A fever is generally defined as a body temperature at or above 100.4°F (38°C). This threshold applies to both adults and children, though what counts as “normal” body temperature is a bit more nuanced than most people realize, and where you measure matters more than you might think.
The 100.4°F Threshold
The widely accepted cutoff for a fever is 100.4°F (38°C). Below that, your temperature is either normal or falls into a gray zone that many providers call a low-grade fever, which sits between roughly 99.5°F and 100.3°F (37.5°C to 37.9°C). A reading in that low-grade range can mean your body is starting to fight something off, or it can simply reflect normal fluctuation throughout the day.
Most people grow up hearing that 98.6°F is the “normal” body temperature, but that number is an average, not a fixed point. Your temperature shifts depending on the time of day, your activity level, your hormones, and even your age. It’s common for healthy adults to run anywhere from about 97°F to 99°F on a typical day, with temperatures naturally peaking in the late afternoon and dipping in the early morning hours.
Fever Severity by Temperature Range
Not all fevers carry the same level of concern. Harvard Health Publishing breaks adult fevers into three tiers:
- Low-grade: 99.1°F to 100.4°F (37.3°C to 38.0°C)
- Moderate-grade: 100.6°F to 102.2°F (38.1°C to 39.0°C)
- High-grade: 102.4°F to 105.8°F (39.1°C to 41.0°C)
A low-grade fever often resolves on its own and may not need any treatment beyond rest and fluids. Moderate fevers typically accompany common infections like the flu or a urinary tract infection. High-grade fevers deserve closer attention, especially if they persist for more than a day or two, or if they come with symptoms like confusion, a stiff neck, difficulty breathing, or a rash. Temperatures above 105.8°F (41°C) are a medical emergency.
Where You Measure Changes the Reading
The number on your thermometer depends on where you place it. Rectal readings tend to run about 0.5°F to 1°F higher than oral readings, while armpit (axillary) readings typically run about 0.5°F to 1°F lower. Ear and forehead thermometers fall somewhere in between but can vary more based on technique.
There’s no reliable formula for converting one measurement site to another. Adding or subtracting a degree to “correct” an armpit or ear reading is a common habit, but the Mayo Clinic advises against it because the relationship isn’t consistent enough to be accurate. The better approach is to use the same method each time so you can spot a meaningful change. If you normally take oral readings, stick with oral readings. That consistency matters more than the specific device you use.
For infants, rectal temperature is the gold standard because it’s the most reliable in small bodies where even a fraction of a degree can change the clinical picture.
Fever Thresholds for Babies and Children
The 100.4°F threshold applies to children too, but the stakes are higher for very young infants. The American Academy of Pediatrics flags any rectal temperature of 100.4°F or above in babies between 8 and 60 days old as something that needs medical evaluation, even if the baby looks perfectly fine otherwise. Young infants have immature immune systems, so a fever can be the only visible sign of a serious bacterial infection.
For older children and toddlers, a fever of 100.4°F is more common and less automatically alarming. How the child is acting often matters more than the number itself. A toddler with a 102°F temperature who is still drinking fluids and playing is generally less concerning than a toddler with a 100.5°F temperature who is lethargic and refusing to eat.
Why Your Body Creates a Fever
A fever isn’t a malfunction. It’s a deliberate immune response. When your body detects an infection, immune cells in the liver and lungs release a signaling molecule that travels through the bloodstream to a small region at the base of the brain responsible for regulating body temperature. This region acts like a thermostat, and the signaling molecule essentially turns the dial up.
Once that set point rises, your brain triggers a cascade of responses to generate heat: your muscles contract (causing shivering), blood vessels near the skin tighten (making you feel cold even though your core temperature is climbing), and your heart rate increases. This is why fevers often start with chills. Your body is actively working to raise its temperature to the new, higher target. The elevated temperature makes your body a less hospitable place for many viruses and bacteria, and it also speeds up certain immune cell functions.
When the infection clears, the thermostat resets. The blood vessels near your skin open back up, you start sweating, and your temperature drops. That “fever breaking” sensation of sudden sweating and relief is the thermostat returning to its normal setting.
Factors That Affect Your Baseline
Several things can shift your normal temperature enough to make fever detection tricky. Older adults tend to run cooler at baseline, sometimes below 97°F, which means a reading of 99°F could actually represent a significant rise for them even though it falls well below the standard fever cutoff. Women’s temperatures also fluctuate with the menstrual cycle, running higher after ovulation. Heavy exercise, hot weather, and even a recent hot meal can temporarily push a reading up without any infection involved.
Certain medications can mask or blunt a fever. Over-the-counter pain relievers that reduce fever can make your temperature appear normal even while an infection is still active. This doesn’t mean you shouldn’t take them for comfort, but it’s worth knowing that a “normal” reading after taking one doesn’t necessarily mean the fever is gone for good. If you stop the medication and the fever returns, the underlying cause is still at work.