The most accurate way to check your temperature depends on your age and the type of thermometer you have, but for most adults, placing a digital thermometer under the tongue gives a reliable reading in under a minute. A temperature of 100.4°F (38°C) or higher is the standard threshold for a fever, as defined by the CDC. Here’s how to get an accurate reading with every common method.
Oral Temperature (Adults and Older Children)
This is the go-to method for anyone old enough to keep their mouth closed around a thermometer for about 30 seconds to a minute. If you’ve been eating, drinking, or smoking, wait at least 30 minutes before taking a reading, since hot coffee or ice water will throw off the number.
Turn on your digital thermometer and place the tip under your tongue, toward the back. Close your mouth and breathe through your nose. Wait until the thermometer beeps, then remove it and read the display. For oral readings, anything at or above 100.4°F (38°C) counts as a fever.
Rectal Temperature (Infants Under 3 Months)
For newborns and babies up to 3 months old, a rectal reading is the most accurate way to check for fever. It measures closer to the body’s true core temperature than any other home method. Use a standard digital thermometer with a flexible tip. Apply a small amount of petroleum jelly to the tip for comfort, lay the baby face-down on your lap or on a firm surface, and gently insert the tip about half an inch to one inch. Hold the thermometer in place until it beeps, keeping the baby still so the probe doesn’t shift.
For children between 3 months and 4 years, a rectal temperature is still considered the gold standard. If you take a reading with another method and the result seems off, a rectal check is the most reliable way to confirm.
Forehead (Temporal Artery) Thermometers
Temporal artery thermometers scan the blood vessel that runs across the forehead. They come in two styles: contact models you slide across the skin and no-touch infrared models you aim from a short distance. Both work by reading heat from the side and front of the forehead.
Follow the directions for your specific model, since placement and distance vary. These thermometers are fast, non-invasive, and recent research suggests they can provide accurate readings even in newborns, making them a practical alternative for parents who find rectal readings stressful.
Ear (Tympanic) Thermometers
Ear thermometers use an infrared sensor to measure heat from the eardrum. They’re convenient and fast, usually giving a reading in one to two seconds. However, they aren’t recommended for babies under 6 months old because the ear canal is too small for the probe to get a reliable reading.
For children over 6 months and adults, gently pull the ear up and back to straighten the ear canal before inserting the probe tip. This helps the sensor line up with the eardrum. A buildup of earwax, an ear infection, or a lot of time spent lying on one ear can all skew the result, so switch to another method if the reading doesn’t match how you feel.
Armpit (Axillary) Temperature
Taking a temperature under the armpit is the easiest method, especially for squirmy toddlers, but it’s also the least accurate. A study published in Global Pediatric Health found that armpit readings ran about 0.8°C (roughly 1.4°F) lower than rectal readings on average, with individual differences ranging even wider. That means a “normal” armpit reading could still be masking a low-grade fever.
To get the best result, place the thermometer tip in the center of the armpit with the arm pressed snugly against the body. Hold it there until the beep. If the number is borderline or you suspect a fever, confirm with an oral or rectal reading. As a rough rule of thumb, adding about 1°F (0.5–0.6°C) to an armpit reading brings it closer to an oral equivalent, though this adjustment isn’t perfectly precise.
Which Method for Which Age
- Birth to 3 months: Rectal (most accurate) or temporal artery.
- 3 months to 4 years: Rectal, temporal artery, or armpit. Ear thermometers are fine from 6 months onward.
- 4 years and older through adulthood: Oral, temporal artery, or ear. Armpit works in a pinch but treat the reading as a rough estimate.
How to Tell Without a Thermometer
If you don’t have a thermometer handy, several physical signs can hint at a fever. Touch the forehead with the back of your hand. If it feels noticeably hot compared to your own skin, a fever is likely. Flushed or reddish cheeks are another visual clue, especially in children.
Dehydration often accompanies a fever. Pinch the skin on the back of your hand and let go. If it snaps back quickly, hydration is probably fine. If it settles slowly, that’s a sign of fluid loss. Unusually dark yellow or orange urine, or producing less urine than normal, points in the same direction.
Other common signs include chills, shivering, sweating, body aches, headache, fatigue, sore eyes, loss of appetite, and swollen lymph nodes. In infants, watch for skin that looks red or flushed, unusual irritability, extreme tiredness, or difficulty feeding. None of these signs replace an actual reading, but together they can tell you it’s worth getting a thermometer.
Keeping Your Thermometer Clean
Wipe the probe with rubbing alcohol (60% to 90% concentration) or a cotton ball soaked in alcohol before and after each use. This is effective for both oral and rectal thermometers. Rinse with cool water afterward and let it air dry. Never share a thermometer between oral and rectal use without thorough disinfection, and ideally, label separate thermometers for each purpose. Store it in its protective case to keep the tip from picking up dust or bacteria between uses.
Tips for a More Accurate Reading
Take your temperature at the same time of day when tracking a fever. Body temperature naturally fluctuates, running lower in the morning and peaking in the late afternoon, so a reading that looks fine at 8 a.m. might register higher by 4 p.m. even without any change in illness.
Avoid taking a reading right after exercise, a hot bath, or bundling up in heavy blankets, all of which can temporarily raise your skin and core temperature. If you get a surprising result, wait five minutes and try again. Two consistent readings are more trustworthy than one outlier.