A fever is generally defined as a body temperature of 100.4°F (38°C) or higher when measured orally or rectally. That single number is the standard threshold used across most medical guidelines for both adults and children. But the full picture is more nuanced, because your normal baseline probably isn’t the 98.6°F you grew up hearing about, and the method you use to take your temperature matters.
The Standard Fever Threshold
The widely accepted cutoff for a fever is 100.4°F (38°C) for oral and rectal readings. For armpit (axillary) measurements, the threshold is lower: 99°F (37.2°C), because the armpit consistently reads cooler than the mouth or rectum.
These thresholds apply to adults and children alike. However, context matters. A temperature of 100.4°F in an otherwise comfortable adult is a very different situation than 100.4°F in a newborn, which is always treated as urgent.
Fever Severity Levels
Not all fevers carry the same weight. Harvard Health Publishing breaks adult fevers into three tiers:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate-grade: 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)
A low-grade fever often doesn’t require treatment and may resolve on its own. Adults with fevers of 103°F (39.4°C) or higher will typically look and act visibly sick. At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency that can damage the brain, heart, lungs, kidneys, and liver if not treated quickly.
Why 98.6°F Isn’t Really “Normal”
The 98.6°F standard dates back to the mid-1800s, and more recent research suggests it’s too high. An analysis of 20 studies published between 1935 and 1999 found the average oral temperature was actually 97.5°F. A separate study of more than 35,000 people placed it at 97.9°F. Over the past 160 years, average human body temperature appears to have gradually dropped by more than one degree.
Your temperature also shifts throughout the day. It tends to be lowest in the morning and rises in the late afternoon and evening. This means a reading of 99.5°F at 7 a.m. could be more significant than the same reading at 5 p.m. Knowing your own typical baseline helps you judge whether a borderline number is actually elevated for you.
How Measurement Site Affects Your Reading
Where you take your temperature changes the number you get. A rectal reading runs 0.5 to 1.0°F higher than an oral reading. An armpit reading runs 0.5 to 1.0°F lower than oral. So if your armpit thermometer says 99.5°F, that likely translates to roughly 100 to 100.5°F orally.
Rectal thermometers are considered the most accurate, which is why they’re the standard recommendation for infants and young children. Oral thermometers work well for older children and adults as long as you haven’t eaten or had something to drink in the last 15 minutes. Forehead (temporal artery) and ear (tympanic) thermometers are convenient but can be slightly less consistent, especially if used incorrectly.
Fever Thresholds for Children by Age
The younger the child, the more seriously a fever is taken. Here’s how pediatric guidelines break down by age:
- Under 3 months: A rectal temperature of 100.4°F (38°C) or higher warrants an immediate call to your pediatrician or a trip to the emergency room, even if the baby seems otherwise fine. Young infants don’t have fully developed immune systems, so a fever can signal a serious infection.
- 3 to 6 months: A temperature up to 101°F (38.3°C) may not require emergency care on its own, but it does if your baby seems unusually irritable, lethargic, or uncomfortable. Anything above 101°F at this age calls for medical attention.
- 6 to 24 months: A temperature above 101°F (38.3°C) that lasts more than one day, even without other symptoms, should be evaluated.
Regardless of temperature, any child with a stiff neck, rash, trouble breathing, inconsolable crying, difficulty swallowing, blue lips or nails, or extreme drowsiness needs emergency care.
Febrile Seizures in Children
Some children experience seizures triggered by fever, called febrile seizures. These are most common between 6 months and 5 years of age, with the highest risk between 12 and 18 months. Even a low-grade fever can trigger one. While terrifying to witness, most febrile seizures are brief and don’t cause lasting harm. If a seizure lasts longer than five minutes, call 911.
Why Older Adults May Not Show a Fever
Fever detection gets trickier with age. Older adults often run a lower baseline temperature, and their bodies produce a weaker fever response when fighting infection. This happens because of reduced production of the chemical signals that trigger fever, a diminished response to those signals, and generally lower immune activity. A temperature that technically falls below 100.4°F can still represent a significant rise for someone whose baseline sits around 97°F. For older adults, a jump of about 2°F above their personal baseline is often treated as clinically meaningful, even if the thermometer doesn’t hit the standard cutoff.
What Actually Happens in Your Body During a Fever
Fever isn’t a malfunction. It’s a deliberate immune response. When your body detects an infection, immune cells release signaling molecules that reach the blood vessels near your brain. These signals trigger the production of a chemical messenger that acts on the brain’s internal thermostat, a region that normally keeps your body hovering around its set point.
That thermostat effectively gets turned up. Your brain now “thinks” your normal temperature is too low, so it kicks off a series of responses to generate and conserve heat. Blood vessels near your skin constrict to prevent heat loss (which is why you might feel cold and shivery even though your temperature is climbing). Your metabolism ramps up to produce more heat. Once your blood temperature matches the new, higher set point, the shivering stops, but you now feel hot.
This elevated temperature isn’t just a side effect. It creates a less hospitable environment for many viruses and bacteria, and it enhances certain immune functions. This is why mild to moderate fevers don’t always need to be suppressed. The discomfort is real, but the mechanism is working in your favor.