What temperature is a fever?

MedlinePlus says over 100.4°F (38°C) most often means a fever.

Is oral or armpit more accurate?

Armpit readings are most often 0.5 to 1°F lower than oral, per MedlinePlus.

What is the best way to take a baby's temperature?

MedlinePlus says rectal temperatures are generally considered more accurate in young children.

Are mercury thermometers safe?

The EPA warns broken mercury thermometers release harmful vapors; digital thermometers are the closest alternative.

Home / Home health monitoring

Digital thermometer: oral, rectal or armpit, and how to read a fever

By the Life in Good Form editors. Updated 2026-10-15.

We research products from makers' specs and health sources; we do not run hands-on tests. Links to Amazon may earn us a commission at no cost to you. How this works.

Quick answerA digital stick thermometer is the simplest replacement for an old mercury thermometer. The EPA says battery-powered digital thermometers can be used orally, rectally or in the armpit. MedlinePlus says a temperature over 100.4°F (38°C) most often means a fever, and that rectal readings are generally considered more accurate for young children. Wait 20 to 30 minutes after eating, drinking or smoking before an oral reading, and clean the tip before and after each use.

Our picks at a glance

Vicks SpeedRead digital thermometer. Best for: Oral, rectal or armpit use for the whole family. See options on Amazon
Braun digital thermometer. Best for: Adults and kids. See options on Amazon

Compare the picks and what to watch out for

Why digital instead of mercury

The EPA explains that a broken mercury thermometer releases mercury vapors that are harmful to health, and that at least 13 states have passed laws restricting the manufacture or sale of mercury fever thermometers. It names battery-powered and solar-powered digital thermometers as the most similar alternatives, usable orally, rectally or in the armpit. MedlinePlus notes the American Academy of Pediatrics recommends against glass mercury thermometers.

How to tell if yours has mercury: the EPA says mercury fever thermometers are glass, about the size of a straw, with a silvery-white liquid inside. Oral, rectal and baby versions hold about 0.61 grams of mercury, and basal thermometers about 2.25 grams. If the liquid is any color other than silver, it is not mercury. If you still have a mercury thermometer, the EPA has guidance on cleanup and on recycling mercury products through hazardous waste programs.

Oral, rectal or armpit

Method	Typical wait	Compared with oral
Oral (under the tongue)	3 minutes or until it beeps	Average normal 98.6°F (37°C)
Rectal (infants and small children)	3 minutes or until it beeps	0.5 to 1°F higher
Armpit	5 minutes or until it beeps	0.5 to 1°F lower

Wait times and comparisons are from MedlinePlus. Many digital models beep when the reading is done; follow the instructions for your model. Always tell your provider which method you used.

How to take an accurate reading

Clean it before and after each use with cool soapy water or rubbing alcohol; see our rubbing alcohol guide.
Wait after activity. MedlinePlus says wait at least 1 hour after heavy exercise or a hot bath, and 20 to 30 minutes after smoking, eating, or drinking something hot or cold.
Oral: tip under the tongue, lips closed around it, breathe through the nose.
Rectal, for babies: use a separate thermometer, put petroleum jelly on the tip, and insert only about 1/2 to 1 inch.
Armpit: keep the arm pressed against the body until it beeps.

Write down the reading, the time and the method, since your provider will want to know which type of measurement you used when discussing a fever.

What counts as a fever

MedlinePlus says normal body temperature varies by person, age, activity and time of day. The average is generally accepted as 98.6°F (37°C), but studies show a normal range from 97°F (36.1°C) to 99°F (37.2°C). A temperature over 100.4°F (38°C) most often means a fever caused by an infection or illness. In adults, temperature is usually lowest in the early morning.

Body temperature that is too high or too low can be serious; contact your provider, especially for infants, as advised for your child's age.

Choosing a digital thermometer

Easy to read. The EPA says choose one that is easy to use and read.
Replaceable battery. The EPA suggests one with a replaceable battery; some models have none, and button-cell batteries may contain a small amount of mercury, so recycle them through a hazardous waste program.
Separate rectal thermometer. If you take rectal temperatures for a baby, keep that thermometer for rectal use only and label it.

For a no-touch option, a forehead thermometer uses infrared; MedlinePlus says electronic forehead thermometers are more accurate than ear thermometers and similar in accuracy to probe thermometers. Plastic strip thermometers are the least accurate, it says, and are not recommended for general home use.

When to call a doctor

MedlinePlus says to contact a child's provider right away for a rectal temperature of 100.4°F (38°C) or higher at 3 months or younger, a fever of 102.2°F (39°C) or higher at 3 to 12 months, or a fever lasting longer than 24 to 48 hours at 2 years or younger. Adults should call for a fever that stays at or above 103°F (39.4°C) or lasts longer than 48 to 72 hours.

Call 911 for a fever with trouble breathing, blue lips, confusion, a stiff neck, a very bad headache, a seizure, or someone who cannot be awakened easily. Keep a thermometer in your first aid kit. See also our pulse oximeter guide.

What to look for when you buy

Vicks SpeedRead digital thermometer

Best for: Oral, rectal or armpit use for the whole family

A digital stick thermometer covers the oral, rectal and armpit uses the EPA describes.

Watch out: Clean before and after use; keep a separate one for rectal use.

See options on Amazon

Braun digital thermometer

Best for: Adults and kids

A simple digital replacement for a mercury thermometer, as the EPA recommends.

Watch out: Check whether the battery is replaceable.