A normal body temperature for most adults falls between 97.3°F and 98.2°F (36.3°C to 36.8°C) when measured orally, with an overall average around 97.9°F. That’s noticeably lower than the 98.6°F number most of us grew up hearing, which dates back to a German physician’s measurements from 1868. Your personal normal depends on your age, sex, body size, time of day, and how you take the reading.
Why 98.6°F Is Outdated
The 98.6°F standard comes from over a million temperature readings collected from roughly 25,000 patients in the mid-1800s. It was solid science for its time, but human body temperature has been steadily dropping since then. Research from Stanford Medicine, analyzing more than 618,000 oral temperature readings from 2008 to 2017, found the current average sits at about 97.9°F. A separate analysis of 20 studies published between 1935 and 1999 placed the average even lower, at 97.5°F.
The decline isn’t a fluke of better thermometers. Men born in the early 1800s ran temperatures roughly 1.06°F (0.59°C) higher than men today, dropping at a steady rate of about 0.05°F per decade. Women show a similar pattern, with temperatures falling about 0.58°F (0.32°C) since the 1890s. The most likely explanation is that people in earlier centuries carried far more chronic inflammation from untreated infections like tuberculosis, syphilis, and gum disease. Modern medicine, sanitation, and nutrition have lowered baseline inflammation, and with it, baseline body temperature.
What Shifts Your Temperature Throughout the Day
Your body temperature isn’t a fixed number. It rises and falls in a predictable daily rhythm. Temperatures tend to be lowest in the early morning and highest in the late afternoon or early evening. This swing can easily span a full degree Fahrenheit, so a reading of 97.4°F before breakfast and 98.4°F after dinner can both be perfectly normal for the same person.
Physical activity raises your temperature temporarily, as does eating a meal. Hormonal cycles play a role too. In women who menstruate, body temperature rises by 0.3°F to 1.0°F (0.18°C to 0.56°C) after ovulation and stays elevated through most of the second half of the cycle, driven by rising progesterone levels. This is the basis of temperature-based fertility tracking.
How Age Affects Normal Temperature
Infants and young children tend to run slightly warmer than adults, partly because their metabolisms are faster relative to body size. For babies, a rectal temperature is the most reliable measurement, and fever is defined as 100.4°F (38.0°C) or higher rectally. Ear thermometers aren’t considered accurate for infants under six months.
Older adults run in the opposite direction. Baseline temperature often drops as you age, for several overlapping reasons: you lose insulating fat under the skin (especially in your hands and feet), your metabolism slows, and your skin becomes drier, all of which increase heat loss. Common medications like beta blockers and certain psychiatric drugs can push temperatures lower still, as can an underactive thyroid. This matters because an older person with a serious infection might register a temperature that looks “normal” on paper but actually represents a significant spike from their personal baseline.
Readings Vary by Thermometer Location
Where you place the thermometer changes the number you get. Oral readings are the most common reference point for adults. Here’s how other methods compare:
- Rectal: reads 0.5°F to 1.0°F higher than oral
- Ear (tympanic): reads 0.5°F to 1.0°F higher than oral
- Armpit (axillary): reads 0.5°F to 1.0°F lower than oral
- Forehead (temporal): reads 0.5°F to 1.0°F lower than oral
There’s no exact, universal conversion between these methods, so it’s worth knowing which type of reading your thermometer gives. If you’re monitoring a fever, stick with the same method each time rather than switching between, say, forehead and oral readings.
Where Normal Ends and Fever Begins
The widely accepted fever threshold for both adults and children is 100.4°F (38.0°C) measured orally, rectally, or via an ear thermometer. For armpit readings, 99.0°F (37.2°C) is considered the cutoff. Temperatures between your personal baseline and these thresholds are sometimes called “low-grade fever,” though the term doesn’t have a strict clinical definition.
At 103°F (39.4°C) or higher, most adults will visibly look and feel sick, with noticeable fatigue, chills, and body aches. This is generally the range where fever becomes more concerning and warrants closer attention.
When Temperature Drops Too Low
On the other end of the spectrum, a core body temperature below 95°F (35°C) is classified as hypothermia. The severity breaks down into three stages:
- Mild (90°F to 95°F): shivering, confusion, and difficulty with coordination
- Moderate (82.4°F to 90°F): shivering may stop, drowsiness increases, and heart rhythm can become irregular
- Severe (below 82.4°F): loss of consciousness and risk of cardiac arrest
Hypothermia doesn’t require extreme cold. Older adults with lower baseline temperatures, people on certain medications, and anyone with prolonged exposure to even mildly cool environments can be at risk. If your thermometer consistently reads below 95°F in a normal indoor setting, that’s worth investigating.
Finding Your Personal Baseline
Because “normal” spans a range rather than landing on a single number, knowing your own baseline is more useful than comparing yourself to a population average. Take your temperature a few times over several days, at different times, when you’re feeling well. Use the same thermometer and the same method each time. Most people will find their readings cluster in a consistent zone somewhere between 97°F and 98.5°F orally. Once you know your personal normal, you’ll have a much better sense of what a meaningful spike actually looks like for you.