98.6 degrees f to c: Why the "Normal" Body Temperature is Changing

You’re feeling a bit sluggish, maybe a little warm. You grab the digital thermometer from the medicine cabinet, slide it under your tongue, and wait for that annoying beep. 37 degrees Celsius pops up. If you grew up in the US, you’re probably doing some quick mental math to figure out what that means. You're likely looking for 98.6 degrees f to c because that's the gold standard we were all taught in grade school.

But here’s the kicker. That "perfect" number is actually a bit of a historical accident.

The Math Behind 98.6 degrees f to c

Converting Fahrenheit to Celsius isn't exactly intuitive unless you’re a math whiz. To get from 98.6°F to Celsius, you subtract 32, multiply by 5, and then divide by 9.

$$C = (F - 32) \times \frac{5}{9}$$

When you plug 98.6 into that formula, you get exactly 37. It’s a clean, round number. That’s not a coincidence. Back in the mid-1800s, a German physician named Carl Reinhold August Wunderlich analyzed millions of temperatures from about 25,000 patients. He was the guy who established 37°C as the mean biological "normal" for humans. When his work was later translated into English, the 37°C figure was converted to 98.6°F.

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The decimal point makes it sound incredibly precise. It feels like a law of physics. In reality, Wunderlich was using a thermometer that was about a foot long and took almost twenty minutes to get a reading. Honestly, his equipment was primitive compared to the infrared sensors we use today.

Is 98.6 Still Actually "Normal"?

Probably not. Most modern medical researchers, including teams at Stanford University, have found that our bodies are actually cooling down.

A massive study led by Dr. Julie Parsonnet at Stanford looked at temperature records dating back to the Civil War. They found that the average body temperature has been dropping by about 0.03°C per decade. If you look at a healthy adult today, they are much more likely to clock in at 97.5°F or 97.9°F than the "standard" 98.6.

Why? We have less inflammation. In the 19th century, people were constantly fighting off chronic infections like tuberculosis, periodontitis, and malaria. Inflammation cranks up your metabolic rate, which raises your temperature. Today, we have better hygiene, vaccines, and antibiotics. We also live in climate-controlled houses. Our bodies don't have to work nearly as hard to maintain homeostasis, so we've literally chilled out.

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Why Converting 98.6 degrees f to c Matters in a Clinical Setting

If you’re traveling abroad or looking at a medical report from a country that uses the metric system, seeing "37.5" might freak you out if you aren't sure of the conversion. Is that a fever?

In the Celsius world, a "fever" is generally considered anything over 38°C (which is 100.4°F). If you are sitting at 37°C, you are exactly at the translated equivalent of 98.6. But because our baseline is dropping, some doctors argue that we should be redefining what a fever actually is. For a person whose "normal" is actually 97.2°F, hitting 98.6 might actually mean they’re fighting something off, even though it’s the "perfect" number on paper.

Factors That Mess With Your Reading

Temperature isn't a static number. It’s a rhythm. Your body isn't a thermostat set to a single point; it's more like a wave.

• Time of Day: You’re coldest at about 4:00 AM and warmest in the late afternoon. Your temperature can swing by a full degree or more throughout the day.
• Age: Older adults tend to have lower body temperatures. Their bodies don't regulate heat as efficiently. This is actually dangerous because an elderly person can have a severe infection but still show a "normal" 98.6°F reading.
• Hormones: Women’s temperatures shift significantly during the menstrual cycle, especially after ovulation.
• Where you measure: Ear (tympanic) and rectal temperatures are usually higher than oral ones. Armpit (axillary) readings? Totally unreliable most of the time.

The Problem With the "Normal" Label

We love categories. We love knowing if we are "in" or "out" of the healthy range. But the obsession with 98.6 degrees f to c has led to some medical dismissal.

Have you ever gone to the doctor feeling like garbage, but they tell you "your vitals are fine" because you’re at 98.6? If your personal baseline is 97.4, then 98.6 is actually an elevated state for you. We need to move toward "personalized baselines" rather than a one-size-fits-all number from the 1800s.

Practical Steps for Tracking Your Health

Since 98.6 is more of a historical landmark than a medical absolute, you should focus on your own trends.

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1. Establish your baseline. Take your temperature for three days in a row when you feel perfectly healthy. Do it once in the morning and once in the evening. Average those numbers. That is your normal.
2. Use the same tool. Don't switch between a forehead scanner and an oral thermometer. They calibrate differently.
3. Wait 30 minutes. If you just drank a hot coffee or an ice-cold water, your oral reading will be useless. Wait for your mouth to return to its natural state.
4. Look at the symptoms, not just the digits. If the thermometer says 37°C (98.6°F) but you have chills, body aches, and a headache, you’re sick. Trust your body over the 150-year-old math.

The conversion of 98.6 degrees f to c is a helpful bit of trivia and a necessary bridge between different measurement systems. However, the most important number isn't the one in the textbook—it's the one that's normal for you.

To stay ahead of your health, start logging your temperature during a week when you feel great. Record it in a health app or a simple notebook. Knowing your personal "normal" will make it much easier to spot when something is actually wrong, regardless of whether you're measuring in Fahrenheit or Celsius.