For adults, a temperature of 100.4°F (38°C) or higher is considered a fever. This is the threshold used by both the CDC and the Mayo Clinic, regardless of whether you measure orally, rectally, or with an ear thermometer. The one exception is armpit readings, where 99°F (37.2°C) or higher signals a fever because the skin surface runs cooler than your core.
Why the Measurement Site Matters
Not all thermometer readings are equal. Rectal and ear thermometers measure closer to your body’s true core temperature, while an armpit thermometer sits on the skin’s surface and typically reads about a degree lower. That’s why the fever cutoff for armpit measurements is set at 99°F rather than 100.4°F.
Oral thermometers fall somewhere in between. Drinking hot or cold liquids within 15 minutes of taking your temperature can throw off the reading. If you’ve just had coffee or ice water, wait before checking. For the most consistent oral reading, place the thermometer under your tongue toward the back of your mouth and keep your lips closed.
Temporal artery thermometers (the kind you swipe across the forehead) use the same 100.4°F threshold as oral and rectal readings. They’re convenient but can be less reliable if you’re sweating heavily, since evaporation cools the skin.
Low-Grade Fever vs. High Fever
Temperatures between 99°F and 100.3°F are often called a “low-grade fever,” though there’s no universally agreed-upon clinical definition for that term. Many people run slightly above or below the old 98.6°F benchmark throughout the day, so a reading of 99.1°F on its own isn’t necessarily a sign of illness. Body temperature naturally rises in the late afternoon and drops in the early morning, sometimes by a full degree.
Once you cross the 100.4°F line, you’re in clear fever territory. Fevers from common infections like the flu tend to land between 100°F and 102°F and last three to four days. A temperature above 103°F (39.4°C) is considered a high fever and warrants closer attention, especially if it doesn’t come down with over-the-counter fever reducers or persists beyond a couple of days. A reading at or above 105°F (40.6°C) is a medical emergency.
What Happens in Your Body During a Fever
Your brain has a built-in thermostat located in a region called the hypothalamus. When your immune system detects an infection, it releases signaling molecules that reach the hypothalamus and effectively turn the thermostat up. The hypothalamus then triggers a cascade of responses: your blood vessels constrict to conserve heat, your metabolism ramps up to generate more of it, and sweating slows down so less heat escapes through your skin.
This is why you feel cold and shivery at the start of a fever, even though your temperature is actually climbing. Your body is trying to close the gap between your current temperature and the new, higher set point. Once your temperature reaches that set point, the chills stop. Later, when your immune system starts winning and the thermostat resets back to normal, you begin sweating as your body works to shed the extra heat. These alternating waves of chills and sweats are a hallmark of fever and a sign the cycle is progressing normally.
Fever Thresholds in Older Adults
Adults over 65 often run a lower baseline temperature than younger adults. Several factors contribute to this: thinning skin and less insulating fat in the extremities, a slower metabolism, and common medications like beta blockers that can suppress body heat. An older adult’s “normal” might sit closer to 97°F or even lower.
This means a reading of 99°F or 100°F in an older person can represent the same degree of immune response that would show up as 101°F or 102°F in a younger adult. A strict 100.4°F cutoff can miss genuine fevers in this group. If you’re caring for someone over 65, pay attention to any temperature that’s notably above their personal baseline, even if it falls below the standard threshold. Combine that with other signs like confusion, fatigue, or loss of appetite to gauge whether something is off.
When a Number Isn’t Available
If you don’t have a thermometer handy, the CDC recognizes several alternatives: feeling warm to the touch (particularly on the forehead or chest), visible facial flushing, chills, or a self-reported history of feeling feverish. These aren’t as precise as a measured reading, but they’re worth noting, especially if you’ve recently taken a fever-reducing medication that could mask a true temperature spike.
Over-the-counter fever reducers can lower your reading by one to two degrees, so a “normal” temperature after taking one doesn’t necessarily mean the fever has resolved. If you’re tracking a fever, try to take your temperature at consistent times and note whether you’ve taken any medication beforehand.
Practical Fever Management
Most fevers in adults are caused by viral infections and resolve on their own within a few days. Staying hydrated is the single most important thing you can do, since fever increases fluid loss through sweat and faster breathing. Water, broth, and electrolyte drinks all help.
You don’t always need to treat a fever with medication. A mild fever is part of your immune system’s defense, and there’s no evidence that letting a low-grade fever run its course slows your recovery. That said, if a fever is making you miserable with headaches, body aches, or disrupted sleep, over-the-counter options can bring relief and help you rest.
Lightweight clothing and a comfortable room temperature are more effective than piling on blankets during the chills phase, even though bundling up feels instinctive. Adding heavy layers traps heat and can push your temperature higher. A lukewarm (not cold) washcloth on the forehead can also help you feel more comfortable without shocking your system.