A normal adult body temperature is about 97°F to 99°F (36.1°C to 37.2°C) (MedlinePlus). Most healthy people sit a little below the famous 98.6°F (37°C). Your own normal depends on your body, the time of day and how you measure. What's typical for you tells you more than any textbook number.
Key takeaways
The 98.6°F figure comes from armpit readings taken in Germany in the 1800s (Protsiv et al., 2020).
Newer studies put the average lower: 98.2°F (36.8°C) in healthy adults aged 18 to 40 (Mackowiak et al., 1992) and 97.9°F (36.6°C) in more than 35,000 people (Obermeyer et al., 2017).
Temperature is lowest around 6 a.m. and highest between about 4 and 6 p.m., with an average swing of 0.9°F (0.5°C) (Mackowiak et al., 1992).
A reading more than 1°F to 1.5°F (0.6°C to 0.8°C) above your own normal counts as a fever (MedlinePlus).
Where did 98.6°F come from?
The number comes from Carl Wunderlich, a German doctor working in Leipzig in the 1800s. He took millions of armpit readings from 25,000 patients, and his average, 98.6°F (37°C), became the standard for well over a century (Protsiv et al., 2020).
Thermometers have improved since then, and doctors now mostly measure by mouth. Researchers who studied the change still think most of it is real: people today simply run cooler (Protsiv et al., 2020). After measuring healthy adults in 1992, one team said 98.6°F "should be abandoned" as the measure of normal (Mackowiak et al., 1992).
What's normal, according to newer studies?
Newer studies put the average adult at about 97.9°F to 98.2°F (36.6°C to 36.8°C), and none of the three large studies below found an average of 98.6°F. In the two biggest, 95% of readings fell in a span about 2.5°F to 2.8°F (1.4°C to 1.6°C) wide.
Study | Who was measured | Average | Range |
|---|---|---|---|
148 healthy adults aged 18 to 40, by mouth | 98.2°F (36.8°C) | upper limit of normal 99.9°F (37.7°C) | |
35,488 adults at doctor visits, 243,506 readings | 97.9°F (36.6°C) | 96.3°F to 99.1°F (35.7°C to 37.3°C) for 95% of readings | |
396,195 adult doctor visits, by mouth | 97.95°F (36.64°C) | 96.7°F to 99.2°F (35.95°C to 37.33°C) for 95% of readings |
In the 2023 study, no group sorted by age, sex, height, weight or time of day averaged 98.6°F (Ley et al., 2023). People may also be running cooler over time. In U.S. records going back to the Civil War, people born in each decade ran about 0.05°F (0.03°C) cooler than those born ten years before (Protsiv et al., 2020).
The most useful finding may be how much people differ. In the 2017 study, measured factors such as age, sex and weight explained only 8.2% of the differences between people (Obermeyer et al., 2017). Most of the differences went unexplained, so your own readings tell you more than your age or weight can.
Why does body temperature change during the day?
Your body runs on a daily cycle, and temperature follows it. By mouth, it's lowest around 6 a.m. and highest between about 4 and 6 p.m., with an average swing of 0.9°F (0.5°C) (Mackowiak et al., 1992). Your core temperature (the temperature deep inside your body) swings a bit more, about 1.4°F to 1.8°F (0.8°C to 1.0°C) (Baker et al., 2020).
For people who menstruate, temperature also rises after ovulation (when an ovary releases an egg). Measured on waking, before getting up, resting temperature runs 0.5°F to 1.3°F (0.3°C to 0.7°C) higher in the second half of the cycle, mostly because of the hormone progesterone (Baker et al., 2020).
That's why a 5 p.m. reading isn't comparable to a 7 a.m. one. Compare mornings with mornings.
Does age change your normal temperature?
A little. In a study of 18,630 white adults aged 20 to 98 at one U.S. health plan, the oldest group ran just 0.3°F (about 0.2°C) cooler than the youngest (Waalen and Buxbaum, 2011). The 2017 study found a drop of about 0.04°F (0.021°C) per decade of age (Obermeyer et al., 2017).
The bigger change shows up in frail older people. Their normal can be lower, and a classic fever doesn't show up in more than half of care home residents with a serious infection (High et al., 2009).
