For adults, a temperature at or above 100.4°F (38°C) taken by mouth is considered a fever. Anything between 99.5°F and 100.3°F falls into a gray zone often called a low-grade fever. Below that, you’re in normal territory, even if you feel lousy. For babies and children, the same 100.4°F threshold applies, but the urgency of a fever changes dramatically depending on age.
Normal Body Temperature Isn’t One Number
The old “98.6°F is normal” number is an average, not a fixed point. Your temperature naturally shifts throughout the day. It’s lowest in the early morning hours and rises through the afternoon, sometimes by a full degree. A small dip also tends to happen between 2 and 4 p.m. for most people. So a reading of 99.1°F at 5 p.m. might be perfectly normal, while the same reading at 6 a.m. could suggest something’s brewing.
Exercise, heavy clothing, hot weather, ovulation, and even a big meal can all push your temperature up without any illness involved. This is why a single thermometer reading doesn’t tell the whole story.
Fever Thresholds for Adults
Most healthcare providers define a fever as 100.4°F (38°C) measured orally. Some use a slightly lower cutoff of 100.0°F (37.8°C), but 100.4 is the most widely accepted line.
A low-grade fever, between 99.5°F and 100.3°F, usually means your immune system is responding to something. It’s not typically dangerous on its own. Fevers below 103°F (39.4°C) in adults are generally not a cause for concern and can be managed at home with rest and fluids. Once a fever climbs above 103°F, it’s worth contacting a healthcare provider. Untreated fevers above 105.8°F (41°C) are genuinely dangerous and require immediate attention.
Fever Thresholds for Babies and Children
Children use the same 100.4°F cutoff, but age changes everything about how seriously to take it. For any baby younger than 3 months old, a temperature of 100.4°F or higher warrants an immediate call to your pediatrician or a trip to the emergency room. At that age, even a modest fever can signal a serious infection because a newborn’s immune system is still immature.
For infants between about 2 and 5 months old, guidelines are slightly less urgent but still call for prompt medical evaluation. For older children, the rules are closer to what adults follow: the number matters less than how the child is acting. A child with a 102°F fever who’s playing and drinking fluids is in a very different situation than a child with a 101°F fever who is limp, confused, or refusing to make eye contact.
Your Thermometer Method Changes the Number
Not all thermometer readings are equal. Where you take the temperature affects what number you see, and you need to account for that when comparing your reading to the 100.4°F standard (which is based on oral measurement).
- Rectal: Reads 0.5 to 1°F higher than oral. A rectal reading of 101°F is roughly equivalent to 100.2°F orally.
- Ear (tympanic): Also reads 0.5 to 1°F higher than oral.
- Armpit (axillary): Reads 0.5 to 1°F lower than oral. So an armpit reading of 99.4°F could actually reflect an oral temperature near 100°F.
- Forehead (temporal): Also reads 0.5 to 1°F lower than oral, similar to armpit readings.
Rectal thermometers are considered the most accurate for infants and young children. For older kids and adults, oral readings are the standard. Forehead and armpit thermometers are convenient but tend to underestimate true core temperature, so keep that offset in mind before you dismiss a borderline reading.
When a Low Temperature Is the Problem
Fever gets all the attention, but a temperature that drops too low is also a sign of trouble. Hypothermia begins when core body temperature falls below 95°F (35°C). This can happen from cold exposure, but it can also occur in elderly adults, newborns, or people with severe infections whose bodies lose the ability to regulate heat properly. If your thermometer reads below 95°F and you haven’t just come in from the cold, that’s a medical concern worth acting on quickly.
Symptoms That Matter More Than the Number
A fever is a symptom, not a diagnosis. The temperature on the thermometer tells you your body is fighting something, but certain accompanying symptoms signal that the situation is more serious, regardless of what the number reads.
In adults, seek immediate medical attention if a fever comes with a stiff neck (especially pain when bending your head forward), mental confusion or altered speech, a new rash, unusual sensitivity to bright light, persistent vomiting, difficulty breathing or chest pain, abdominal pain, pain during urination, or seizures. Several of these are hallmarks of meningitis or other conditions that can escalate fast.
In children, watch for listlessness, poor eye contact, repeated vomiting, a severe headache, or seizures associated with the fever. A child who had a fever after being left in a hot car needs emergency care immediately, as that represents heat stroke rather than an infection.
A 100.5°F fever with no other symptoms in an otherwise healthy adult is almost never an emergency. A 101°F fever paired with a stiff neck and confusion is. The context around the number matters far more than the number itself.