A fever in adults is a body temperature at or above 100.4°F (38°C), measured orally. That’s the standard threshold used across most medical guidelines. Anything between 99.1°F and 100.4°F is considered a low-grade fever, which signals your body is responding to something but hasn’t crossed into true fever territory yet.
Why 98.6°F Isn’t the Whole Story
Most people grow up hearing that 98.6°F is “normal,” but body temperature naturally fluctuates throughout the day. It tends to be lowest in the early morning and highest in the late afternoon. Exercise, hormonal cycles, heavy meals, and even the weather can nudge your baseline up or down. A reading of 99°F at 4 p.m. after a long walk is probably not meaningful, while that same number at 6 a.m. when you’re feeling lousy tells a different story.
What matters more than any single number is context: how you feel, when you took the reading, and where on your body you measured.
How Measurement Site Changes the Number
The temperature you get depends on where you place the thermometer. Rectal and ear readings run about 0.5 to 1°F higher than an oral reading. Armpit and forehead readings run about 0.5 to 1°F lower than oral. So if your forehead thermometer says 99.5°F, that could be equivalent to an oral reading closer to 100°F.
Here’s a quick guide to fever thresholds by measurement site for adults:
- Oral: 100.4°F (38°C)
- Rectal or ear: approximately 100.9°F to 101.4°F (38.3 to 38.6°C)
- Armpit or forehead: approximately 99.4°F to 99.9°F (37.4 to 37.7°C)
Rectal thermometers are the most accurate, but oral thermometers provide similar accuracy and are far more practical for adults. Forehead thermometers are convenient but can give misleading results in direct sunlight, cold air, or if the forehead is sweaty.
Fever Thresholds for Babies and Children
The rules change significantly for young children, especially infants. For a baby under 3 months old, a rectal or forehead temperature of just 100.4°F (38°C) warrants a call to the doctor. That same threshold that’s routine in adults is treated seriously in newborns because their immune systems are still developing, and infections can escalate quickly.
For children between 3 months and 3 years, the concern level shifts upward. A rectal or forehead reading of 102°F (38.9°C) or higher is the point to contact your pediatrician. Ear thermometers follow the same 102°F threshold but are only reliable for children older than 6 months. For any child at any age, an armpit temperature above 103°F or any temperature above 104°F calls for medical attention regardless of how the child seems to be acting.
What Actually Happens in Your Body During a Fever
A fever isn’t your body malfunctioning. It’s a deliberate defense strategy. When your immune system detects an infection, it releases signaling molecules that travel to the hypothalamus, a small region at the base of your brain that acts as your internal thermostat. These signals raise the thermostat’s “set point,” essentially telling your body that 98.6°F is too cool and it needs to heat up.
Your body then does two things to reach that new target. First, it constricts blood vessels near the skin’s surface and diverts blood inward to reduce heat loss. That’s why you might feel cold and shivery even though your temperature is climbing. Second, shivering generates heat through rapid muscle contractions. Once your blood temperature reaches the new set point, the shivering stops and you feel hot instead. The whole process is your body creating a less hospitable environment for viruses and bacteria, which tend to reproduce less efficiently at higher temperatures.
Low-Grade Fever vs. High Fever
Not all fevers carry the same weight. A low-grade fever, between 99.1°F and 100.4°F, is common with mild viral infections, allergies, or even stress. It often resolves on its own within a day or two and rarely needs treatment beyond rest and fluids. Many people with a low-grade fever feel only slightly off, or don’t notice it at all.
A moderate fever, roughly 100.4°F to 103°F, is the range most people associate with being sick. You’ll likely feel achy, fatigued, and warm to the touch. This range is typical for the flu, common bacterial infections, and many childhood illnesses. Over-the-counter fever reducers can help with comfort, but the fever itself is doing useful work for your immune system.
High fevers above 103°F in adults deserve closer attention, particularly if they last more than a couple of days or come with severe symptoms like a stiff neck, rash, persistent vomiting, or difficulty breathing.
When a Fever Becomes Dangerous
The true danger zone starts at 106.7°F (41.5°C), a condition called hyperpyrexia. This is a medical emergency. At this temperature, the body’s own proteins and enzymes can begin to break down, potentially damaging the brain, heart, and other organs. Symptoms include confusion, rapid heart rate, seizures, muscle stiffness, and loss of consciousness.
Hyperpyrexia is rare from ordinary infections. It’s more commonly linked to heatstroke, certain drug reactions, or conditions affecting the brain’s temperature-regulation center. If someone has a very high fever and suddenly becomes confused, dizzy, or unresponsive, that warrants emergency care immediately, not a wait-and-see approach.
Getting an Accurate Reading
The thermometer you choose and how you use it both affect accuracy. Digital oral thermometers are the best balance of convenience and reliability for adults and older children. Place the tip under the tongue, toward the back, and keep your mouth closed for the full measurement time. Avoid eating, drinking, or exercising for at least 15 minutes beforehand, since all three can temporarily skew the reading.
Forehead thermometers are popular because they’re fast and non-invasive, but they’re the most sensitive to environmental conditions. A reading taken in a cold room or right after coming inside on a hot day may not reflect your actual core temperature. If a forehead reading seems surprisingly high or low, confirm it with an oral thermometer.
For infants under 3 months, rectal thermometers remain the gold standard because they provide the most accurate core body temperature, and accuracy matters most when the stakes are highest.