A fever in adults is generally any oral temperature at or above 100°F (37.8°C), though normal body temperature varies from person to person, ranging anywhere from 97°F to 99°F. Checking for a fever is straightforward with a digital thermometer, but the method you use, where you place it, and a few simple preparation steps all affect how accurate your reading will be.
What Counts as a Fever
The old benchmark of 98.6°F as “normal” is just an average. Your own baseline may sit a bit higher or lower, and your temperature naturally shifts throughout the day, peaking in the late afternoon around 4 p.m. and dipping in the early morning. Women also tend to have slightly more temperature variation than men, partly due to hormonal cycles.
Because of this natural range, the threshold for a fever depends on where you measure it:
- Mouth: 100°F (37.8°C) or higher
- Rectum, ear, or forehead: 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
Armpit readings run the lowest because the sensor isn’t inside a body cavity. Rectal readings are the highest and most accurate reflection of core body temperature. Oral falls in between. Knowing which method you used matters when interpreting the number.
Choosing a Thermometer
Digital thermometers are the standard for home use. Mercury glass thermometers are no longer recommended because of the toxicity risk if they break. Beyond that, your main choices come down to where the thermometer measures.
Oral digital thermometers are the most common option for older children and adults. They offer similar accuracy to rectal thermometers and are far less invasive. Rectal thermometers give the most accurate reading and are the preferred method for infants and toddlers, since babies can’t hold a thermometer under their tongue. Ear (tympanic) thermometers are fast and convenient, but earwax, ear infections, and an imperfect fit in the ear canal can all throw off the reading. Forehead (temporal artery) thermometers work by scanning the skin’s surface, making them the least disruptive option for sleeping children. However, they can be less accurate if the forehead is sweaty, if the room is very cold, or if you’re outdoors in direct sunlight.
How to Take an Oral Temperature
If you’ve eaten or had something to drink, wait 30 minutes before measuring. Hot coffee or ice water can temporarily change the temperature inside your mouth enough to give a misleading number.
Turn on the digital thermometer and place the tip under your tongue, toward the back. Close your mouth around it and breathe through your nose. Keep it in place until the thermometer beeps, then remove it and read the display. For adults and children old enough to cooperate, this is the simplest and most reliable everyday method.
How to Take a Rectal Temperature
For babies under three months, a rectal reading is the gold standard because even a small inaccuracy matters at that age. Apply a small amount of petroleum jelly to the tip of the thermometer 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, keeping the baby still, until it beeps. Never force it or leave the baby unattended during the reading.
Use a thermometer that’s clearly labeled or designated for rectal use only, and keep it separate from any thermometer you use orally.
How to Use a Forehead Thermometer
Forehead thermometers require a bit more technique than most people realize. Place the sensor flat against the center of the forehead, away from hair or clothing. Press the scan button and slowly glide the thermometer in a straight line across the forehead toward the ear, keeping the sensor in direct contact with the skin the entire time. Some models then ask you to touch the sensor briefly to the soft spot just behind the earlobe to verify the reading. Make sure that area is free of hair or earrings.
The key mistakes that lead to bad readings: holding the thermometer too far from the skin, scanning too quickly, or measuring right after the person has been outside in cold or hot weather. Let them acclimate indoors for a few minutes first.
Factors That Skew Your Reading
A number of everyday situations can push your temperature up without an actual infection. Exercise raises body temperature for 30 minutes or more afterward. Heavy clothing or blankets can artificially warm the skin. Hot baths have the same effect. Time of day matters too: a reading taken at 6 a.m. will typically be lower than one taken at 4 p.m., so a borderline number in the afternoon may not mean much.
If you get a surprising result, wait 15 to 20 minutes and measure again. A single reading that seems off is worth rechecking before you worry about it.
Checking for Fever Without a Thermometer
If you don’t have a thermometer handy, you can get a rough sense by placing the back of your hand on the person’s forehead or neck. Skin that feels noticeably hot compared to your own suggests a fever, but this method can’t give you a number and is unreliable for detecting low-grade fevers. Other clues include flushed cheeks, chills, sweating, glassy eyes, and general achiness. These signs can prompt you to get a proper reading, but they aren’t a substitute for one.
Cleaning Your Thermometer
Wipe the thermometer tip with rubbing alcohol or an alcohol swab before and after each use. Research on hospital thermometer disinfection found that a simple alcohol wipe is at least as effective as soaking in disinfectant solutions. Rinse with cool water afterward (hot water can damage the sensor). If your household shares a thermometer, cleaning between users is especially important.
Fever in Babies and Young Children
Any fever of 100.4°F or higher in a baby younger than three months old warrants an immediate call to your pediatrician, regardless of how the baby seems otherwise. At that age, a fever can signal a serious infection that’s hard to detect by observation alone.
For older infants and children, the temperature number matters less than how the child is acting. A child with 101°F who is playing and drinking fluids is generally less concerning than one with 100.5°F who is limp, confused, or making poor eye contact. Watch for repeated vomiting, a severe headache, a stiff neck, or a seizure associated with the fever. A fever that develops after a child has been left in a hot car is a medical emergency.
Warning Signs in Adults
Most fevers in adults resolve on their own within a few days. But certain symptoms alongside a fever point to something more serious:
- Stiff neck with pain when bending your head forward
- Severe headache or unusual sensitivity to bright light
- Difficulty breathing or chest pain
- Mental confusion, altered speech, or strange behavior
- Persistent vomiting
- New rash
- Pain when urinating
- Seizures
Any of these paired with a fever calls for prompt medical attention. A fever above 103°F in an adult also warrants a call to your doctor, even without additional symptoms.