A fever in adults and children is generally defined as a body temperature at or above 100.4°F (38°C). That’s the threshold used by the CDC, the American Academy of Pediatrics, and most clinical guidelines worldwide. But the number that counts as a fever depends on where you measure, how old the person is, and even what time of day it is.
The Standard Fever Threshold
For adults and children alike, 100.4°F (38°C) is the most widely accepted cutoff for fever. Anything below that but above your normal baseline falls into a gray zone sometimes called low-grade fever, typically defined as 99.5°F to 100.3°F (37.5°C to 38.3°C). A reading in that range can still signal that your immune system is responding to something, but it isn’t classified as a true fever by most medical standards.
Once temperature climbs above 104°F (40°C), the situation becomes more urgent. A temperature at or above 106°F (41.1°C) is classified as hyperpyrexia and is a medical emergency requiring immediate care.
Why 98.6°F Isn’t Really “Normal” Anymore
The familiar 98.6°F (37°C) benchmark dates back to 1851, when a German physician took millions of armpit temperature readings from 25,000 patients. It stuck for over a century, but large-scale modern research tells a different story. A study analyzing temperature data spanning nearly 200 years found that average body temperature in the United States has dropped steadily since the Industrial Revolution. Men born in the early 1800s ran about 1°F (0.59°C) warmer than men today, and women’s temperatures have declined by about 0.6°F (0.32°C) since the 1890s.
The practical takeaway: a healthy adult’s resting temperature today is closer to 97.5°F to 97.9°F (36.4°C to 36.6°C) on average. That means a reading of, say, 99.5°F might represent a more meaningful rise from your personal baseline than it would have for someone living 150 years ago.
Fever Thresholds for Babies and Children
The standard 100.4°F threshold still applies to infants and children as a general rule, but younger bodies warrant more caution at lower temperatures. Research on age-specific thresholds suggests the following breakdown:
- Newborns up to 3 months: A temperature above 99.4°F (37.4°C) is considered a fever. Any reading at or above 100.4°F (38°C) in this age group warrants prompt medical evaluation, because very young infants can’t fight infections the way older children can.
- 3 months to 3 years: Fever begins above 99.6°F (37.6°C), and a high fever is anything above 101.3°F (38.5°C).
- Over 3 years: Fever starts above 99.9°F (37.7°C), and a high fever is above 103°F (39.4°C).
The younger the child, the lower the bar for concern. A temperature that would be unremarkable in a ten-year-old can signal a serious bacterial infection in a newborn.
Where You Measure Changes the Number
Not all thermometer placements give you the same reading. Rectal temperatures run about 0.8°F (0.43°C) higher than armpit (axillary) readings, and oral temperatures run about 0.5°F (0.25°C) higher than armpit readings. So if you’re using an armpit thermometer and get 99.5°F, the equivalent oral temperature would be closer to 100°F, and the rectal equivalent closer to 100.3°F.
If you’re taking an armpit temperature, a reading above 99.5°F (37.5°C) best corresponds to an oral fever of 100.4°F. For rectal readings, the standard 100.4°F cutoff applies directly since that’s the method most fever guidelines are built around, particularly for infants.
Which Thermometer Is Most Accurate
A comparison study testing seven commercial thermometers against a medical-grade reference found that ear (tympanic) thermometers came closest to the gold standard, with readings off by less than 0.1°F on average. Infrared forehead thermometers were reasonably close, typically within 0.3°F to 0.4°F. Digital oral thermometers also performed well, landing within about 0.3°F of the reference.
Forehead-scanning temporal artery thermometers and thermal imaging cameras were the least reliable, drifting as much as 0.8°F to 1°F from accurate readings. If precision matters, which it does when you’re trying to decide whether a baby has crossed the fever line, an ear thermometer or a digital oral thermometer is your best bet for home use. For infants under 3 months, rectal thermometers remain the recommended method because they’re the most consistent.
Your Temperature Shifts Throughout the Day
Body temperature follows a daily cycle driven by your internal clock. It’s lowest in the early morning, typically bottoming out around 4 to 6 a.m., and peaks in the late afternoon or early evening. The average swing is about 0.6°F (0.33°C), though some people fluctuate by nearly a full degree Fahrenheit (0.53°C) over 24 hours.
This means a temperature of 99.8°F at 7 a.m. is more significant than the same reading at 5 p.m. If you’re monitoring a fever, taking readings at consistent times gives you a more accurate picture of whether things are improving or getting worse.
When a Fever Needs Medical Attention
In adults, a fever above 104°F (40°C) is reason to call a doctor. A fever at any level that lasts more than a few days, or one that goes away and comes back, also deserves evaluation. Certain accompanying symptoms elevate the urgency regardless of the temperature reading: seizures, confusion, stiff neck, trouble breathing, severe pain, or loss of consciousness all call for immediate care.
For infants under 3 months, any temperature at or above 100.4°F (38°C) is treated as potentially serious. Their immune systems are still developing, and infections that would be minor in older children can escalate quickly. In children between 3 months and 3 years, a fever above 101.3°F (38.5°C) that persists or comes with lethargy, rash, or refusal to drink warrants a call to a pediatrician.