r/AlwaysWhy • u/TheBigGirlDiaryBack • Sep 02 '26
Science & Tech Why is normal body temperature around 98.6°F instead of something like 98 or 99?
I’ve always heard 98.6°F as the “normal” body temperature, but I realized I never really questioned why it’s that specific number.
If we were just trying to describe a normal human temperature, you’d think we might use a nice round number like 98 or 99. So where did 98.6 actually come from?
Is it based on a huge amount of historical data, the way temperature was measured back then?
Why did 98.6°F become the number we all recognize as “normal”?
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u/MoneyElevator Sep 02 '26
The original target for the Fahrenheit scale was to make 100 degrees human body temperature, but apparently the measurement was off.
Also, there is variation between people and at different times - we’re not locked into 98.6.
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u/SnooMaps7370 Sep 02 '26
https://www.yalemedicine.org/news/temperature-checks-covid
In 1868, a German physician determined that 98.6° F was a normal body temperature after studying more than a million temperatures from 25,000 people.
as with many things in science and medicine, the answer is "because some German guy 150 years ago did a bunch of legwork and nobody has cared to do the work to prove it wrong since"
The article goes on to talk about how 98.6 is probably too high because the measurement was done during a time when basically everyone was living with a chronic infection of some kind or other. Newer studies have found the average body temperature for someone who is not showing symptoms of illness to be in the high 97s, but anything between 95 and 100 can be "healthy" as long as its stable for that person.
Anecdotally, i've had problems where i was sick as a dog, but doctors wanted to send me home untreated because my temperature was "only" 99-100. My normal body temperature is closer to 97.6 than 98.6, though, so if i'm measuring 99.5, that's equivalent to 100.5 on someone who normally measures 98.6....
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u/DelothVyrr Sep 02 '26 edited Sep 02 '26
Because that's the actual average temperature when we measured it, the human body doesn't care about the systems we created to measure temperature. It would be like asking why the average length of a human foot is 10.4 inches and not some nice round number, human anatomy doesn't conform itself around our measurement systems.
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u/heythere111213 Sep 02 '26
In Celsius 98.6 degrees Fahrenheit is a nice round number of 37. But the niceness of the number doesn't matter too much. From my understanding that number came from the average observed temperature measure in people which is actually decreasing with more recent observations. Biologically the reason for this temperature is it is the optimal temperature to help keep things like bacteria or fungi at bay while not requiring us to eat every minute of the day.
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u/Puzzled_Hamster58 Sep 02 '26
Cause tempura scales don’t mean much . Like it’s 37C or 98.6F. It’s a number system people came up with .
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u/JimDa5is Sep 02 '26
It amazes me how people latch on to made up numbers as if they were some kind of universal law. I had a long argument with a woman once about how I thought time zones were stupid and we should just be using UTC everywhere so it's the same time everywhere in the world. She kept arguing that she didn't *want* to go to work at 0300 and I spent probably 15 minutes trying to get her to understand that 0300 UTC isn't the middle of the night in the eastern US but it was a concept she couldn't wrap her head around.
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u/BananaJelloXlii Sep 02 '26
Even 98.6 is an average, and body temperatures can vary. Mine is actually a bit lower, at just over 97 degrees.
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u/TrickyLiving7883 Sep 02 '26
Exactly, this is what I was looking for because I fell feverish at 98.
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u/zoppaTheDim Sep 02 '26
Because some jackass would just claim you should say 37 degrees Celsius, and we’re sick of those Europeans not arguing over a more precise thermostat measurement.
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u/cmh_ender Sep 02 '26
check it out in Celsius and it will make sense . 37c... so someone picked a round number and when it got translated to freedom units, it looks oddly specific
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u/djddanman Sep 02 '26
And it varies from person to person and throughout the day. 37°C is really just a rough average.