How much does the thermometer matter?
Quite a bit. The same body gives different numbers in different places (MedlinePlus):
Rectal and ear readings usually run 0.5°F to 1°F (0.3°C to 0.6°C) higher than a reading by mouth.
Armpit and forehead readings usually run 0.5°F to 1°F (0.3°C to 0.6°C) lower.
Timing matters too. Wait 20 to 30 minutes after eating, drinking something hot or cold, or smoking. After hard exercise or a hot bath, wait at least an hour (MedlinePlus).
Everyday thermometers also miss some fevers. In a review of 75 studies, ear, forehead, armpit and mouth thermometers caught 64% of the fevers that a probe inside the body found (Niven et al., 2015). If you feel feverish but the reading looks normal, don't let one number settle it.
How do you find your own normal temperature?
Take your temperature on a few mornings in a row when you feel well, before you eat or drink. Use the same thermometer and the same spot each time, since each spot reads differently (MedlinePlus). The middle of those readings is a fair guess at your normal. It's the same idea as reading any number against your own past: a change from your own normal tells you more than a textbook figure.
When is a temperature a fever?
A reading over 100.4°F (38°C) most often means a fever from an infection or illness (MedlinePlus). That line isn't universal. Another MedlinePlus page says an adult probably has a fever above 99°F to 99.5°F (37.2°C to 37.5°C) by mouth, depending on the time of day (MedlinePlus). Oddly, 100.4°F began as the top of the normal range in the 1800s (Mackowiak et al., 1992).
Your own normal is a better guide. A reading more than 1°F to 1.5°F (0.6°C to 0.8°C) above your normal, taken the same way, counts as a fever (MedlinePlus). For frail older adults in care homes, a doctors' guideline counts any of these as a fever (High et al., 2009):
one reading over 100°F (37.8°C) by mouth
repeated readings over 99°F (37.2°C) by mouth
a rise of more than 2°F (1.1°C) over that person's normal
Contact a doctor right away if a fever reaches 105°F (40.6°C), stays above 103°F (39.4°C), or lasts longer than 48 to 72 hours. Call too if fevers keep coming and going for a week or more. The same goes for any fever if you have a serious health condition or a weakened immune system. Call 911 if a fever comes with confusion, trouble waking up, trouble breathing, a stiff neck, a very bad headache or a seizure (MedlinePlus).
What about temperature from a watch or ring?
Watches and rings measure your skin overnight, not the inside of your body. So they show a change from your own normal rather than a reading, usually after several nights of wear. One watchmaker says plainly that its temperature feature is not a thermometer, and a ring maker shows skin temperature as a plus or minus from your personal normal.
A rise can show up before you feel sick. In one small study, 38 of 50 people who reported having COVID-19 had a fever-like day on their ring sometime in the 45 days before their symptoms began (Smarr et al., 2020). Their infections weren't confirmed with tests, and the ring's maker paid for the study, so it needs bigger, independent studies. A rise on your wrist is a nudge to take a real temperature, not a reading in itself.
Frequently asked questions
Is 97°F a normal temperature? Yes, for many adults. Studies show a normal range of about 97°F to 99°F (36.1°C to 37.2°C) (MedlinePlus), and a 2017 study of more than 35,000 people put the average at 97.9°F (36.6°C) (Obermeyer et al., 2017).
Is 99°F a fever? Usually not, by the common 100.4°F line. But if your normal is around 97.5°F (36.4°C), 99°F is a real rise for you. One MedlinePlus page also says an adult probably has a fever above 99°F to 99.5°F by mouth (MedlinePlus).
Can you have an infection without a fever? Yes. Frail older adults often don't run a classic fever, even with a serious infection (High et al., 2009). Everyday thermometers can also miss a fever (Niven et al., 2015).
Are forehead thermometers accurate? They usually read 0.5°F to 1°F (0.3°C to 0.6°C) lower than a reading by mouth, so compare forehead readings only with other forehead readings (MedlinePlus).
How Knit shows your temperature
Knit puts every temperature on one chart in the Record tab. That includes readings you add, readings from Apple Health and readings from your medical records, so you can see what's normal for you and spot a real change.
