A body temperature at or above 100.4°F (38°C) is considered a fever. This is the standard threshold used by the CDC, the Mayo Clinic, and most medical guidelines. But the number that counts as a fever can shift depending on where you measure, your age, and how your body normally runs.
The Standard Fever Threshold
For healthy adults, 100.4°F (38°C) is the widely accepted cutoff. Anything at or above that number, measured orally, rectally, or with an ear thermometer, qualifies as a fever. Normal body temperature hovers around 98.6°F (37°C), though individual baselines vary. Some people run a little warmer or cooler throughout the day, with temperatures typically lowest in the early morning and highest in the late afternoon.
The CDC uses 100.4°F as its clinical definition but also recognizes that a thermometer isn’t always handy. Feeling warm to the touch, looking flushed, or having chills can all signal a fever when you can’t get a reading.
How Measurement Site Changes the Number
Not every thermometer reads the same, because different parts of your body sit at slightly different temperatures. The Mayo Clinic breaks it down this way:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral (under the tongue): 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
Armpit readings run about a degree lower than oral readings, which is why the fever threshold drops to 99°F for that method. Rectal temperatures are the most accurate and are considered the gold standard for infants. If you’re using a no-contact infrared thermometer, research shows it’s just as accurate as oral or rectal readings when used correctly, though internal thermometers remain preferred in clinical settings.
Fever Ranges: Low-Grade to Dangerous
Not all fevers carry the same weight. A temperature of 100.4°F feels very different from one pushing past 104°F, and the medical response changes accordingly.
Low-grade fevers fall roughly between 100.4°F and 102.2°F. These are common with mild infections and often resolve on their own. High-grade fevers, classified by Harvard Health as 102.4°F to 105.8°F (39.1°C to 41°C), signal a more significant immune response and deserve closer attention. If your temperature climbs above 104°F (40°C), that’s a good reason to call your doctor.
At the extreme end, temperatures above 106.7°F (41.5°C) are classified as hyperpyrexia. This is a medical emergency. Symptoms at that level can include confusion, seizures, a racing heart rate, loss of consciousness, and severe muscle cramps. If someone has a very high temperature and seems dizzy, confused, or “off,” they need emergency care immediately.
Why Your Body Creates a Fever
A fever isn’t a malfunction. It’s your immune system deliberately raising your body’s thermostat. When your body detects an infection, immune cells release signaling molecules that travel to the brain’s temperature control center, a region in the hypothalamus. These signals trigger the production of a chemical messenger (a type of prostaglandin) that resets your internal thermostat to a higher point.
Once that new set point is established, your brain orchestrates a coordinated response: your metabolism speeds up, blood vessels near the skin constrict to trap heat, and sweating slows down. That’s why you feel chilled even though your temperature is climbing. Your body is working to reach its new, higher target. The elevated temperature helps activate immune cells and creates a less hospitable environment for many viruses and bacteria.
Children and Infants
For babies and young children, the same 100.4°F (38°C) threshold applies, but the urgency is different. The American Academy of Pediatrics uses a rectal temperature of 100.4°F or higher as the defining cutoff for fever in infants. For babies between 8 and 60 days old, any fever at that level warrants prompt medical evaluation, even if the baby appears well. Young infants have immature immune systems, and a fever can be the only visible sign of a serious infection.
For older children and toddlers, fevers are extremely common and usually tied to routine viral infections. The temperature number matters less than how the child is behaving. A child with a 102°F fever who is drinking fluids and playing is generally less concerning than a lethargic child with a lower reading.
Why the Threshold Is Lower for Older Adults
People over 65 tend to run cooler at baseline, which means the standard 100.4°F cutoff can miss real infections. Research from the University of Maryland School of Medicine suggests using 99°F (37.2°C) as the fever threshold for older adults instead. That single adjustment increases the ability to detect bacterial infections from 40% to 83%, while still being accurate 89% of the time. If you’re caring for an older adult, a temperature that would seem unremarkable in a younger person could be a meaningful signal.
Older adults are also less likely to mount a high fever in response to infection, so waiting for 100.4°F can mean missing the early window when treatment is most effective. Changes in behavior, appetite, or alertness alongside even a modest temperature increase are worth taking seriously.
Getting an Accurate Reading
The type of thermometer matters less than using it correctly. Digital stick thermometers placed under the tongue are reliable and inexpensive. Temporal artery (forehead) and tympanic (ear) thermometers are convenient and accurate when positioned properly. Forehead strips and feeling someone’s skin can give you a rough sense but aren’t precise enough to guide decisions.
A few practical tips: wait at least 15 minutes after eating or drinking before taking an oral temperature, since hot or cold foods will skew the result. For ear thermometers, a buildup of earwax can cause inaccurate readings. And if you’re tracking a fever over time, use the same thermometer and the same body site each time so your numbers are comparable.