Normal body temperature is closer to 97.5°F (36.4°C) than the familiar 98.6°F (37°C) most of us grew up hearing. That old number dates back to the 1860s, and large modern studies consistently show that average human body temperature has dropped since then. Your own “normal” also shifts throughout the day, varies by age and sex, and depends on where you take the reading.
Why 98.6°F Is Outdated
The 98.6°F standard comes from a German physician named Carl Wunderlich, who published a massive analysis of roughly 25,000 patients in 1868. For over a century, that number stuck. But modern researchers have found a significant problem: the thermometers Wunderlich used were calibrated 2.9°F to 3.4°F higher than instruments used today. His readings were skewed from the start.
Even setting the equipment aside, human body temperature has genuinely declined over time. A Stanford University analysis spanning 157 years of data, covering Civil War veterans through patients seen between 2007 and 2017, found that average body temperature has dropped about 0.05°F per decade. Over the full period, that adds up to more than a full degree. Researchers believe this likely reflects reductions in chronic inflammation, thanks to better sanitation, dental care, antibiotics, and overall improvements in public health.
An analysis of 20 studies published between 1935 and 1999 pegged the average oral temperature at 97.5°F. A more recent study of over 35,000 people landed at 97.9°F. Either way, 98.6°F is no longer the midpoint. It’s above average.
What Counts as a Normal Range
Rather than one fixed number, normal body temperature falls within a range of roughly 97.0°F to 99.0°F (36.1°C to 37.2°C) for most healthy adults. Where you land inside that window depends on several factors, including the time of day you check.
Your body temperature follows a predictable daily cycle. It hits its lowest point in the early morning, typically around 4 to 6 a.m., about two hours before you wake up. It climbs through the day and peaks in the early evening. The difference between your daily low and high can range from 0.5°F to 1.9°F. So a reading of 97.2°F first thing in the morning and 98.8°F after dinner could both be perfectly normal for the same person.
Readings Vary by Measurement Site
Where you place the thermometer changes the number you get, sometimes by a meaningful amount. Rectal readings run highest because they measure core body temperature most directly. Oral readings come in slightly lower. Armpit (axillary) readings are the lowest and least accurate of common methods.
This matters when you’re checking for a fever. The thresholds differ by site:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher indicates a fever
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
For infants and young children, rectal readings are the most reliable. If you take an armpit or forehead reading on a child and the number seems off, a rectal check gives you the most trustworthy confirmation.
Age, Sex, and Other Factors
Children tend to run warmer than adults, partly because their metabolisms are faster. Older adults trend cooler, often reading below 97°F without anything being wrong. This is worth knowing because a temperature of 99.5°F in an elderly person might represent a more significant immune response than the same number in a 30-year-old.
For people who menstruate, body temperature shifts predictably across the cycle. After ovulation, basal body temperature rises by 0.4°F to 1.0°F and stays elevated until the next period begins. This shift is small but consistent enough that some people use daily temperature tracking as a fertility awareness method.
Several other things can nudge your baseline reading up or down on any given day: poor sleep, stress, alcohol, travel across time zones, breastfeeding, and certain medications. Vigorous exercise raises body temperature for a period afterward as well. None of these represent illness, but they can make a single reading misleading if you’re trying to figure out whether you have a fever.
When a Temperature Becomes a Fever
The CDC defines fever as a measured temperature of 100.4°F (38°C) or greater. This threshold applies regardless of age and is the standard used in clinical and public health settings. Temperatures between your personal baseline and 100.4°F are sometimes called “low-grade fevers,” though that term has no strict clinical definition.
Because everyone’s baseline differs, it helps to know your own normal. If you typically run at 97.4°F, a reading of 99.5°F represents a two-degree jump, which may feel like a fever even though it falls below the official cutoff. Taking your temperature a few times when you’re feeling well, at the same time of day and with the same method, gives you a personal reference point that’s more useful than any population average.
Getting an Accurate Reading
Consistency matters more than precision. Use the same thermometer, the same body site, and roughly the same time of day when you’re comparing readings. Oral thermometers should go under the tongue with your mouth closed for the full measurement period. Avoid eating, drinking, or exercising for at least 15 minutes beforehand, since all of these temporarily shift oral temperature.
Digital thermometers are the standard for home use. Glass mercury thermometers are no longer recommended due to the risk of breakage and mercury exposure. Forehead (temporal artery) thermometers offer a quick, noninvasive option, though they can be thrown off by sweating or ambient temperature. Ear (tympanic) thermometers work well for older children and adults but can give unreliable readings if earwax is blocking the canal or the probe isn’t angled correctly.
If you get a reading that surprises you, wait a few minutes and try again. A single number, taken once, is a snapshot. Trends over hours or days tell you far more about what’s actually going on.