A fever starts at 100.4°F (38°C) when measured rectally, which is the standard threshold used by most medical organizations. For an oral (mouth) thermometer, readings of 100°F (37.8°C) or higher generally indicate a fever. The number shifts depending on where you take the temperature, your age, and even the time of day.
The Standard Fever Threshold
The widely accepted medical definition of a fever is a core body temperature at or above 100.4°F (38°C), measured rectally. This is the number the American Academy of Pediatrics uses, and it’s the benchmark in most clinical guidelines. If you’re using an oral thermometer, the cutoff is slightly lower: 100°F (37.8°C) or higher is generally considered a fever.
These numbers exist because “normal” body temperature isn’t actually the neat 98.6°F most of us learned. A large systematic review combining data from 36 studies found the true average is closer to 97.9°F (36.6°C), with a normal range spanning roughly 97.3°F to 98.7°F depending on the person. Your baseline can sit anywhere in that window and still be perfectly healthy.
How the Measurement Site Changes the Number
Not all thermometers read the same temperature, even when used at the exact same moment. Rectal readings run the hottest because they measure core body temperature most directly. Armpit (axillary) readings run the coolest. Here’s how normal ranges break down by site:
- Rectal: 98.2–100.8°F (36.8–38.2°C)
- Oral: 96.4–99.1°F (35.8–37.3°C)
- Ear (tympanic): 97.0–100.2°F (36.1–37.9°C)
- Armpit (axillary): 96.4–97.3°F (34.8–36.3°C)
- Forehead (temporal): 95.4–98.6°F (35.2–37.0°C)
This matters in practice. A reading of 99.5°F under the arm is more significant than 99.5°F from an ear thermometer, because armpit temperatures normally run about a full degree lower. If you’re using a forehead or armpit thermometer and want to compare your reading to the standard 100.4°F rectal threshold, you’ll typically need to add about 1°F to an armpit reading or roughly 0.5–1°F to a forehead reading, though these conversions are rough estimates.
Why Your Temperature Fluctuates Naturally
Body temperature follows a daily rhythm. It’s lowest in the early morning, often dipping below 97.5°F, and peaks in the late afternoon or evening, sometimes reaching 99.5°F or slightly higher. This swing of about 1–1.5°F is completely normal and doesn’t indicate illness. It’s one reason a reading of 99°F at 7 a.m. is more noteworthy than the same reading at 5 p.m.
Exercise, heavy clothing, hot drinks, and ovulation can all temporarily push your temperature up without any infection. If you suspect a fever, wait 15 minutes after physical activity or a hot or cold drink before taking an oral reading.
Fever Ranges: Low-Grade to Dangerous
Not all fevers carry the same urgency. Harvard Health breaks adult fevers into tiers that help you gauge how concerned to be:
- Low-grade fever: 100.4–102.2°F (38–39°C). Common with mild infections like colds. Usually manageable at home.
- Moderate fever: 102.2–104°F (39–40°C). Your body is mounting a stronger immune response. Worth monitoring closely.
- High-grade fever: 102.4–105.8°F (39.1–41°C). Call your doctor if your temperature exceeds 104°F (40°C).
Any fever above 104°F in an adult warrants a call to your doctor. If the fever comes with confusion, seizures, a stiff neck, trouble breathing, or loss of consciousness, seek emergency care regardless of the number on the thermometer.
Fever Thresholds for Children and Infants
The same 100.4°F rectal threshold applies to children, but the urgency depends heavily on age. The AAP clinical guidelines focus specifically on infants 8 to 60 days old with temperatures at or above 100.4°F, because young babies can’t fight infections the way older children can. A rectal temperature of 100.4°F or higher in a baby under 3 months old is a reason to call your pediatrician right away, even if the baby seems fine otherwise.
For older children and toddlers, fever is common and usually signals the immune system doing its job. The height of the fever alone doesn’t always predict how serious the illness is. A child with 103°F who is drinking fluids and playing is often less concerning than a child with 101°F who is limp and unresponsive. How your child looks and acts matters as much as the number.
Why Fever Is Different in Older Adults
People over 65 tend to run cooler at baseline. Their resting body temperature may sit a full degree lower than a younger adult’s. This means a reading of 99°F in an 80-year-old could represent the same immune response that produces 101°F in a 30-year-old. Medical references note that lower fever thresholds apply to frail elderly persons, with a morning oral temperature above 99°F or an afternoon reading above 99.9°F potentially qualifying as a fever in this group.
This is a practical concern because older adults with serious infections, including pneumonia and urinary tract infections, sometimes never develop a “classic” fever. A temperature that looks borderline on paper may be significant if the person also seems more confused, fatigued, or off than usual.
What Happens in Your Body During a Fever
Fever isn’t a malfunction. It’s a deliberate response coordinated by a small region at the base of your brain that acts as your internal thermostat. When your immune system detects an invader, it releases signaling molecules that travel to this thermostat and trigger the production of a chemical messenger called prostaglandin E2. This messenger essentially turns the dial up, resetting your “normal” temperature to a higher target.
Once the set point rises, your body treats its current temperature as too cold. That’s why you shiver and feel chilled at the start of a fever: your body is generating heat and constricting blood vessels near the skin to reach its new, higher target. When the infection clears and the set point drops back to normal, the process reverses. You sweat, flush, and kick off the blankets as your body works to dump the excess heat. This is also why fever-reducing medications work: they block the production of prostaglandin E2, lowering the set point back toward normal.