A temperature of 100.4°F (38°C) or higher is considered a fever in both adults and children. That’s the widely accepted clinical threshold, regardless of how you measure it. Below that number, you’re in normal territory, even if you feel lousy.
Why 98.6°F Isn’t the Magic Number
The idea that 98.6°F (37°C) is “normal” body temperature has been around for over a century, and it’s still the generally accepted average. But normal actually spans a range: anywhere from 97°F (36.1°C) to 99°F (37.2°C) qualifies as healthy in most people.
Your body temperature shifts throughout the day. It tends to be lowest in the early morning and highest in the late afternoon. A small dip also happens for most people between 2 and 4 p.m. Physical activity, hormonal cycles, age, and even what you recently ate or drank can push the number up or down within that normal range. So a reading of 99°F at 5 p.m. after a workout is not the same thing as 99°F first thing in the morning.
Fever Thresholds by Measurement Method
Not all thermometers give you the same number. Readings vary depending on where you take the temperature, and the differences are predictable enough to account for.
- Oral (under the tongue): 100.4°F (38°C) or higher is a fever.
- Rectal or ear: 100.4°F (38°C) or higher. These methods tend to read 0.5 to 1°F higher than an oral thermometer.
- Armpit (axillary): 99°F (37.2°C) or higher. Armpit readings run about 0.5 to 1°F lower than oral readings.
If you’re using an armpit thermometer and get 99.5°F, that’s roughly equivalent to an oral reading of 100°F or slightly above, which puts you right at the fever line. Rectal thermometers are the most accurate, which is why they’re recommended for infants.
What’s Happening Inside Your Body
A fever isn’t a malfunction. It’s your immune system deliberately raising your internal thermostat. A region of the brain called the hypothalamus normally keeps your body temperature steady, but when you get an infection, the immune system releases signaling molecules that tell the hypothalamus to set the target temperature higher. Your body then works to reach that new set point by constricting blood vessels near the skin to trap heat and, in some cases, triggering shivering to generate more warmth. That’s why you can feel freezing cold even though your temperature is climbing.
Once the infection starts to clear, the set point drops back down, and your body switches to cooling mode. That’s when the sweating kicks in.
Low-Grade vs. High Fever
Not all fevers carry the same weight. A temperature between 100.4°F and about 102°F is generally considered a low-grade fever. It’s common with viral infections like colds and the flu, and it usually resolves on its own within a few days. You may feel achy, tired, and generally miserable, but a low-grade fever on its own is typically not dangerous for otherwise healthy adults.
Temperatures between 102°F and 104°F signal a more significant immune response. You’ll likely feel noticeably worse, and it’s worth paying closer attention to other symptoms at this point. A fever above 104°F (40°C) warrants a call to your doctor, as it can indicate a more serious infection or complication.
When a Fever Needs Medical Attention
For adults, the temperature alone matters less than the full picture. A fever over 104°F (40°C) is reason enough to contact a doctor. Below that, watch for these red-flag symptoms alongside the fever: trouble breathing, chest pain, a bad headache with a stiff neck, confusion, seizures, loss of consciousness, or severe pain anywhere in the body. Any of these combinations calls for prompt medical care.
The rules are stricter for babies and young children. An infant under 3 months old with a rectal temperature of 100.4°F or higher needs immediate medical evaluation, even if the baby seems fine otherwise. For babies between 3 and 6 months, a temperature up to 101°F may be manageable at home if the baby is acting normally, but anything above that, or any sign of unusual irritability or lethargy, should prompt a call to the pediatrician.
How to Get an Accurate Reading
Timing and technique both matter. Wait at least 15 minutes after eating, drinking, or exercising before taking an oral temperature, since all of these can temporarily skew the reading. For the most consistent results, take your temperature at the same time of day if you’re tracking it over several hours or days. Remember that late afternoon readings will naturally run a bit higher than early morning ones.
If you’re monitoring a fever at home, the trend matters more than any single reading. A temperature that steadily climbs over several hours tells you something different than one that spikes briefly and comes back down. Write down your readings and the times you took them so you can share an accurate picture with a doctor if needed.