Normal body temperature for most adults is closer to 97.9°F (36.6°C), not the 98.6°F number you probably learned growing up. That old standard comes from a German physician’s measurements in 1868, and large-scale modern studies show the average has dropped since then. Your personal normal can fall anywhere from about 97.3°F to 98.2°F depending on your age, sex, body size, and time of day.
Why 98.6°F Is Outdated
The 98.6°F figure came from over a million temperature readings taken from 25,000 patients in the mid-1800s. It was solid work for its era, but human body temperature has been steadily declining since then. Researchers at Stanford Medicine analyzed more than 618,000 oral temperature readings from adult patients seen between 2008 and 2017 and found the current average sits at 97.9°F. The decline works out to roughly 0.05°F per decade, likely because modern living conditions, better nutrition, and lower rates of chronic infection have reduced the background level of inflammation in our bodies.
This matters practically. If your thermometer reads 97.5°F and you feel fine, nothing is wrong. That’s well within the normal adult range. Treating 98.6°F as the one “correct” number can make people worry unnecessarily or miss a mild fever because the reading doesn’t seem far enough above the old benchmark.
What Changes Your Temperature Throughout the Day
Your body temperature isn’t fixed. It follows a daily rhythm, running lowest in the early morning (often dipping below 97.5°F) and rising through the afternoon into the early evening. This swing can span a full degree or more, so a reading of 98.5°F at 6 p.m. may be completely normal for you even though it’s above the traditional average.
Several other factors shift your baseline:
- Age and body size. Together with sex, height, weight, and time of day, these factors account for about 25% of the variability in a person’s normal temperature readings.
- Physical activity. Exercise raises core temperature, sometimes significantly. A reading taken right after a workout won’t reflect your resting baseline.
- Menstrual cycle. After ovulation, rising progesterone levels push basal temperature up compared to the first half of the cycle. This is the principle behind temperature-based fertility tracking.
- Older age. Seniors tend to run cooler than younger adults, which can make it harder to detect fevers using standard cutoffs.
Where You Measure Makes a Difference
Rectal readings are closest to true core body temperature and run about 0.5°F to 1°F higher than oral readings. Armpit (axillary) readings tend to run about 0.5°F to 1°F lower than oral. Forehead and ear thermometers are convenient but can be less consistent, especially if the sensor isn’t positioned correctly or if you’ve just been outdoors in cold or hot weather. For infants under three months, rectal temperature is the standard because accuracy matters most at that age.
When a Temperature Counts as a Fever
An oral temperature of 100°F (37.8°C) or higher is generally considered a fever. The CDC uses 100.4°F (38°C) as its threshold. In practical terms, anything in the 99°F range can be a low-grade fever or simply the upper end of your personal normal, especially later in the day. Context matters: if you feel achy and chilled at 99.5°F, your body is likely fighting something even though the number looks modest.
For adults, a temperature of 103°F (39.4°C) or higher warrants a call to your doctor. Seek immediate care if a fever comes with a severe headache, stiff neck, rash, confusion, difficulty breathing, chest pain, persistent vomiting, or seizures.
Fever Thresholds for Babies and Children
Young children run warmer than adults, and the rules for when to act are stricter:
- Under 3 months: A rectal temperature of 100.4°F (38°C) or higher needs prompt medical attention, regardless of how the baby appears.
- 3 to 6 months: A rectal temperature above 102°F (38.9°C), or a lower fever paired with unusual irritability or sluggishness, calls for a doctor visit.
- 7 to 24 months: A rectal temperature above 102°F that lasts longer than one day without other symptoms should be evaluated.
- Older children: Seek care for fevers lasting more than three days, repeated vomiting, severe headache, or any seizure. Call 911 if a seizure lasts longer than five minutes.
When Temperature Drops Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. Even mild hypothermia (90°F to 95°F) causes shivering, confusion, and poor coordination. Moderate hypothermia (roughly 82°F to 90°F) brings drowsiness, slurred speech, and a slowing heart rate. Below 82°F is severe and life-threatening, with risk of cardiac arrest. Hypothermia doesn’t only happen in extreme cold. Older adults, very young children, and people who are wet or underdressed in moderately cold conditions are all vulnerable.
Finding Your Personal Baseline
Because normal temperature varies so much from person to person, the most useful thing you can do is learn your own baseline. Take your temperature a few times over several days when you’re feeling well, using the same thermometer and the same method each time. Note the time of day. After a handful of readings, you’ll have a personal range to compare against when you suspect a fever. A jump of 1.5°F or more above your usual number is meaningful, even if the thermometer doesn’t hit the textbook fever threshold.