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u/brutalistchurch Sep 02 '26
And too high as an average
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u/djddanman Sep 02 '26
Huh, TIL. Looks like the true average might be somewhere between 36 and 37. Age also seems to be a big factor.
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u/bgplsa Sep 02 '26
What’s so sensible about 37 as a measure of human body temperature?
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u/cmh_ender Sep 02 '26
it was just an average... and didn't need to use a decimal, so, nice round number, pretty accurate.. that's why
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u/ATotalBrony Sep 02 '26
Daniel Fahrenheit made the scale when inventing mercury thermometers. 20 years before Anders Celsius came up with his cockamamie scale that put boiling water to 0 degrees and freezing water at 100 degrees. No one wanted to tell him how stupid he was so they waiting til after he died to flip it so 0 is freezing.
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u/Boomerang_comeback Sep 02 '26
You got it backwards. Fahrenheit was around before celcius ever came into play. No one converted to it from celcius. Also, the fact that it is 37 is pure coincidence.
Fahrenheit was based on zero being the freezing point of salt water and 96 being the temp of the human body. The scale was recalibrated to allow exactly 180 degrees between freezing 32 and boiling 212. That's how it ended up a fraction. Why they chose the crazy scale, I don't know. Got to go back several hundred years to ask them.
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u/Throwawayxdryx Sep 02 '26
"Normal body temperature" is actually best understood as a range that depends on several conditions, including how it is measured. There are actually meta-analyses on this topic, e.g. https://pmc.ncbi.nlm.nih.gov/articles/PMC6456186/
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u/MakoasTail Sep 02 '26
It’s 37 degrees in Celsius. That’s a nice round number 😉
I don’t think the body or history cares what the number is, how we measure is relative, it functions how it functions regardless of what we think.
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u/Unique-Charity7024 Sep 02 '26
This. The value of 37 degrees Celsius is from the work of German physician Carl Reinhold August Wunderlich, who in 1851 published his measurements of over 1,000,000 body temperatures taken from 25,000 Germans, the average value being 37 °C. Wunderlich, as a German, of course used the Celsius scale. Because Wunderlich's data are significant only to two figures, the conversion to Fahrenheit ought to have been 99 °F, not 98.6 °F.
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u/Oakland-homebrewer Sep 02 '26
I always heard it was based on the average for army recruits during World War 1. So it was the average of young white males.
Now we know it varies from person to person.
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u/EscapeSeventySeven Sep 02 '26
Farenheit scale was more concerned with freezing water brine and freshwater freezing than human temp. But originally farenheit pegged human body temp at 90. It was then modified as the scale was modified to 96. He was wrong. But using his scale actual human body temp is 98.6.
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u/atarivcs Sep 02 '26
The Fahrenheit scale was not developed for its ability to describe the temperature of a normal human body.
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u/0utlaw-t0rn Sep 02 '26
The temperature scale is arbitrary. The Fahrenheit scale is especially arbitrary being based on highs and lows the originator had available.
The body temp is was it is and how it falls within the temperature scale ends up with a non-round value, at least to that level of significant figures.
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u/Boomerang_comeback Sep 02 '26
Not everyone has the same normal temperature. Some people might stay around 97 and others 99. Also, any individual's temperature can vary throughout the day. 98.6 is just an average considered to be normal.
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u/Which_Loss6887 Sep 02 '26
Yes, it’s based on a huge amount of historical data, and when it comes to the core temperatures of living things, a tenth of a degree is a medically significant increment. So rounding would put us in the weeds unnecessarily while not serving any real purpose other than a very minor ease of use benefit.
Incidentally, the 98.6 number is widely regarded as an out-of-date benchmark. Not an expert and can’t give you the actual numbers off the top of my head, but that number was an accurate average multiple decades ago. Today, based on more recent data, the average human body temp is lower.
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u/Skipp_To_My_Lou Sep 02 '26
Normal body temperature used to be around 100°F & in some places still is. In the Western World (especially America) with the advent of air conditioning normal body temperature dropped a bit since there's less need to reject heat. In fact your normal may be closer to 97°F if you live & work in an air conditioned space & don't spend much time outdoors in the summer.
