To check your temperature, place a digital thermometer under your tongue, close your lips, and wait for the beep. That oral reading is the most common method for adults and older children, and a result above 100.4°F (38°C) generally indicates a fever. But the best technique depends on who you’re measuring and what type of thermometer you have. Here’s how to get the most accurate reading with each method.
What Counts as Normal and What Counts as a Fever
The classic “98.6°F” is an average, not a rule. Normal body temperature actually ranges from about 97°F to 99°F (36.1°C to 37.2°C) depending on the person, time of day, and activity level. Your temperature is naturally lowest in the early morning and rises slightly through the afternoon.
A reading of 100.4°F (38°C) or higher is the widely accepted threshold for fever in both adults and children. For babies three months and younger, even a lower reading, around 99.4°F (37.4°C), can signal a serious problem and warrants immediate medical attention. Children between 3 and 6 months old with temperatures reaching 102°F or above are also at higher risk for serious illness.
Oral Temperature (Adults and Children Over 4)
This is the go-to method for most people. Turn on your digital thermometer, then place the tip under your tongue as far back as you comfortably can. Close your lips around it and breathe through your nose. The thermometer will beep when it has a stable reading, usually within 30 to 60 seconds.
One important detail: if you’ve eaten or had something to drink, wait 30 minutes before taking your temperature. Hot coffee or ice water can throw the reading off significantly. Oral readings also tend to run about 1°F lower than rectal readings on average, though the gap can be as large as nearly 3°F in some cases. So if your oral temperature seems borderline, it’s worth knowing that your actual core temperature may be somewhat higher.
Rectal Temperature (Infants Under 3)
Rectal measurement is the most accurate way to estimate core body temperature, and pediatric guidelines recommend it for children under 3. It’s the gold standard that other methods are compared against.
To do it safely, apply a small amount of petroleum jelly to the thermometer tip and your baby’s bottom. Lay the baby face-down on your lap or on a firm surface. Gently slide the thermometer in no more than half an inch for babies under 6 months, and no more than 1 inch for older infants. If you feel any resistance, stop immediately. Hold the baby still and wait for the beep, which takes about 10 seconds with most digital models.
Never force the thermometer, and always hold it in place rather than letting go. A reading of 100.4°F or higher in an infant counts as a fever.
Ear (Tympanic) Temperature
Ear thermometers use an infrared sensor to read the temperature of the eardrum, and they give results in just a couple of seconds. The trick is straightening the ear canal so the sensor has a clear path.
For adults and older children, gently pull the top of the ear up and back before inserting the probe. For children under 3, pull the earlobe down instead. This small difference in technique matters because the ear canal is shaped differently at younger ages. Without straightening it, the sensor reads the canal wall instead of the eardrum, and the result can be off by more than a degree in either direction.
Avoid using an ear thermometer if the person has an ear infection, excessive earwax, or has been lying on that ear. All of these can skew the reading.
Forehead (Temporal Artery) Temperature
Contactless forehead thermometers scan infrared heat from the temporal artery just beneath the skin. They’re fast and comfortable, especially for sleeping children, but they’re the least precise option in many situations.
Hold the thermometer about half an inch to 2 inches from the center of the forehead (check your model’s instructions, since the recommended distance varies by brand). Keep the forehead dry, because sweat interferes with the reading. Direct sunlight and cold outdoor temperatures also reduce accuracy. If you’ve just come inside from the cold, wait a few minutes before scanning.
Because of these limitations, a forehead reading is best used as a quick screening. If it shows a borderline or surprising number, confirm it with an oral or rectal measurement.
Underarm (Axillary) Temperature
Placing a thermometer in the armpit is the least invasive option, but it’s also the least accurate. Axillary readings run lower than core temperature, sometimes by a full degree or more, so they’re really only useful as a rough estimate.
To get the best possible result, place the thermometer tip in the deepest part of the armpit with the arm pressed snugly against the body. Keep the arm down for the full measurement cycle. This method can be helpful for a quick check when oral measurement isn’t practical, but don’t rely on it for making decisions about fever in young children.
Which Method to Use by Age
- Newborns to 3 years: Rectal is the recommended first choice. Ear thermometers work as a secondary option for children over about 6 months.
- Children 4 and older: Oral is the standard. Ear and forehead thermometers are convenient alternatives.
- Adults: Oral is most practical and reliable for home use. Ear thermometers are a reasonable backup.
Keeping Your Thermometer Clean
Clean the thermometer before and after every use, especially if more than one person uses it. Rubbing alcohol at a concentration of 60% to 90% is effective for disinfecting both oral and rectal thermometers. Wipe the tip thoroughly with an alcohol-soaked cotton ball or pad, then let it air dry or rinse with cool water before the next use. Some digital thermometers come with disposable probe covers, which reduce the need for deep cleaning but don’t eliminate it entirely.
If you use the same thermometer for rectal and oral readings (which is best avoided), label it clearly and disinfect it with alcohol each time. A better approach is simply keeping two separate thermometers.
Common Mistakes That Skew Readings
The most frequent source of error is taking an oral reading too soon after eating, drinking, or breathing through the mouth. Hot or cold food and beverages shift the temperature inside your mouth for up to 30 minutes. Placing the thermometer in front of your teeth instead of under the back of your tongue also produces a lower, less accurate number.
With ear thermometers, failing to pull the ear into the correct position is the biggest issue. With forehead scanners, holding the device too far away or scanning through sweat leads to unreliable results. And with any method, using a low battery can produce erratic readings, so replace batteries when the display starts to dim or the thermometer takes noticeably longer to register.