What Is Considered a Fever? Adult & Child Thresholds

That number on the thermometer can feel ambiguous—are you actually running a fever, or just a little warm? The difference between normal body temperature and a clinical fever is surprisingly narrow: most adults sit around 97°F–99°F, and it takes only about 1–2 degrees more before most doctors consider it a fever. The CDC and Mayo Clinic both put the threshold at 100.4°F (38°C), but measurement method matters, and guidelines differ slightly for children versus adults. This guide walks through the exact thresholds you need to know.

Adult fever threshold (oral): 100°F (37.8°C) · CDC fever threshold: 100.4°F (38°C) · Low-grade fever range: 99°F to 100.4°F (37.2°C to 38°C) · High fever in adults: 103°F (39.4°C) · Normal average temperature: 98.6°F (37°C)

The table below summarizes fever thresholds across different measurement methods, with guidance on when each applies and what accuracy level to expect.

What’s considered a fever and when to see a doctor

Adult thresholds

The CDC defines a fever as a measured temperature of 100.4°F (38°C) or greater. Johns Hopkins Medicine notes that most healthcare providers consider a fever 100.4°F (38°C) or higher. Cleveland Clinic similarly defines fever as 100.0°F (37.8°C) or 100.4°F (38°C), acknowledging slight variation in how different institutions frame the threshold.

For adults, Mayo Clinic considers an oral temperature of 100°F (37.8°C) or higher as a fever. This means you don’t necessarily need to hit the CDC’s 100.4°F mark if you’re taking an oral reading—the 100°F threshold is enough to signal that your body is fighting something.

Measurement methods

Different body sites produce different readings. Rectal temperatures run about 0.5°F to 1°F higher than oral readings, which is why pediatric guidelines use 100.4°F rectally while adult oral guidelines start at 100°F. Ear (tympanic) and forehead (temporal) measurements fall somewhere in between, though forehead strips tend to be less reliable. If you get a borderline reading, verify with an oral thermometer for adults or rectal for young children.

Mayo Clinic reports that fevers caused by infection typically do not exceed 105–106°F (40.6–41.1°C) without treatment—a useful ceiling to keep in mind when deciding whether to monitor at home or seek care.

“A fever – when the body’s temperature is above 100.4 degrees Fahrenheit – is typically a sign that your body is fighting off an infection.”

— Dr. Tina Ardon, Mayo Clinic News Network

Does 37.5 mean a fever?

Celsius vs Fahrenheit

A reading of 37.5°C converts to 99.5°F—still below the clinical fever threshold for adults. The standard fever cutoff across CDC, Mayo Clinic, and Johns Hopkins sits at 38°C (100.4°F), which means 37.5°C falls into what many sources call “low-grade” territory.

Normal body temperature range

Ubie Health notes that normal adult temperature ranges from 97°F to 99°F (36.1°C to 37.2°C). Mayo Clinic places the average body temperature at 98.6°F (37°C), with natural variation throughout the day—it’s typically lower in the morning and higher in the late afternoon and evening.

What this means: if you’re checking your temperature and see 37.5°C, you’re technically normal—perhaps on the higher end of normal for that time of day. Recheck in a few hours, and if it’s climbed to 38°C or above, that’s when you can say you have a fever.

Is 99.7 having a fever?

Low-grade fever definition

A reading of 99.7°F (37.6°C) sits in the low-grade fever range. Ubie Health defines low-grade fever in adults as 99.1°F to 100.4°F (37.3°C to 38°C). This is not yet a clinical fever by most definitions, but it’s elevated enough that your body is responding to something.

Fahrenheit equivalents

The numbers can be confusing. Here’s how the thresholds stack up:

• 99.7°F = 37.6°C (low-grade territory)
• 100.4°F = 38°C (clinical fever threshold)
• 103°F = 39.4°C (high fever; seek medical care)

The catch: low-grade fevers often resolve on their own with rest and fluids. They’re worth monitoring but not panic-inducing. The exception is when they persist—Ubie recommends seeking medical evaluation if fever lasts more than 3 days in adults.

What is a low-grade fever?

Range and symptoms

Low-grade fever spans 99°F to 100.4°F (37.2°C to 38°C). According to Ubie Health, this range is where your body is actively fighting something but the immune response is moderate. Common accompanying symptoms include mild fatigue, slight body aches, and a flushed feeling—but nothing severe.

When it persists

Mayo Clinic advises calling a healthcare professional if fever in children lasts more than 48-72 hours without an obvious cause. For adults, Ubie recommends evaluation if fever persists beyond 3 days. Persistent low-grade fever can indicate underlying infections, inflammatory conditions, or medication reactions that warrant professional assessment.

The pattern: low-grade fever is the body’s early warning signal. It doesn’t necessarily mean you’re seriously ill, but it’s your cue to rest, hydrate, and monitor—whether it climbs or fades over the next 24-48 hours will tell you whether to keep self-managing or call a doctor.

What is considered a fever in a child

Child vs adult thresholds

Children run hotter than adults, and their thresholds differ accordingly. Mayo Clinic states that for infants and young children, a rectal temperature of 100.4°F (38°C) or higher constitutes a fever. The American Academy of Pediatrics similarly defines fever as above 38.0°C (>100.4°F) for all ages, with rectal measurement recommended for children under 3 years.

For infants under 3 months, the recommendation is uniform across sources: any rectal temperature of 100.4°F (38°C) or higher requires immediate medical evaluation. Mayo Clinic advises calling a healthcare professional right away, and Stamford Health confirms this threshold for immediate concern.

For children 3 to 6 months, Mayo Clinic recommends calling a professional if rectal temperature exceeds 102°F (38.9°C). For children 6 to 24 months, seek care if fever exceeds 102°F rectally and persists—acetaminophen or ibuprofen (for children 6 months and older) can be given while monitoring.

High fever warning signs

Studies cited in NIH research establish that in children 3 to 36 months, a rectal temperature of 102.2°F (39.0°C) or higher marks high-grade fever that warrants evaluation. Other red flags at any age include difficulty breathing, persistent crying, unexplained rash, stiff neck, or inability to wake easily.

“Fever in and of itself does not necessarily need to be treated.”

— Dr. Tina Ardon, Mayo Clinic News Network

When to see a doctor

The decision framework is different for each age group:

• Infants under 3 months: Any rectal temperature 100.4°F (38°C)+ → immediate medical evaluation
• Children 3-6 months: Rectal temp 102°F (38.9°C)+ → call pediatrician
• Children 6-24 months: Rectal temp 102°F+ persisting → seek care; give medication if comfortable
• Adults 18+: Fever 103°F (39.4°C)+ or lasting more than 3 days → medical attention
• Any age: Seizures, confusion, difficulty breathing, rash that doesn’t blanch → emergency care

Stamford Health notes that high-grade fever in adults is 103°F or higher—a level where most medical guidelines recommend contacting a healthcare provider. For children, Ubie adds that fever persisting beyond 24-48 hours warrants evaluation, especially without an obvious source like a cold.

For those wondering about the “24-hour fever rule” used by many schools and workplaces: the standard recommendation is that a person should be fever-free (without medication) for at least 24 hours before returning to normal activities. This prevents spreading illness and ensures the immune response is stabilizing.

What this means: the thermometer number is a guide, not the only factor. A fever of 101°F that climbs to 103°F within hours warrants faster action than one that plateaus at 101°F. Track both the reading and how it’s changing—you’re looking for direction, not just height.