A temperature of 100.4°F (38°C) or higher is considered a fever in both adults and children. This is the threshold used by the CDC, the Mayo Clinic, and most medical professionals. Your normal body temperature hovers around 98.6°F (37°C), though it naturally fluctuates throughout the day, running slightly lower in the morning and higher in the late afternoon.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s your body deliberately turning up the heat. A small region deep in your brain acts as your internal thermostat, constantly monitoring and adjusting your core temperature. When your immune system detects an infection, it releases signaling molecules that effectively raise the thermostat’s set point to a higher level.
Once that set point rises, your body treats its current normal temperature as “too cold” and responds accordingly. Blood vessels near the skin constrict to reduce heat loss, your metabolism ramps up to generate more warmth, and you may start shivering. That’s why you can feel freezing cold even as your temperature climbs. The chills are your body’s heat-generating engine working to reach the new, higher target.
Fever Thresholds by Measurement Method
Where you take a temperature matters. Different parts of the body run at slightly different temperatures, so the number that qualifies as a fever depends on the thermometer you’re using.
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral (under the tongue): 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
Rectal readings run about half a degree higher than oral readings, while armpit readings run about a degree lower. If your armpit thermometer shows a borderline number, it’s worth confirming with an oral or rectal reading, since armpit measurements are the least accurate of the three methods.
How Accurate Are Forehead Thermometers?
Forehead (temporal artery) thermometers are popular because they’re fast and non-invasive, but their accuracy has real limits. A clinical comparison found that forehead readings could differ from rectal readings by as much as 1.7°C (about 3°F) in either direction. The study calculated that forehead thermometers correctly identified a fever only 67% of the time, meaning they missed roughly one in three actual fevers. They were much better at confirming when someone did not have a fever, with 96% accuracy on that front.
For a quick screening at home, a forehead thermometer is convenient. But if the reading seems off, or if you’re checking a very young child or someone who seems clearly ill, follow up with an oral or rectal measurement.
Fever Ranges: Low, High, and Dangerous
Not all fevers carry the same level of concern. Doctors generally think of fevers in tiers:
- Low-grade fever: 100.4°F to about 102.2°F (38°C to 39°C). Common with mild infections and often resolves on its own.
- High-grade fever: 102.4°F to 105.8°F (39.1°C to 41°C). More likely to cause discomfort, fatigue, and body aches.
- Dangerous territory: Above 104°F (40°C) in an adult. This warrants a call to your doctor, and above 105.8°F (41°C) is a medical emergency.
The severity of a fever doesn’t always match the severity of the illness. A mild viral infection can spike a high fever, while a serious bacterial infection in an older adult might produce only a modest temperature rise.
Fever in Babies and Children
The 100.4°F (38°C) threshold applies to children too, but the stakes are higher at younger ages. For infants under 3 months, any rectal temperature of 100.4°F or above needs prompt medical attention, even if the baby seems otherwise fine. Young infants have immature immune systems, and a fever can be the only visible sign of a serious infection.
For babies under 3 months, rectal thermometers are the recommended method because they give the most reliable core temperature reading. Oral thermometers aren’t practical until a child is old enough to hold one under the tongue with their mouth closed, typically around age 4 or 5.
Why Older Adults Run Cooler
Baseline body temperature tends to drop with age. Many adults over 65 run closer to 97°F or even lower as their normal resting temperature. This means a reading of 99°F or 100°F, which wouldn’t raise an eyebrow in a younger person, could represent a meaningful fever in an older adult. The standard 100.4°F cutoff can miss infections in this age group because their starting point is already lower. If an older person feels unwell and their temperature is even a degree or two above their personal baseline, that’s worth paying attention to.
What Affects Your Temperature Reading
Several everyday factors can nudge your temperature up or down, making a single reading less reliable than you might expect. Exercising, drinking hot or cold liquids, eating a meal, and even the time of day all influence the number on the thermometer. Body temperature typically bottoms out in the early morning hours and peaks in the late afternoon, with a natural swing of about 1°F over the course of the day.
For the most accurate oral reading, wait at least 15 minutes after eating or drinking anything. If you’ve just been physically active, give yourself time to cool down before checking. And if you get a surprising number, take it again in 15 to 20 minutes before deciding what to do next.