A fever in adults and children is a temperature of 100.4°F (38°C) or higher when measured orally, rectally, or in the ear. This is the standard threshold used by the CDC and most medical guidelines. But the number you see on your thermometer depends on where and when you take it, and understanding those differences matters for getting an accurate reading.
The Standard Fever Threshold
The widely accepted cutoff for a fever is 100.4°F (38°C). This applies to oral, rectal, ear, and temporal artery (forehead) readings. Armpit readings run lower, so the fever threshold there is 99°F (37.2°C). These numbers apply to both adults and children.
Below the official fever line, there’s a gray zone. Temperatures between 99.1°F and 100.4°F (37.3 to 38.0°C) are considered a low-grade fever. You might feel warm, slightly off, or achy, but your body hasn’t mounted a full fever response yet. Many viral infections hover in this range for a day or two before climbing higher or resolving on their own.
Why Your Reading Depends on Where You Measure
Not all thermometer placements give the same number. Rectal temperatures read the highest because they measure your core body temperature most directly. Oral readings run about 0.5°F (0.25°C) lower than rectal, and armpit readings fall roughly 0.8°F (0.43°C) below rectal. This is why armpit readings use a lower fever threshold of 99°F rather than 100.4°F.
Armpit measurements are the least reliable. If you get an armpit reading that seems borderline, it’s worth confirming with an oral or ear thermometer. For infants under 3 months, rectal thermometers are the gold standard because the stakes of missing a fever at that age are high.
Normal Temperature Isn’t Always 98.6°F
The old benchmark of 98.6°F as “normal” is an average, not a fixed number. Your body temperature naturally fluctuates by 1 to 1.5°F over the course of a day. It tends to be lowest in the early morning and peaks in the late afternoon or evening. So a reading of 99°F at 6 p.m. might be perfectly normal for you, while the same reading at 7 a.m. could signal the beginning of a fever.
Age, activity level, and hormonal cycles also shift your baseline. Older adults often run cooler than younger people, which means a temperature of 100°F in someone over 65 may represent a more significant immune response than the number alone suggests.
Getting an Accurate Reading
If you’re taking an oral temperature, wait 20 to 30 minutes after eating, drinking, or smoking. A cup of hot coffee or a glass of ice water will temporarily skew the reading in either direction. Place the thermometer under your tongue, toward the back, and keep your mouth closed until it beeps.
For ear thermometers, gently tug the ear up and back to straighten the ear canal before inserting the probe. Forehead thermometers are convenient but can be affected by sweat, ambient temperature, or skin products. If you get a reading that doesn’t match how you feel, try again or switch methods.
Fever Levels and What They Mean
Not all fevers carry the same weight. Here’s how the range breaks down for adults:
- Low-grade (99.1 to 100.4°F): Common with mild infections and often resolves without treatment. You may feel slightly warm or fatigued.
- Moderate (100.4 to 103°F): A clear sign your immune system is actively fighting something. Rest, fluids, and over-the-counter fever reducers can help manage discomfort.
- High (103 to 106°F): Warrants close attention. At this level you may experience dizziness, nausea, a fast heart rate, or significant fatigue.
- Hyperpyrexia (above 106.7°F / 41.5°C): A medical emergency. This level of heat can cause confusion, seizures, loss of consciousness, and organ damage without rapid treatment.
Fever Thresholds for Children
The 100.4°F threshold applies to children too, but the urgency depends heavily on age. Any fever in a baby under 3 months old requires immediate medical attention, even if the baby seems otherwise fine. The immune system at that age is too immature to reliably fight certain infections, and a fever may be the only visible sign of something serious.
For children between 3 months and 3 years, a fever that lasts more than a couple of days, goes above 102°F, or comes with symptoms like repeated vomiting, stiff neck, or unusual drowsiness deserves a call to your pediatrician. Older kids and teens can generally be managed at home with fluids and rest unless the fever climbs above 103°F or they develop concerning symptoms.
When a Fever Signals Something Urgent
The number on the thermometer matters, but so does the full picture. In adults, a fever above 103°F that doesn’t respond to fever-reducing medication, or any fever paired with confusion, a severe headache, chest pain, stiff neck, or a rash, points toward something that needs prompt evaluation. Persistent vomiting or diarrhea alongside a fever raises the risk of dehydration, which can become its own problem quickly.
After exposure to extreme heat (like being in a hot car), a high temperature without sweating is especially dangerous. This pattern suggests the body’s cooling system has failed, which is a hallmark of heatstroke rather than a typical infection-driven fever.