Normal body temperature for most adults averages about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us grew up hearing. That familiar figure dates back to 1868, when a German physician named Carl Wunderlich analyzed over one million temperature readings from roughly 25,000 patients. His work set the standard for more than a century, but modern research tells a different story.
Why 98.6°F Is Outdated
A Stanford Medicine study analyzing more than 618,000 oral temperature readings from adult outpatients between 2008 and 2017 found that normal body temperature actually ranges from 97.3°F to 98.2°F, with an overall average of 97.9°F. The same research team discovered that average body temperature in the U.S. has been dropping by about 0.05°F per decade since the 1800s. The likely explanation: improvements in health, sanitation, and living conditions have reduced chronic inflammation, which slightly lowers baseline temperature.
So if your thermometer consistently reads 97.5°F or 98.1°F, that’s perfectly normal. There is no single number that applies to everyone.
What Affects Your Normal Temperature
Your body temperature shifts throughout the day following a predictable pattern. It dips to its lowest point in the early morning, between 6 and 8 a.m., and peaks in the evening, around 7 to 9 p.m. This daily swing can easily account for a full degree of variation without anything being wrong.
Age, sex, height, weight, and time of day together account for about 25% of the variation in any individual’s normal readings. People who menstruate see an additional shift tied to their cycle: after ovulation, basal body temperature rises by 0.5°F to 1°F and stays elevated through the second half of the cycle. This is the basis of temperature-based fertility tracking.
Older adults tend to run cooler than younger people, which is worth knowing because it means a temperature that looks “normal” on a thermometer could actually represent a fever in someone over 65.
Where You Measure Matters
Not all thermometer placements give you the same number. Oral readings are the standard reference point, but other methods read slightly higher or lower:
- Rectal and ear (tympanic): typically 0.5°F to 1°F higher than an oral reading
- Armpit (axillary): typically 0.5°F to 1°F lower than an oral reading
- Forehead (temporal artery): generally close to oral, though accuracy varies by device
This means a rectal reading of 99.2°F and an armpit reading of 97.6°F could both reflect the same core temperature. If you’re comparing numbers over time, use the same method each time.
For infants and young children, rectal thermometers are considered the most accurate. In older kids and adults, oral or forehead thermometers are practical and reliable enough for home use.
When Temperature Becomes a Fever
A fever is generally defined as a temperature at or above 100.4°F (38°C) when measured rectally, by ear, or with a temporal artery thermometer. For oral readings, that same 100.4°F threshold applies. An armpit temperature of 99°F (37.2°C) or higher is considered a fever because of the lower baseline at that site.
Low-grade fevers, roughly in the 99°F to 100.4°F range orally, are common with mild infections and often resolve on their own. Adults with temperatures reaching 103°F (39.4°C) or higher will typically look and feel noticeably sick. In children, the same fever thresholds apply, though younger children, especially under 3 months, warrant more caution at lower temperatures simply because their immune systems are still developing.
When Temperature Drops Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. The severity breaks down into three stages:
- Mild hypothermia: 89.6°F to 95°F (32°C to 35°C), causing shivering and difficulty with coordination
- Moderate hypothermia: 82.4°F to 89.6°F (28°C to 32°C), where shivering may actually stop as the body loses its ability to warm itself
- Severe hypothermia: below 82.4°F (28°C), a life-threatening emergency
Hypothermia doesn’t require extreme cold. Prolonged exposure to cool, wet conditions or even air conditioning in frail or elderly individuals can gradually lower core temperature. Older adults are particularly vulnerable because they often have a lower baseline temperature to begin with and may not shiver as effectively.
Finding Your Personal Baseline
Because normal temperature varies so much from person to person, knowing your own baseline is more useful than memorizing a single number. Take your temperature a few times over several days at the same time, using the same method. Most people will find their readings cluster in a narrow range, and that range becomes your personal reference point for spotting when something is off.
If your normal runs around 97.5°F and you suddenly read 99.5°F, that two-degree jump could be more meaningful than someone else hitting 100°F when their baseline is 98.5°F. Temperature is personal, and the trend matters more than any single reading.