A fever in adults is a body temperature of 100.4°F (38°C) or higher. The most reliable way to know if you have one is with a thermometer, but your body also sends clear signals, like chills, aching muscles, and feeling hot to the touch, that can tip you off even without one.
Fever Temperature Ranges
Not all fevers are equal. The severity breaks down like this:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
A low-grade fever often resolves on its own and is your body doing its job. A high-grade fever needs closer attention and usually calls for a fever-reducing medication.
Why Your “Normal” Might Not Be 98.6°F
The 98.6°F number comes from a 19th-century study, and modern research shows that normal body temperature varies from person to person. Your temperature also shifts throughout the day. It tends to be lowest in the early morning and rises in the late afternoon. Most people experience a small dip between 2 p.m. and 4 p.m. as well. This means a reading of 99°F at 6 p.m. could be completely normal for you, while the same reading at 7 a.m. might signal the start of a fever.
The best way to know your personal baseline is to take your temperature a few times on a day you feel healthy. That gives you a reference point so a slightly elevated number later doesn’t cause unnecessary worry.
How to Take Your Temperature Accurately
A digital oral thermometer is the most practical option for adults. To get a reliable reading, wait at least 30 minutes after eating or drinking anything. Place the thermometer tip under your tongue, close your mouth, and keep it there until the device beeps. Read the number immediately.
Temperatures vary slightly depending on where you measure them. An underarm (axillary) reading tends to run lower than an oral one, while a rectal reading runs higher. There’s no exact formula for converting between them, so pick one method and stick with it. Comparing an oral reading from this morning to a forehead reading tonight won’t tell you much. Consistency is what makes temperature tracking useful.
Signs of a Fever Without a Thermometer
If you don’t have a thermometer handy, your body gives you several clues. The most common signs include:
- Chills and shivering, even when the room is warm
- Sweating, sometimes alternating with chills
- Headache
- Muscle soreness and aching
- Fatigue and difficulty concentrating
- Loss of appetite
- Sore or heavy-feeling eyes
- Swollen lymph nodes in the neck or armpits
One of the most telling signs is feeling simultaneously hot and cold. You might be flushed and warm to the touch while shivering under a blanket. That combination is a strong indicator of fever rather than simply being overheated. You can place the back of your hand on your forehead or neck to check for unusual warmth, but this method is imprecise. It’s more useful when someone else checks you, since your own hands may also be warm from the fever.
What’s Actually Happening in Your Body
A fever isn’t a malfunction. It’s a deliberate response controlled by the part of your brain that acts as a thermostat. When your immune system detects an infection, it releases chemical signals that raise your brain’s target temperature. Your body then works to reach that new, higher set point the same way it would try to warm up on a cold day: blood vessels near your skin constrict to trap heat, your metabolism speeds up to generate more warmth, and your muscles contract rapidly (shivering) to produce heat.
This is why you feel cold at the start of a fever. Your actual body temperature hasn’t caught up to the new target yet, so your brain perceives a gap and triggers heat-seeking behavior. You pile on blankets, curl up, and shiver. Once your temperature reaches the higher set point, the chills ease. When the fever breaks, the thermostat resets back to normal, and suddenly you’re too warm. That’s when the sweating kicks in as your body dumps the excess heat.
Fever vs. Overheating
A fever and overheating (hyperthermia) can look similar on a thermometer, but they work in opposite ways. A fever is your brain intentionally raising your temperature as part of an immune response. Overheating happens when your body can’t cool itself fast enough, typically from intense exercise in hot weather, prolonged sun exposure, or being in an overheated environment.
The practical difference matters. Fever-reducing medications like acetaminophen work on fevers because they lower the brain’s temperature set point. They do not work on overheating, because the brain’s thermostat isn’t the problem. If you suspect overheating rather than fever, especially if it follows physical exertion or heat exposure, cooling the body directly with cold water, shade, and airflow is what helps.
Fever Thresholds for Children
Children, especially infants, have different rules. For babies under 3 months old, any fever at all (or a temperature below 97.7°F rectally) warrants a call to a healthcare provider. In children older than 3 months, the concern level rises if the temperature exceeds 104°F (40°C) or if the fever doesn’t respond to fever-reducing medication. A fever lasting more than five days in any child may need further investigation regardless of how high it is.
When a Fever Becomes Serious
Most fevers in otherwise healthy adults are harmless and resolve within a few days. Certain symptoms alongside a fever, however, signal something more urgent. A high-grade fever above 103°F that doesn’t come down with medication, confusion or unusual drowsiness, a stiff neck, a rash that doesn’t fade when you press on it, difficulty breathing, or persistent vomiting are all reasons to seek immediate care. These symptoms can point to infections that need treatment beyond what your immune system can handle on its own.
A fever that keeps returning over weeks, even if it’s low-grade, can sometimes indicate an underlying condition like an autoimmune disorder or, less commonly, certain cancers. If you’re dealing with recurring fevers without an obvious cause like a cold or flu, it’s worth getting checked out rather than assuming it will pass.