A fever in adults is any body temperature above 100.4°F (38°C), measured with a thermometer. That single number is the standard threshold used across medical guidelines, regardless of age. If you don’t have a thermometer handy, certain physical signs can suggest a fever, but a thermometer is the only way to confirm one.
What Counts as a Fever
The old benchmark of 98.6°F as “normal” body temperature dates back to an 1868 study. Modern research puts the actual average closer to 97.9°F (36.6°C), and healthy people routinely sit anywhere between 97°F and 99°F depending on the time of day, their age, and how active they’ve been. Your temperature is naturally lowest in the early morning and peaks in the late afternoon.
Because normal runs across a range, a single reading of 99.5°F doesn’t necessarily mean you’re sick. The widely accepted cutoff is 100.4°F (38°C). Anything between your baseline and that threshold is sometimes called a low-grade fever, which can reflect mild illness, ovulation, heavy exercise, or simply a warm environment. A reading above 103°F (39.4°C) in an adult is considered a high fever and warrants closer attention.
Choosing the Right Thermometer
Not all thermometers are equally reliable. In a study comparing several commercially available devices against a laboratory-grade standard, ear (tympanic) thermometers came out on top for accuracy, with an average error of less than 0.1°F. Oral (under-the-tongue) thermometers performed well at stable body temperatures but tended to underread during temperature changes. Forehead (temporal artery) thermometers consistently overestimated temperature, sometimes by more than 1°F. Infrared forehead guns and thermal cameras were the least accurate overall.
For most households, a digital oral thermometer or an ear thermometer is a solid choice. Rectal thermometers give the closest reading to true core body temperature and are recommended for infants, but most adults find oral or ear measurement more practical.
How to Take Your Temperature
Oral (Under the Tongue)
Place the tip of the thermometer under your tongue, toward the back, and close your lips around it. Breathe through your nose and wait for the beep. If you’ve eaten, had something to drink, or smoked, wait at least 30 minutes before measuring. Hot coffee or ice water can throw off the reading significantly. Oral readings typically run about 1°F lower than rectal, so a reading of 99.5°F orally could correspond to a core temperature closer to 100.5°F.
Ear (Tympanic)
Pull your ear gently up and back to straighten the ear canal, then insert the probe tip snugly. The reading takes just a few seconds. Use the same ear each time for consistency, as readings can vary slightly between sides. Earwax buildup or an ear infection can affect accuracy.
Forehead (Temporal)
Swipe the thermometer across your forehead or hold a no-contact device a few inches away, following the manufacturer’s directions. These are fast and convenient, especially for sleeping children, but keep in mind they tend to read a bit high. Sweat on the forehead can also skew results downward.
Armpit (Axillary)
Place the thermometer tip in the center of your armpit and press your arm snugly against your body. This method is the least accurate for adults, typically reading 1 to 2°F below core temperature. It’s better than nothing when other options aren’t available, but add roughly a degree to the displayed number for a more realistic estimate.
Checking Without a Thermometer
If you don’t have a thermometer, your body gives several clues. The most common signs of fever include chills and shivering (your body’s way of generating heat), sweating, headache, muscle aches, loss of appetite, and a general feeling of weakness. Feeling hot to the touch, especially on the forehead or back of the neck, is a classic indicator, though the hand doing the touching needs to be at a normal temperature itself for this to be meaningful.
None of these symptoms are definitive on their own. Chills can come from cold exposure, and sweating can come from anxiety. But when several of these show up together, especially alongside an obvious illness like a sore throat or cough, a fever is likely. Still, you can’t gauge the actual number without a thermometer, which matters when deciding whether a fever is mild or high enough to need medical attention.
Fever Thresholds in Children
The same 100.4°F cutoff applies to children, but the stakes are different depending on age. For babies under 3 months, any temperature at or above 100.4°F is considered urgent and needs prompt medical evaluation. At that age, a fever can be the only visible sign of a serious infection. For babies between 3 and 6 months, temperatures above 102.2°F (39°C) put them in a higher-risk category. For children over 3, a fever above 103°F (39.4°C) is the threshold for heightened concern.
Rectal temperature is the gold standard for infants because it’s closest to core body temperature. Forehead or ear thermometers are reasonable for older children, but if you get a borderline reading and your child seems unwell, a rectal check gives the most trustworthy number.
Getting an Accurate Reading
A few common mistakes can make your reading unreliable. Taking your temperature right after drinking hot tea or exercising will give you a falsely high number. Measuring immediately after coming in from cold weather can produce an artificially low one. For the most accurate result, sit calmly indoors for 15 to 30 minutes before measuring.
Time of day matters too. Your body temperature can be a full degree lower in the morning than in the evening. If you’re tracking a fever over time, try to measure at roughly the same times each day so you can compare readings meaningfully.
Clean your thermometer before and after each use. Wiping the probe with a cotton ball or cloth soaked in 70% isopropyl alcohol, then rinsing under cool water and drying, is sufficient for digital and glass thermometers. For non-contact models, a gentle wipe with an alcohol pad over the sensor works. Avoid submerging any thermometer that isn’t labeled waterproof.
What a Fever Feels Like as It Rises and Breaks
When a fever is climbing, you’ll typically feel cold even though your body temperature is actually going up. That’s because your brain has temporarily reset its internal thermostat higher, so your current temperature feels “too low” by comparison. This triggers shivering and the urge to pile on blankets. Once your temperature reaches the new set point, the chills usually stop and you feel hot instead.
When the fever breaks, the reverse happens. Your brain resets downward, and your body sheds heat through sweating. You might soak through your clothes or sheets. This is a normal part of the cycle, not a sign that something is wrong. Staying hydrated throughout is important because both sweating and the fever itself increase fluid loss.