The most accurate way to tell your temperature is with a digital thermometer placed under the tongue, in the ear, or on the forehead. But the “normal” number you’re aiming for has changed: modern research puts average body temperature closer to 97.5°F (36.4°C), not the 98.6°F (37°C) figure most of us learned growing up. That old number dates back to a single study from the 1860s. An analysis of 20 studies spanning decades found average oral temperature had dropped by more than a full degree, and a separate study of over 35,000 people landed on 97.9°F.
Knowing what’s normal for you matters, because it changes how you interpret a reading. Here’s how to get an accurate temperature using different methods, what counts as a fever, and how to estimate temperature when you don’t have a thermometer at all.
Which Thermometer Type Is Most Accurate
Ear (tympanic) thermometers tend to be the most accurate consumer option. In testing against a gold standard, a tympanic thermometer averaged just 0.04°C off from the true reading, which falls within the clinical ideal of ±0.1°C accuracy. Oral digital thermometers come in close behind, typically reading about 0.19°C lower than core body temperature.
Forehead and temporal artery thermometers are convenient, especially for children, but they’re the least consistent. Different models in the same study ranged from 0.18°C below to 0.43°C above true temperature. Infrared thermal cameras performed even worse, especially at greater distances. If you’re using a forehead scanner, it’s a reasonable screening tool, but don’t rely on it to catch a borderline fever.
Mercury glass thermometers are no longer recommended. Mercury is toxic to both people and the environment, and most medical institutions have phased them out. If you still have one, replace it with a standard digital thermometer or an alcohol-filled glass thermometer (the ones with red or blue liquid inside).
How to Take an Oral Temperature Correctly
Place the tip of a digital thermometer under your tongue, toward the back of your mouth, and keep your lips closed until it beeps. That sounds simple, but several things can throw off the reading significantly.
Drinking hot or cold liquids before measuring will skew the result. So will breathing through your mouth, being in a very hot or cold room, or even having a warm compress on your head. One study found that exposure to cold air (around −5°C) dropped oral readings by about 0.7°C compared to rectal temperature, likely because inhaling cold air cools the mouth directly. For the most reliable number, wait at least 15 minutes after eating, drinking, or coming in from extreme temperatures before taking your reading.
How to Use a Forehead Thermometer
The FDA recommends holding a non-contact infrared thermometer perpendicular to the forehead, with the sensing area pointed at clean, dry skin. The exact distance varies by manufacturer, so check your device’s instructions. Make sure the forehead isn’t covered by hair, a headband, or sweat, and avoid taking a reading right after someone has been wearing a hat or using facial cleansing wipes. The person being measured should hold still during the scan.
What Temperature Counts as a Fever
The standard medical threshold for fever is 100.4°F (38.0°C), regardless of age. But context matters, especially for young children.
- Babies 3 months and younger: Any reading at or above 100.4°F is considered urgent. Some clinical guidelines flag temperatures as low as 99.4°F in this age group as worth watching.
- Infants 3 to 36 months: Fever starts around 99.6°F, with high fever above 101.3°F (38.5°C).
- Children over 3, teens, and adults: Mild fever begins around 99.9°F. High fever is anything above 103.0°F (39.4°C).
Keep in mind that body temperature naturally fluctuates throughout the day. It’s typically lowest in the early morning and highest in the late afternoon, with a swing of about 1°F. A reading of 99.2°F at 4 p.m. may be completely normal for you, even though it would look slightly elevated at 6 a.m.
How to Tell if You Have a Fever Without a Thermometer
No thermometer gives you a number, but your body offers several clues that together paint a reliable picture.
Touch the forehead with the back of your hand (not your palm, which is less heat-sensitive). If the skin feels noticeably hot compared to your own, that’s a strong signal. This works best when someone else checks, since your own hands may also be warm if you’re running a fever.
Look in a mirror. Flushed or unusually red cheeks often accompany a fever, particularly in children. In infants, the skin may look generally reddened, and the child may be unusually irritable or have trouble feeding.
Check for dehydration. Pinch the skin on the back of your hand and release it. If it snaps back immediately, you’re reasonably hydrated. If it moves slowly, that can signal the fluid loss that comes with fever. Dark yellow or orange urine with a strong odor points in the same direction.
Other common signs include chills and shivering (which happen as your body raises its internal thermostat), sweating, headache, body aches, fatigue, difficulty concentrating, and swollen lymph nodes along the jaw or neck. None of these alone confirms a fever, but a cluster of them is a fairly dependable indicator.
Tracking Basal Body Temperature
If you’re measuring temperature for fertility tracking rather than illness, the rules are stricter. Basal body temperature is your lowest resting temperature, and it shifts by only about 0.2 to 0.5°F after ovulation. To catch that small change, take your temperature every morning before getting out of bed, at the same time each day, after at least three hours of uninterrupted sleep. Use a digital oral thermometer or a dedicated basal body temperature thermometer, which reads to an extra decimal place. Even sitting up or walking to the bathroom can raise your temperature enough to mask the ovulation shift.
Measuring Room Temperature Accurately
If your search was about air temperature rather than body temperature, placement is everything. The National Institute of Standards and Technology recommends keeping a thermometer away from walls, windows, direct sunlight, and pavement or concrete surfaces, all of which absorb and radiate heat that will inflate your reading. The thermometer should have good airflow around it. Professional weather stations use slatted enclosures that let air circulate freely while shielding the sensor from the sun. At home, placing a thermometer on an interior wall away from windows and heating vents gives the most representative reading of actual room temperature.