A fever starts at 100.4°F (38°C) for both adults and children. That’s the standard threshold used across major medical guidelines, regardless of whether you measure orally, rectally, or with an ear thermometer. But the number on your thermometer can shift depending on where you measure, your age, and even the time of day.
The Standard Fever Threshold
For adults and children alike, 100.4°F (38°C) is the widely accepted cutoff. Anything at or above that reading counts as a fever. Below that but above your normal baseline is sometimes called an “elevated temperature” or low-grade fever, though it doesn’t meet the clinical definition.
That said, “normal” body temperature isn’t really the 98.6°F (37°C) you probably learned growing up. That number dates back to a study from 1868. A large systematic review of modern research found that the true average is closer to 97.9°F (36.6°C), with a normal range spanning roughly 97.1°F to 98.6°F (36.2°C to 37.0°C). So your personal baseline may sit well below the old standard, which means a reading of 99.5°F might already feel “feverish” to you even if it’s not technically a fever.
How Measurement Method Changes the Number
Not all thermometer readings are equal. A rectal temperature runs about 0.5°F to 1°F higher than an oral reading. Ear (tympanic) thermometers are similarly higher. Forehead and armpit readings, on the other hand, tend to run 0.5°F to 1°F lower than oral.
This matters when you’re checking a number against the 100.4°F threshold. If you’re using a forehead scanner and it reads 99.5°F, the “oral equivalent” could actually be around 100°F to 100.5°F. Rectal thermometers are considered the most accurate, which is why they’re the preferred method for infants. If you use an armpit or forehead thermometer regularly, keep the lower offset in mind before assuming a reading is normal.
Fever Thresholds for Babies and Children
The 100.4°F standard applies to all ages, but younger children, especially infants, warrant closer attention at lower temperatures. Age-adjusted guidelines break it down further:
- Newborns (3 months and under): A reading above 99.4°F (37.4°C) oral equivalent is considered elevated. Any rectal temperature of 100.4°F or higher in this age group is treated seriously and usually warrants a call to a pediatrician right away.
- Infants and toddlers (3 to 36 months): Elevated starts above 99.6°F (37.6°C), and a high fever is anything over 101.3°F (38.5°C).
- Children over 3 years: Elevated begins above 99.9°F (37.7°C), with high fever defined as over 103°F (39.4°C).
The younger the child, the more important it is to take even a mildly elevated temperature seriously. A newborn’s immune system handles infections very differently than an older child’s, and a fever in the first three months of life can signal something that needs prompt evaluation.
Fever Thresholds for Older Adults
Adults over 65 often have a lower baseline body temperature, which means a standard fever threshold can be misleading. A reading of 99°F might represent a significant spike for someone whose resting temperature normally sits around 97°F. Medical references note that lower fever thresholds apply to frail elderly persons, and infections in this group sometimes produce little or no fever at all. If an older adult feels unwell and has a temperature even slightly above their usual baseline, it’s worth paying attention.
Why Your Temperature Shifts Throughout the Day
Your body temperature isn’t static. It follows a daily cycle driven by your internal clock. Temperatures are lowest in the early morning hours, bottoming out around 4:00 a.m., and peak in the late afternoon and early evening. This swing can span a full degree or more in healthy people.
This means a temperature of 99.5°F at 7 a.m. is more noteworthy than the same reading at 5 p.m., when your body naturally runs warmer. Medical guidelines account for this: a morning oral temperature above 99.2°F (37.2°C) or a late afternoon oral temperature above 99.9°F (37.7°C) can both indicate fever, even though the numbers are different. If you’re tracking a fever at home, measuring at the same time each day gives you the most useful comparison.
When a Fever Becomes Serious
Most fevers in adults are uncomfortable but not dangerous. They’re your immune system’s way of fighting off infection, and they typically resolve within a few days. The temperature itself becomes a concern at certain levels.
For adults, a fever over 104°F (40°C) warrants a call to your doctor. Regardless of the exact number, a fever paired with certain symptoms signals a need for immediate medical attention: seizures, confusion, loss of consciousness, a stiff neck, difficulty breathing, or severe pain anywhere in the body. These combinations can indicate a serious underlying infection or condition that needs rapid treatment.
For children, the threshold for concern depends on age. A fever of any level in a baby under 3 months old is worth a call to the pediatrician. In older children, temperatures above 103°F (39.4°C) or fevers lasting more than a couple of days are the typical trigger points for seeking medical advice, especially if the child appears unusually lethargic or irritable.