Normal body temperature for an adult falls within a range, not at a single number. For people ages 11 to 65, the typical range is 97.6°F to 99.6°F (36.4°C to 37.6°C). The old standard of 98.6°F is a reasonable midpoint, but most people run slightly above or below it on any given day.
Why 98.6°F Isn’t the Whole Story
The 98.6°F benchmark dates back to a German study from the 1800s. It’s not wrong, exactly, but it suggests a precision that doesn’t exist. Your body temperature shifts constantly based on time of day, your activity level, and your individual biology. Treating 98.6°F as a fixed target can make a perfectly healthy reading of 97.8°F or 99.2°F seem abnormal when it isn’t.
More recent research from Stanford Medicine confirms that normal body temperature is personal. Some people consistently run cooler or warmer than others, and that baseline is influenced by factors like age, sex, body size, and even the time you take the reading. The single biggest influence on your temperature at any moment is the time of day: body temperature is lowest in the early morning and peaks around 4 in the afternoon. That swing can account for a full degree of difference without anything being wrong.
How Age Changes Your Baseline
Adults over 65 tend to run cooler than younger adults. The typical range for people older than 65 is 96.4°F to 98.5°F (35.8°C to 36.9°C), roughly a full degree lower at both ends compared to younger adults. This happens because metabolism slows with age and the body becomes less efficient at generating and retaining heat.
This lower baseline matters for two reasons. First, an older adult with a reading of 99°F may actually have a significant fever relative to their personal normal, even though that number looks unremarkable on paper. Second, older adults are more vulnerable to hypothermia because their starting temperature is already closer to dangerous territory.
Readings Vary by Measurement Site
Where you take your temperature changes the number you get. Oral readings (under the tongue) are the most common reference point, and the ranges above are based on oral measurement. Other methods read slightly higher or lower:
- Rectal: 0.5°F to 1°F higher than oral. This is the closest to true core body temperature.
- 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.
So if an armpit reading shows 97.2°F, that likely corresponds to an oral temperature around 97.7°F to 98.2°F, which is perfectly normal. Knowing these offsets helps you avoid unnecessary worry over a number that’s simply reflecting the measurement site rather than an actual problem.
Getting an Accurate Reading
If you’ve been eating or drinking, wait 30 minutes before taking an oral temperature. Hot coffee or ice water can temporarily shift the reading in your mouth by a degree or more, which is enough to make a normal temperature look like a low-grade fever or vice versa. For the most consistent results, take your temperature at roughly the same time of day each time you check, ideally before physical activity or a hot shower.
When a Temperature Becomes a Fever
A fever is defined as a body temperature of 100.4°F (38°C) or higher. That said, temperatures above your personal normal but below 100.4°F still count as a low-grade fever and can signal that your immune system is responding to something. The severity breaks down like this:
- Low-grade fever: 99.1°F to 100.4°F (37.3°C to 38.0°C)
- Moderate fever: 100.6°F to 102.2°F (38.1°C to 39.0°C)
- High fever: 102.4°F to 105.8°F (39.1°C to 41.0°C)
Low-grade fevers are common with mild infections and often resolve on their own. A high fever, especially one that persists or is accompanied by confusion, stiff neck, or difficulty breathing, warrants prompt medical attention.
When a Temperature Is Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. This isn’t just a cold-weather concern. Older adults, people with certain chronic conditions, and anyone exposed to cool environments for extended periods can develop hypothermia even indoors. The stages progress as temperature drops:
- Mild hypothermia: 90°F to 95°F (32.2°C to 35°C). Shivering, confusion, and poor coordination.
- Moderate hypothermia: 82.4°F to 90°F (28°C to 32.2°C). Shivering may stop, drowsiness increases.
- Severe hypothermia: Below 82.4°F (28°C). Loss of consciousness, life-threatening.
Because older adults already have a lower baseline (potentially as low as 96.4°F), a reading in the mid-95s deserves attention even if they feel fine. Persistent readings below 96°F in any adult are worth investigating.