A body temperature at or above 100.4°F (38°C) is generally considered a fever in both adults and children. That single number is the most widely used clinical threshold, but the full picture is more nuanced. Your normal baseline, your age, the time of day, and where on your body you measure all affect what counts as a true fever.
The Standard Fever Threshold
For most adults and children, 100.4°F (38°C) is the line. A reading at or above that point, taken rectally in young children or orally in older children and adults, signals a fever. An oral temperature of 100°F (37.8°C) is sometimes used as a slightly lower threshold for adults, since oral readings tend to run a bit cooler than rectal ones.
That said, the old idea that “normal” body temperature is exactly 98.6°F (37°C) is outdated. Studies over the past few decades have found that average healthy body temperature runs closer to 97.5°F to 97.9°F for most people. Your own baseline could be higher or lower, which is why a temperature that looks borderline on paper might already represent a meaningful rise for you personally.
Fever Severity Ranges
Not all fevers carry the same weight. Harvard Health breaks them into three tiers for adults:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
Low-grade fevers often accompany minor infections and usually resolve on their own. Fevers below 104°F (40°C) from common viral illnesses like the flu can actually help your immune system fight the infection and are generally not harmful. A temperature of 103°F (39.4°C) or higher in an adult, however, warrants a call to your doctor.
Why the Numbers Differ for Children
Fever thresholds get stricter the younger the child. For babies under 3 months, any rectal temperature of 100.4°F (38°C) or higher is treated seriously and needs prompt medical attention, even if the baby seems fine otherwise. At that age, an immature immune system means infections can escalate quickly.
For children between 3 and 6 months, the concern level rises at temperatures above 101°F (38.3°C), particularly if the child seems unusually irritable, sluggish, or uncomfortable. Between 6 and 24 months, a temperature above 101°F that lasts longer than a day without other symptoms is worth a call to your pediatrician. Rectal thermometers are the most reliable method for infants and toddlers because other measurement sites are less accurate at that age.
Why Older Adults Need a Lower Bar
If you’re over 65 or caring for someone who is, the standard 100.4°F threshold can be misleading. Older adults tend to run cooler at baseline. Data from nursing home residents in North Carolina found an average well temperature of just 97.7°F, nearly a full degree below the traditional “normal.” A reading of 99°F in an older adult could represent a significant rise, even though it wouldn’t qualify as a fever by the usual definition.
The most reliable approach for older adults is to know their individual baseline and flag any increase of 1.4°F or more above that number. When a baseline isn’t known, 99°F and above is a reasonable rule of thumb. Because temperature naturally shifts throughout the day, comparing two readings taken about four hours apart gives a clearer picture than a single measurement.
Where You Measure Matters
A rectal reading runs about 0.5 to 1°F higher than an oral reading. An armpit (axillary) reading typically runs 0.5 to 1°F lower than oral. Forehead and ear thermometers fall somewhere in between but can be less consistent. Forehead scanners, for instance, often require a correction factor of roughly 2°F to approximate what a contact thermometer would show.
If you’re using an armpit thermometer and get 99.4°F, that could translate to roughly 100.4°F orally, which would meet the fever threshold. Knowing which site your thermometer measures helps you interpret the number correctly. Most packaging or instruction manuals will note whether the device is calibrated for oral-equivalent readings or raw skin temperature.
What’s Actually Happening in Your Body
A fever isn’t a malfunction. It’s your brain deliberately turning up the heat. A region at the base of your brain acts as a thermostat, and during an infection, immune signals push that thermostat’s set point higher. Your body then works to reach the new target: blood vessels near your skin constrict to trap heat, your metabolism ramps up, and you may start shivering. That’s why you can feel freezing cold even though your temperature is climbing.
This is different from overheating due to heat stroke or heavy exertion. In those cases, your thermostat hasn’t changed. Instead, your body simply can’t shed heat fast enough, and the cooling system fails. A fever responds to standard treatments like rest and fever reducers. Heat-related overheating does not, and it requires cooling the body externally.
Normal Fluctuations That Aren’t Fever
Your body temperature isn’t static. It follows a daily rhythm, dipping to its lowest point in the early morning hours and peaking in the late afternoon or early evening. A small drop also happens for many people between 2 and 4 p.m. This means a reading of 99°F at 5 p.m. could be completely normal, while the same reading at 6 a.m. might be more meaningful.
Exercise, heavy meals, hormonal shifts during the menstrual cycle, and even emotional stress can temporarily push your temperature up without any infection present. A single borderline reading in isolation doesn’t necessarily mean you’re sick. A pattern of elevated readings, especially paired with symptoms like body aches, chills, or fatigue, is a more reliable signal.