A temperature of 100.4°F (38°C) or higher is the standard medical threshold for a fever in both adults and children. That number applies when you measure from the mouth, ear, rectum, or forehead. Armpit readings run lower, so 99°F (37.2°C) from the armpit is considered a fever.
Why 100.4°F Is the Cutoff
Your body doesn’t sit at one fixed temperature all day. The old standard of 98.6°F actually dates back to the 1800s, and modern data suggests it’s too high. A Stanford Medicine analysis of over 618,000 oral temperature readings found that the average adult body temperature today is closer to 97.9°F, with a normal range of about 97.3°F to 98.2°F. Better living conditions and lower rates of chronic infection over the past two centuries have likely driven this gradual decline, roughly 0.05°F per decade.
Despite that shift in baseline, 100.4°F remains the widely accepted fever line because it reliably signals that the body’s internal thermostat has been deliberately turned up in response to illness or inflammation.
How Your Thermometer Location Changes the Number
Not all thermometer readings are equal. Rectal and ear measurements run about 0.5 to 1°F higher than an oral reading, because they’re closer to your core body temperature. Armpit and forehead readings run 0.5 to 1°F lower than oral. That means if your forehead scanner says 99.5°F, your actual core temperature could be a full degree higher.
Here’s what counts as a fever by measurement site:
- Mouth, ear, rectum, or forehead: 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
For the most accurate home reading, oral thermometers are reliable for adults and older children. Rectal thermometers are the gold standard for infants and toddlers.
What Actually Happens When You Get a Fever
A fever isn’t a malfunction. It’s your immune system deliberately raising your body’s temperature set point. When your white blood cells detect an infection, they release signaling molecules called cytokines. These travel through the bloodstream to the brain’s temperature control center, the hypothalamus, which acts like a thermostat. The hypothalamus responds by raising the target temperature, just like turning up the dial on your home heating system.
Your body then works to reach that new, higher set point. Blood vessels near the skin constrict to trap heat, your metabolism speeds up, and your muscles contract (which is why you shiver). You feel cold even though your temperature is rising, because your body is trying to close the gap between your current temperature and the new target. Once the infection is under control, the set point drops back down and you start sweating to release the extra heat.
Normal Temperature Fluctuations
Your body temperature isn’t constant even when you’re perfectly healthy. It follows a daily cycle, dipping to its lowest point during sleep and rising through the morning. Most people also experience a small temperature dip between 2 p.m. and 4 p.m. By late afternoon or early evening, your temperature typically peaks, sometimes reaching half a degree or more above your morning reading.
This means a temperature of 99.5°F in the evening might be completely normal for you, while that same reading first thing in the morning could be more meaningful. Exercise, heavy clothing, hot drinks, and hormonal changes (including ovulation) can also push your temperature up temporarily without any illness involved.
Fever Thresholds for Babies and Children
The 100.4°F threshold applies to children too, but the urgency changes dramatically with age. For babies younger than 3 months, a rectal temperature of 100.4°F or higher needs immediate medical evaluation, even if the baby seems fine otherwise. At that age, the immune system is immature enough that a fever can signal a serious infection with few other visible symptoms.
For babies 3 to 6 months old, a temperature above 102°F (38.9°C) warrants a call to their doctor, as does a lower fever paired with unusual irritability, lethargy, or discomfort. For children 7 months to 2 years, a rectal temperature above 102°F that lasts more than a day without other symptoms is worth checking on. In older children, the number matters less than how the child looks and acts. A child who is alert, drinking fluids, and playing with a temperature of 102°F is generally less concerning than a listless child at 101°F.
When a Fever Becomes Dangerous
For adults, a temperature of 103°F (39.4°C) or higher is the point where medical guidance is recommended even if you otherwise feel okay. Most fevers from common infections peak well below this and resolve within a few days.
Certain symptoms alongside any fever deserve urgent attention: a stiff neck with pain when bending your head forward, a rash, confusion or altered speech, persistent vomiting, difficulty breathing, chest pain, or seizures. These combinations can point to serious conditions like meningitis or sepsis that need fast treatment.
At the extreme end, a body temperature above 106.7°F (41.5°C) is a medical emergency called hyperpyrexia. This is rare with ordinary infections and more commonly results from heatstroke, drug reactions, or certain brain injuries. At that level, the heat itself can damage organs, and emergency cooling is critical. If someone’s temperature reaches this range, call 911 immediately.
Low-Grade Fever vs. True Fever
You’ll often hear the term “low-grade fever” for temperatures between about 99°F and 100.3°F (measured orally). This range sits above the modern average but below the official fever threshold. It can show up with mild viral infections, after vaccination, during ovulation, or even after intense exercise. A low-grade fever that comes and goes for a few days usually isn’t cause for concern on its own, but one that persists for more than two weeks without explanation is worth investigating, as it can occasionally signal autoimmune conditions or other chronic issues.