes after eating or drinking anything, since food and beverages can skew the reading. The same goes for hot or cold fluids, which alter mouth temperature directly.

Place the thermometer tip under the tongue, toward the back of the mouth where it’s warmest. Close your lips around it and breathe through your nose. Hold the device steady in that spot, keeping your mouth closed, until you hear the beep or see the completion signal.

What About Rectal, Underarm, and Forehead Readings?

Rectal, underarm, and forehead routes each have their own technique, and getting them right means the difference between a useful number and a misleading one.

For rectal temperatures, use a thermometer designated for that purpose only. Apply lubricant and insert gently — about half an inch to an inch in a child — stopping immediately if you feel resistance. Never force it. Hold it in place until the beep, then remove and clean it thoroughly.

Underarm readings are the simplest but least accurate. Place the probe tip directly against skin in the armpit — not clothing — and press your arm firmly against your body until the device signals it’s done. Because this method runs cooler than the body’s core, don’t be surprised if it reads slightly lower than an oral measurement.

Forehead infrared thermometers follow a different protocol entirely.

Using a Food Thermometer Correctly in the Kitchen

A food thermometer is a different product from a body thermometer, but it still demands the right technique to give an accurate reading. Insert the probe into the thickest part of the food, avoiding bone, fat, and gristle, which read differently than the meat itself. For liquids, stir first so the temperature is even throughout.

Make sure the sensing area — the lower portion of the probe — is fully inserted past the dimple or indentation on the stem. Wait at least 15 seconds for the reading to steady before trusting it. If you’re cooking, only use probes that are rated oven-safe when you leave them in meat during cooking.

If you’re tracking temperatures around your home, a dedicated wireless thermometer can make the job easier. Our roundup of the best wireless indoor outdoor thermometer models covers tested options for monitoring conditions at home.