A fever starts at 100.4°F (38°C) when measured rectally, at the forehead, or in the ear. If you’re using an oral thermometer (the most common method for adults), a reading of 100°F (37.8°C) or higher counts as a fever. An armpit reading of 99°F (37.2°C) or higher qualifies. The threshold depends on where you take the measurement because different parts of the body run at slightly different temperatures.
Why the Number Depends on the Thermometer
Your core body temperature is highest deep inside your body and cooler at the surface. A rectal thermometer sits closest to your core, so it gives the highest and most accurate reading. An oral thermometer reads about 0.5 to 1°F lower than rectal. An armpit thermometer reads another 0.5 to 1°F lower than oral. Ear (tympanic) thermometers tend to read close to rectal, while forehead scanners typically run 0.5 to 1°F lower than an oral reading.
This is why the fever cutoff shifts depending on the method:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
The CDC uses 100.4°F (38°C) as its standard threshold for public health screening. If you’ve ever been temperature-checked at an airport or clinic entrance, that’s the number they were looking for.
What Counts as “Normal” Body Temperature
The textbook number of 98.6°F (37°C) dates back to the 1800s and is still used as a general reference point, but normal body temperature actually spans a range. Healthy people can sit anywhere from 97°F (36.1°C) to 99°F (37.2°C) and be perfectly fine.
Your temperature also shifts throughout the day. It tends to be lowest in the early morning and highest in the late afternoon or evening, with a typical swing of 0.5 to 1.9°F over a 24-hour cycle. So a reading of 99°F at 4 p.m. may be completely normal for you, while the same reading at 6 a.m. could signal the start of a fever. Older adults also tend to run cooler at baseline, which means a temperature that looks “normal” on paper could actually represent a meaningful rise for them.
Low-Grade, Moderate, and High Fevers
Not all fevers carry the same weight. Harvard Health classifies them into three tiers:
- Low-grade: 99.1 to 100.4°F (37.3 to 38°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41°C)
A low-grade fever is the kind you might barely notice, perhaps feeling slightly warm or a little off. Moderate fevers are where most people start feeling genuinely sick, with chills, body aches, and fatigue. High-grade fevers are more intense and more likely to need active management, though the fever itself is rarely dangerous in adults until it climbs past 103°F or so.
How a Fever Actually Works
A fever isn’t a malfunction. It’s a deliberate response orchestrated by your brain. When your immune system detects an infection, white blood cells release signaling proteins that travel through the bloodstream to a temperature-control center in the brain called the hypothalamus. These signals trigger the release of a chemical messenger that essentially turns up the thermostat, raising the target temperature your body tries to maintain.
Once the new set point is established, your body acts as though it’s too cold. Blood vessels near the skin constrict to conserve heat (that’s why you feel chilled), and your muscles may shiver to generate warmth. You pile on blankets even though you’re technically running hot. This continues until your blood temperature matches the new, higher set point. The elevated temperature helps your immune system fight infection more effectively, which is why mild fevers are generally considered beneficial rather than harmful.
When to Treat a Fever
A fever below 102°F in an otherwise healthy adult or older child usually doesn’t need medication. Rest and fluids are enough. The fever is doing useful work, and suppressing it doesn’t make you recover faster.
Fever-reducing medication like acetaminophen or ibuprofen makes more sense once the temperature climbs above 102°F (38.9°C), especially if you’re uncomfortable. For children between 6 months and 17 years, the same 102°F threshold applies. Children under 6 months should only take acetaminophen, not ibuprofen. The goal of medication isn’t to get your temperature back to 98.6°F. It’s to bring it down a degree or two so you can rest, sleep, and stay hydrated.
Fever Thresholds for Babies and Young Children
The rules change significantly for infants. A baby under 28 days old with a rectal temperature of 100.4°F or higher needs emergency care immediately, even if the baby looks fine. At that age, a fever can signal a serious infection, and the immune system is too immature to fight it reliably on its own.
For babies between 1 and 3 months old, a rectal temperature of 100.4°F or higher warrants a call to the pediatrician right away. If you can’t reach them or be seen quickly, head to the emergency department rather than an urgent care center. Urgent care facilities often aren’t equipped to run the tests a young infant may need.
For children 3 months and older, the urgency shifts. A fever of 105°F (40.5°C) or above is the threshold for concern, particularly if it doesn’t drop by 1 to 2 degrees after fever-reducing medicine. But regardless of the number on the thermometer, certain symptoms alongside any fever warrant emergency evaluation: seizures, difficulty breathing, a stiff neck, blue-tinted lips or nails, extreme sleepiness or difficulty waking, a rash with purple spots, or nonstop inconsolable crying.
Getting an Accurate Reading
The most accurate method for adults is an oral thermometer placed under the tongue with the mouth closed for the recommended time (usually 30 to 60 seconds for digital models). Avoid eating or drinking anything for 15 minutes beforehand, since hot coffee or ice water will throw off the reading.
For babies and toddlers, rectal thermometers remain the gold standard because they’re the most reliable at that age. Forehead and ear thermometers are faster and more convenient for older kids and adults, but they’re slightly less precise. Armpit readings are the least accurate of all, so if an armpit reading looks borderline, it’s worth rechecking with an oral or rectal thermometer before deciding how to respond.
If you’re tracking a fever over time, use the same thermometer and the same body site each time. Comparing an armpit reading from the morning to an oral reading in the evening doesn’t give you useful information about whether the fever is rising or falling.