The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) figure most of us learned growing up. That classic number dates back to 1851, when a German physician took millions of armpit readings from 25,000 patients. Modern studies with hundreds of thousands of measurements consistently find the true average is lower.
Why 98.6°F Is Outdated
The 98.6°F standard came from Dr. Carl Wunderlich’s massive study in the mid-1800s. At the time, it was groundbreaking work, and the number stuck for over 150 years. But a large-scale analysis of American body temperatures across two centuries found that men born in the early 1800s ran about 1.06°F (0.59°C) warmer than men today. Women showed a similar decline of about 0.58°F (0.32°C) since the 1890s. The drop has been steady: roughly 0.05°F per decade of birth.
A study of more than 35,000 British patients with nearly 250,000 temperature readings confirmed the lower modern average, finding a mean oral temperature of 97.9°F (36.6°C). The leading explanation is that reduced rates of chronic infection and inflammation, along with changes in metabolic rate and living conditions since the Industrial Revolution, have gradually lowered the body’s resting temperature.
The Normal Range
There is no single “normal” temperature. Healthy adults typically fall somewhere between 97°F (36.1°C) and 99°F (37.2°C). Your temperature shifts throughout the day, running lowest in the early morning and peaking in the late afternoon or evening. This daily rhythm is driven by your internal clock and can account for a swing of roughly 1°F over the course of a day.
How Age Affects Body Temperature
Adults under 60 average about 98°F (36.69°C), while adults 60 and older run cooler at roughly 97.7°F (36.5°C), a difference of about 0.4°F. This matters because an older person with a serious infection may not spike as high a fever as a younger person would, making it easier to miss. Fever thresholds for frail elderly adults are set lower for exactly this reason.
Gender differences, by contrast, are negligible. Women average about 97.97°F (36.65°C) and men about 98.04°F (36.69°C), a gap so small it has no practical significance.
How Your Body Maintains Its Temperature
A small region at the base of the brain called the hypothalamus acts as your body’s thermostat. It receives signals from two types of temperature sensors: those in your skin, which track how warm or cold your surroundings are, and those deeper in your core organs and spinal cord, which monitor internal heat. When these sensors detect a shift away from your set point, the hypothalamus triggers responses to correct it, like sweating to cool down or shivering to generate warmth.
Where You Measure Matters
Different spots on the body give different readings. Rectal temperatures run about 0.8°F (0.43°C) higher than armpit readings and are considered the closest reflection of true core temperature. Oral readings fall in between, roughly 0.45°F (0.25°C) above armpit measurements. If you’re comparing your reading to a standard, it helps to know which method produced it.
Timing matters too. Eating or drinking anything hot or cold within 15 minutes of an oral reading can throw it off. Ear thermometers can give inaccurate results if you’ve just come in from very hot or cold weather, so wait about 15 minutes before using one.
When a Temperature Becomes a Fever
The most widely used cutoff is 100.4°F (38°C), measured orally. Above that threshold, you most likely have a fever driven by infection or illness. But context matters depending on the time of day. A morning oral temperature above 99°F (37.2°C) or a late afternoon reading above 99.9°F (37.7°C) can also qualify as a fever, since your body naturally runs cooler in the morning.
Beyond those thresholds, fevers are sometimes graded by severity. Temperatures between 100.4°F and 101.1°F (38°C to 38.4°C) are considered slight, 101.3°F to 102.2°F (38.5°C to 39°C) moderate, and anything above 103.1°F (39.5°C) a high fever.
When Body Temperature Drops Too Low
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia ranges from 90°F to 95°F (32°C to 35°C), moderate from 82°F to 90°F (28°C to 32°C), and severe hypothermia is anything below 82°F (28°C). Even mild hypothermia can impair judgment and coordination, which is why it’s a serious concern in cold-weather emergencies.