In adults, a temperature of 100.4°F (38°C) or higher is considered a fever when measured rectally, in the ear, or on the forehead. An oral reading of 100°F (37.8°C) or higher also qualifies. The difference comes down to where on your body you take the measurement, since some sites run slightly warmer than others.
Fever Thresholds by Measurement Method
Not all thermometer readings are interchangeable. A rectal, ear (tympanic), or temporal artery (forehead) thermometer registers a fever at 100.4°F (38°C). An oral thermometer hits the fever mark a bit lower, at 100°F (37.8°C), because the mouth is slightly cooler than your core. An armpit reading of 99°F (37.2°C) or higher suggests a fever, but armpit measurements are the least reliable of the bunch. If your armpit reading seems off, confirm it with a different method.
For most adults checking at home, an oral thermometer is the simplest and most practical option. Place it under your tongue, keep your mouth closed, and wait for the beep. If you’ve had something hot or cold to drink, wait about 15 minutes before taking a reading.
Why “Normal” Isn’t Always 98.6°F
The classic 98.6°F benchmark dates back to a 19th-century study, and modern research shows it’s more of an average than a fixed rule. Your body temperature shifts throughout the day in a predictable cycle. It dips to its lowest point around 6 a.m. and peaks around 8 p.m., with a normal swing of about 0.5°F to 1.9°F over the course of a day. That means a reading of 99°F in the evening could be perfectly normal, while the same number at 6 a.m. might hint at something brewing.
Exercise, heavy meals, hormonal shifts during the menstrual cycle, and warm clothing can all nudge your temperature up temporarily without meaning you’re sick.
Older Adults Run Cooler
If you’re over 65, your baseline body temperature is typically lower than a younger adult’s, ranging from about 96.4°F to 98.5°F. Core body temperature decreases with age, which means even a modest rise can signal infection. Someone with a normal baseline of 97°F who reaches 99.5°F may be mounting the same immune response as a younger person at 101°F. For older adults, paying attention to how far the temperature has climbed from your personal baseline matters more than hitting a specific number.
Low-Grade vs. High Fever
Fevers aren’t all-or-nothing. A low-grade fever generally falls between 100°F and 102°F orally. You might feel mildly achy or warm but can typically manage it at home with rest and fluids. Temperatures between 102°F and 104°F are moderate fevers that usually accompany a more active infection. You’ll likely feel noticeably unwell, with chills, fatigue, and muscle aches.
A fever over 104°F (40°C) is considered high and warrants a call to your doctor. At this level, your body is working extremely hard, and the underlying cause may need specific treatment rather than just symptom management.
When a Fever Becomes Dangerous
A temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency. At that extreme, the heat itself starts damaging organs. Your brain, heart, lungs, liver, and kidneys can all struggle to function. Without rapid treatment, hyperpyrexia can cause brain swelling, permanent brain damage, or coma. This level of fever is rare and typically tied to severe infections, drug reactions, or heat-related illness rather than a common cold or flu.
Below that extreme, certain symptoms alongside any fever should prompt immediate medical attention: confusion, seizures, loss of consciousness, a stiff neck, trouble breathing, severe pain anywhere in the body, or painful urination with foul-smelling urine. These can signal serious infections like meningitis, sepsis, or organ involvement that need urgent care regardless of how high the thermometer reads.
Managing a Fever at Home
Most fevers in adults are the body’s natural response to infection and resolve on their own within a few days. A fever is actually your immune system’s way of creating an inhospitable environment for viruses and bacteria, so a mild one doesn’t always need to be suppressed. The main reasons to treat a fever are comfort and preventing dehydration.
Stay hydrated, since sweating and an elevated metabolic rate burn through fluids faster than usual. Water, broth, and electrolyte drinks all help. Light clothing and a comfortable room temperature let your body release heat naturally. Over-the-counter fever reducers can bring relief when you feel miserable, but they’re treating symptoms, not the underlying cause.
A fever that persists for more than two days, keeps climbing despite home care, or comes back after seeming to resolve is worth discussing with a healthcare provider. The duration and pattern of a fever often tell a clinician more than the peak number alone.