To check for a fever with a digital thermometer, turn it on, place the tip under your tongue slightly off-center, close your mouth, and wait for the beep. A reading of 100°F (37.8°C) or higher taken orally means you have a fever. The process takes under a minute with most modern digital thermometers, but a few preparation steps beforehand make the difference between an accurate reading and a misleading one.
Before You Take a Reading
Wait 20 to 30 minutes after eating, drinking, or smoking before placing a thermometer in your mouth. Hot coffee or ice water can temporarily shift the temperature inside your mouth enough to throw off your reading. This is the most common reason people get numbers that seem too high or too low.
Check that your thermometer has a fresh battery. A low or dying battery can produce incorrect readings, and you may not realize the number is off. If the display looks dim or the thermometer responds sluggishly when you press the power button, replace the battery before relying on the result.
How to Take an Oral Temperature
Oral readings are the most practical option for anyone over age four. Here’s how to get an accurate one:
- Turn on the thermometer. Most models display a startup indicator or flash to show they’re ready.
- Place the tip under your tongue, slightly to one side of center. That pocket of tissue at the base of your tongue, near the back, sits closest to blood vessels and gives the most reliable reading.
- Close your lips around the thermometer. Breathing through your nose keeps room air from cooling the sensor.
- Wait for the beep. Digital thermometers signal when the reading is final. Removing it early can give you a number that’s a degree or more too low.
A normal oral temperature hovers around 98.6°F (37°C), though healthy people can run anywhere from about 97°F to 99°F depending on the time of day. You have a fever at 100°F (37.8°C) or above.
Taking an Armpit Temperature
Armpit (axillary) readings are less accurate than oral ones, but they’re useful for young children or anyone who can’t hold a thermometer in their mouth. The technique matters more here because the sensor isn’t surrounded by tissue the way it is under the tongue.
Remove any clothing covering the arm so the thermometer tip touches bare skin directly. Turn on the thermometer, lift the arm, and place the tip in the center of the armpit, right in that pocket that’s hidden when the arm rests at the side. Lower the arm snugly against the body and hold it there until the thermometer beeps. For squirmy toddlers, gently holding their arm in place helps prevent the sensor from slipping out of position.
Armpit readings run lower than oral ones. Add 0.5 to 1°F (0.3 to 0.6°C) to get a better estimate of actual body temperature. An armpit reading of 99°F (37.2°C) or higher suggests a fever.
Rectal Readings for Infants
For babies under three months, a rectal temperature is the gold standard because it’s the closest measure of core body temperature. Use a thermometer with a flexible tip designed for rectal use. Apply a small amount of petroleum jelly to the tip for comfort, lay the baby face-down across your lap or on a firm surface, and insert the tip gently about half an inch to one inch. Hold the thermometer in place (never let go of it) until you hear the beep.
A rectal reading of 100.4°F (38°C) or higher counts as a fever. In a baby younger than three months, that number warrants a call to your pediatrician right away, even if the baby seems fine otherwise.
Fever Thresholds at a Glance
The number that counts as a fever depends on where you measure:
- Oral: 100°F (37.8°C) or higher
- Rectal or ear: 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
These thresholds apply to both children and adults. If you’re comparing readings from different methods, keep in mind that armpit readings sit at the low end and rectal readings at the high end. Switching methods between checks can make it look like a fever is rising or falling when it hasn’t actually changed.
Cleaning Your Thermometer
Clean your thermometer before and after every use, especially if more than one person shares it. Wipe the tip with rubbing alcohol (isopropyl alcohol in the 60% to 90% range works best), or wash it with cool soapy water and rinse thoroughly. The CDC confirms that alcohol at these concentrations effectively disinfects both oral and rectal thermometers. Let the tip dry completely before storing it in its case.
Never use hot water to clean a digital thermometer. It can damage the sensor and shorten the device’s lifespan.
When a Fever Needs Medical Attention
Most fevers in adults resolve on their own within a few days. But a temperature over 104°F (40°C) calls for a conversation with your doctor. And certain symptoms alongside any fever signal something more serious. Seek immediate medical help if a fever comes with a seizure, loss of consciousness, confusion, a stiff neck, trouble breathing, severe pain, or swelling anywhere in the body. Pain during urination or unusual vaginal discharge paired with a fever can also point to an infection that needs prompt treatment.
For children, the rules tighten. Any fever in a baby under three months, a fever lasting more than a day in a child under two, or a fever above 102°F in an older child that doesn’t respond to over-the-counter fever reducers is worth a call to the pediatrician.