A body temperature of 100.4°F (38°C) or higher is generally considered a fever. That’s the threshold most medical guidelines use, regardless of whether you’re measuring orally, rectally, or with an ear thermometer. But the full picture is more nuanced than a single number, because “normal” body temperature isn’t what most people think it is, and where you take the reading matters.
Normal Body Temperature Is Lower Than You Think
The 98.6°F benchmark dates back to the mid-1800s, and it’s outdated. An analysis of 20 studies published between 1935 and 1999 found the average oral temperature was actually 97.5°F. A more recent study of over 35,000 people put the average at 97.9°F. A large 2020 analysis spanning nearly 160 years of data showed that average oral temperature has gradually dropped by more than a full degree over that period. Harvard Health Publishing now suggests that something closer to 97.5°F is a more accurate “normal.”
This means your baseline temperature on a healthy day might sit anywhere from about 97°F to 99°F, and that’s perfectly fine. Several factors push it around throughout the day. Your body temperature rises during the last hours of sleep and peaks in the late afternoon or early evening. It drops at night as your body prepares for sleep, and most people experience a small dip between 2 p.m. and 4 p.m. as well. Women also tend to run slightly warmer during parts of their menstrual cycle, and older adults often run cooler than younger people.
How Your Body Controls Its Temperature
A small region deep in your brain acts as your internal thermostat. When you’re exposed to heat, specialized neurons in this area activate and send signals that suppress heat production and physical activity, cooling you down. When you’re cold, a different set of neurons fires up to increase energy expenditure and muscle activity, generating warmth. These two systems work in opposition, constantly fine-tuning your core temperature like a seesaw. When you’re sick, your brain essentially raises the thermostat’s set point, which is why you feel chilled even though your temperature is climbing.
Where You Measure Changes the Number
Not all thermometer readings are equal. The average normal oral temperature is 98.6°F (37°C), and other measurement sites read higher or lower relative to that baseline:
- Rectal: 0.5°F to 1°F higher than oral
- Ear (tympanic): 0.5°F to 1°F higher than oral
- Armpit (axillary): 0.5°F to 1°F lower than oral
- Forehead (temporal): 0.5°F to 1°F lower than oral
This is why fever thresholds differ slightly depending on where you check. The Mayo Clinic considers these readings a fever: a rectal, ear, or temporal reading of 100.4°F (38°C) or higher, an oral reading of 100.4°F (38°C) or higher, or an armpit reading of 99°F (37.2°C) or higher. If you get an armpit reading that seems off, it’s worth confirming with another method, since armpit measurements are the least reliable.
Fever Ranges in Adults
Once your temperature crosses 100.4°F, you’re in fever territory. Not all fevers carry the same weight, though. A reading between 100.4°F and about 102°F is a low-grade fever. Your body is fighting something, but this range is common with ordinary viral infections and often resolves on its own. You might feel achy, tired, or slightly warm to the touch.
At 103°F (39.4°C) and above, most adults visibly look and act sick. This level of fever warrants closer attention and is a reasonable point to seek medical advice, especially if it persists for more than a couple of days or comes with severe symptoms like confusion, chest pain, or difficulty breathing.
Fever Thresholds for Children
Children follow the same basic cutoffs, with a few important distinctions. A child has a fever with a rectal, ear, or temporal reading of 100.4°F (38°C) or higher, an oral reading of 100°F (37.8°C) or higher, or an armpit reading of 99°F (37.2°C) or higher.
For infants under three months, any rectal temperature of 100.4°F or above is taken seriously, because very young babies can’t fight infections the way older children and adults can. Rectal readings are the gold standard for babies because they’re the most accurate at that age. For toddlers and older children, ear or forehead thermometers are practical alternatives, though armpit readings remain the least precise option.
Low Body Temperature
While most people search for the fever threshold, temperatures that drop too low also matter. A core body temperature below 95°F (35°C) is classified as hypothermia. At this level, your body loses heat faster than it can produce it, and normal metabolic processes start to falter. Mild hypothermia causes shivering and confusion. As temperature drops further, shivering may actually stop, coordination worsens, and the situation becomes dangerous. Hypothermia can result from prolonged cold exposure, but it also sometimes signals an underlying medical condition, especially in older adults who may not feel cold as intensely.
Getting an Accurate Reading
A few practical details help you get a number you can trust. Wait at least 15 minutes after eating or drinking something hot or cold before taking an oral reading, since food and beverages temporarily shift mouth temperature. If you’ve been exercising or bundled under blankets, give yourself time to cool down before checking. Digital thermometers are the current standard for home use. Glass mercury thermometers are no longer recommended because of the toxicity risk if they break.
When tracking a fever over time, use the same thermometer and the same body site for every reading. Switching between an ear thermometer and an armpit thermometer makes it impossible to compare numbers meaningfully, since each site has its own baseline. Write down the time of each reading too, because your temperature naturally runs higher in the evening than in the morning, and that context helps you (and your doctor) spot real trends versus normal daily fluctuation.