A normal body temperature for most adults falls between 97.3°F and 98.2°F (36.3°C to 36.8°C), with an average around 97.9°F. That’s notably lower than the 98.6°F number most of us grew up hearing. The old standard came from a German physician who took over a million temperature readings in the 1860s, but large-scale research from Stanford Medicine shows that average human body temperature has dropped by about 0.05°F per decade since then, likely because improvements in health and living conditions have reduced chronic inflammation.
Why 98.6°F Is Outdated
The 98.6°F benchmark was published in 1868 and stuck around for more than 150 years. Researchers at Stanford analyzed over 618,000 oral temperature readings from adult outpatients between 2008 and 2017 and found the real average sits closer to 97.9°F. The traditional number isn’t dangerous or wrong in a clinical sense, but if you see 97.5°F on your thermometer and worry something is off, it’s almost certainly fine.
That said, medical guidelines still use 98.6°F as a general reference point, and the fever threshold of 100.4°F (38°C) hasn’t changed. So while your “normal” may be lower than the textbook number, the line where doctors start considering a fever remains the same.
Your Temperature Changes Throughout the Day
Body temperature isn’t static. It follows a daily rhythm, dipping to its lowest point in the early morning (typically between 4 a.m. and 6 a.m.) and peaking in the early evening. The difference between your daily low and high can range from 0.5°F to nearly 2°F. This means a reading of 99°F at 7 p.m. may be perfectly normal for you, while the same number at 6 a.m. could signal something is brewing.
Factors That Shift Your Baseline
Age, sex, body size, and even the time you take your temperature all influence the reading. Together, these factors account for about 25% of the variability in a single person’s temperature over time. Still, the person-to-person differences are large enough that there’s no single “correct” number for everyone.
For people who menstruate, basal body temperature rises by 0.5°F to 1°F after ovulation and stays elevated through the second half of the cycle. This is normal and is actually the basis for temperature-based fertility tracking. If you’re charting your temperature for any reason, readings during the luteal phase will consistently run higher than in the first half of your cycle.
Older adults tend to run cooler overall, and their daily temperature rhythm shifts. In younger adults, the lowest temperature of the day occurs around 4 to 6 a.m., but in people aged 70 to 80, the trough can shift to as late as noon. This means a fever in an older person might not register as dramatically on a thermometer, even when an infection is present.
When a Temperature Becomes a Fever
The widely accepted fever threshold is 100.4°F (38°C) for oral, rectal, ear, or temporal artery readings. An armpit reading of 99°F (37.2°C) or higher is also considered a fever, since armpit measurements tend to run about a degree lower than oral ones.
Not all fevers feel the same or carry the same urgency. A low-grade fever between 100.4°F and about 102°F is common with minor infections and often resolves on its own. Adults with temperatures of 103°F (39.4°C) or higher typically look and feel noticeably sick. In infants, any rectal temperature of 100.4°F or above warrants prompt medical attention, since young babies can’t fight infections the way older children and adults can.
When a Temperature Is Too Low
A body temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia ranges from 90°F to 95°F and typically causes shivering, confusion, and clumsiness. Moderate hypothermia (roughly 82°F to 90°F) brings more severe confusion, drowsiness, and loss of shivering. Below 82°F is severe and life-threatening.
Hypothermia doesn’t only happen in extreme cold. Older adults, very young children, and people with certain medical conditions can develop low body temperature indoors, especially in poorly heated environments. If your thermometer consistently reads below 96°F without an obvious explanation like just coming in from the cold, it’s worth bringing up with a doctor.
How Measurement Method Matters
Where you take your temperature affects the number you get. Oral readings are the most common for adults and are what most “normal” ranges are based on. Rectal readings run about 0.5°F to 1°F higher than oral, which is why they’re considered the most accurate, particularly for infants. Armpit (axillary) readings run about a degree lower than oral and are the least precise, though they’re convenient for a quick check. Ear and forehead (temporal artery) thermometers fall somewhere in between and can be influenced by ambient temperature or technique.
If you’re trying to figure out whether a specific reading is normal, knowing which method you used is just as important as the number itself. A forehead reading of 99.5°F means something different than an armpit reading of 99.5°F.