A temperature of 100.4°F (38°C) or higher is considered a fever. That’s the standard threshold used by the CDC and most medical guidelines. But the number that counts as a fever actually shifts depending on where you take your temperature, your age, and even the time of day.
The Standard Fever Threshold
The widely accepted cutoff is 100.4°F (38°C), measured rectally, in the ear, or with a temporal artery (forehead) thermometer. If you’re taking your temperature by mouth, a reading of 100°F (37.8°C) or higher indicates a fever. An armpit reading of 99°F (37.2°C) or higher also qualifies, since the armpit runs cooler than internal body temperature.
These differences matter more than most people realize. Armpit readings can be a full degree or more below your core temperature, so a number that looks “normal” under your arm might actually be a fever if measured orally or rectally. Rectal temperatures are the most accurate reflection of core body temperature, which is why they’re the standard for infants and young children.
Why 98.6°F Isn’t Really “Normal” Anymore
The idea that 98.6°F is normal body temperature dates back to an 1851 study, and it’s been gradually falling out of favor. A large analysis of over 35,000 British patients found the average oral temperature was actually 97.9°F (36.6°C). Research published in eLife found that average human body temperature has been slowly dropping by about 0.05°F per decade since the Industrial Revolution. The reasons aren’t entirely clear, but reduced rates of chronic infection and changes in living conditions likely play a role.
This means many people walk around with a baseline closer to 97.5°F or 98°F. If your personal baseline is lower than the old 98.6°F standard, a reading of 99.5°F might feel feverish to you even though it falls below the official threshold.
Low-Grade, Moderate, and High Fevers
Not all fevers carry the same level of concern. Harvard Health breaks fever into three ranges:
- Low-grade: 99.1°F to 100.4°F (37.3°C to 38°C). This is the gray zone. You may feel warm or slightly off, and your body is mounting an early immune response. Many doctors don’t consider this a true fever, but it’s worth monitoring.
- Moderate: 100.6°F to 102.2°F (38.1°C to 39°C). This is the most common fever range during infections like colds, flu, or COVID. It’s uncomfortable but generally not dangerous in otherwise healthy adults.
- High: 102.4°F to 105.8°F (39.1°C to 41°C). Fevers in this range warrant closer attention, especially if they persist or are accompanied by confusion, stiff neck, difficulty breathing, or severe pain.
Why Your Temperature Changes Throughout the Day
Your body temperature isn’t a fixed number. It follows a daily rhythm, hitting its lowest point in the early morning (around 4 to 6 a.m.) and peaking in the early evening. The difference between your daily low and high can range from 0.5°F to nearly 2°F. This is why you might feel fine in the morning and feverish by dinnertime, even though the infection hasn’t changed. You’re not imagining it. Your body’s natural temperature cycle is amplifying the fever in the evening hours.
Other factors push your temperature up temporarily too: exercise, hot drinks, heavy clothing, and ovulation in women can all raise readings by a degree or more without any illness involved. If you’re checking for a fever, take your temperature after sitting calmly for a few minutes.
How Fever Works in Your Body
Fever isn’t a malfunction. It’s a deliberate response. When your immune system detects an infection, it releases signaling molecules that act on the brain’s temperature control center. This region essentially raises the thermostat, making your body think its current temperature is too low. That’s why you shiver and feel cold at the start of a fever: your brain is trying to generate more heat to reach the new, higher set point.
This elevated temperature appears to help your immune system work more efficiently, making it harder for some bacteria and viruses to replicate. That said, the fever itself can make you miserable, and very high fevers can become dangerous on their own.
Fever Thresholds for Babies and Children
The 100.4°F threshold applies to children as well, but the stakes are higher for infants. Any baby under 3 months old with a rectal temperature of 100.4°F or higher needs immediate medical evaluation, even if the baby seems otherwise fine. Young infants have immature immune systems, and fever can be the only visible sign of a serious bacterial infection.
For older children, the number on the thermometer matters less than how the child is behaving. A toddler with a 102°F fever who is still drinking fluids and playing is generally less concerning than one with a 101°F fever who is listless, inconsolable, or refusing to drink. The fever’s trajectory and accompanying symptoms tend to be more telling than the exact number.
Which Thermometer to Use
Digital thermometers are the current standard for home use. Forehead (temporal artery) thermometers are convenient and reasonably accurate. Ear thermometers work well for children over 6 months but can give unreliable readings if earwax blocks the canal or the probe isn’t angled correctly. Oral thermometers are reliable for older children and adults, though eating or drinking within 15 minutes before a reading can skew the result. Armpit measurements are the least accurate but can still be useful as a quick screen.
For infants under 3 months, a rectal thermometer remains the gold standard because it provides the most accurate core temperature reading. If you get a concerning reading from a forehead or ear thermometer, confirming with a rectal measurement gives you the clearest answer.