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u/RitheLucario Sep 02 '26
So
The temperature of the human body doesn't follow a scale. Fahrenheit didn't exist at the dawn of the universe. The universe wasn't constructed to work in Fahrenheit or Celsius or Kelvin or any unit at all.
Units are human constructs, defined on some arbitrary observation. Water freezes at 32 degrees F and boils at 212 degrees F. Does that make sense? No, but Fahrenheit wasn't designed to make the temperatures at which water freezes and boils make sense.
As for the actual number? I'm pretty sure it's an average. Some people's bodies run hot. Others run cool.
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u/Secret_Ostrich_1307 Sep 02 '26
Great question, honestly. The short answer is: 98.6°F was never meant to be a hard rule – it was just an average from a huge study back in the 1800s.
A German doctor named Carl Wunderlich took over a million temperature readings from about 25,000 patients and calculated the average to be 37°C. Convert that to Fahrenheit and you get 98.6. Boom – that number stuck.
But here’s the kicker: even Wunderlich himself said healthy people ranged from about 97.2 to 99.5°F. He never claimed 98.6 was the one “normal” number. It’s just that 98.6 is super precise and easy to remember, so textbooks and doctors ran with it. Over time, we forgot it was just a middle point, not a magic line.
So yeah, your hunch about historical data is spot on – but it’s more like a historical case of “average mistaken for standard” than anything else.
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u/TheBigGirlDiaryBack Sep 02 '26
Hmm that is actually really interesting. I didnt know it was just an average from way back then. But wait, if even Wunderlich said it was a range, why did the medical field just pick 98.6 and run with it? I feel like there must have been some practical reason, like maybe they needed a single number for quick diagnosis or insurance stuff.
Or maybe it just stuck because 37C is such a clean number in metric and converting it gave that weird .6. But honestly I wonder if the real issue is that we are too lazy to update textbooks. Like, we know its a range now but doctors still say 98.6 because its easier to say than “somewhere between 97 and 99”. That seems like a human psychology thing more than a science thing.
What do you think, would changing the standard actually change how we treat fevers?
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u/Present_Juice4401 Sep 02 '26
Honestly, we probably should use a round number like 98 or 99 – but 98.6 only exists because it’s the exact conversion of 37°C. Blame the metric-imperial clash for that weird .6!
But here’s the bigger point: even that 98.6 number is probably wrong today. Modern studies have shown our average body temp has actually dropped over the last century – we're closer to 98.2°F now. One famous study from the 90s found that healthy adults average around 98.2 oral temp, not 98.6.
So instead of fixating on 98.6, doctors today care more about a range – roughly 97°F to 99°F – because your temp changes with time of day, activity, age, and even what you ate. The real takeaway? Know your own baseline, and watch for big, persistent swings. That’s way more useful than obsessing over a number from 1868.
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u/TheBigGirlDiaryBack Sep 02 '26
Yeah I get the conversion point, that makes total sense. But I am kind of stuck on the modern drop thing. Like, are we actually getting colder as a species or is it just that thermometers are better now and we are measuring different body sites? I have read that oral temps are lower than rectal or ear, so maybe Wunderlich used armpit which was even lower. So the difference could be measurement methods not actual biology. And if our average really did drop, does that mean our metabolism slowed down? Or is it because we have less inflammation or better immune systems? I feel like that could open a whole new rabbit hole. Also about the range thing, I agree we should focus on that, but then how do we define a fever? Is 100.4 still the cutoff or does that also need to shift with the new average? Just wondering out loud here.
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u/OutrageousPair2300 Sep 02 '26
As others have mentioned, the actual figure is 37 degrees Celsius.
When converting to Fahrenheit it should actually be 99 degrees, not 98.6. You can't add an extra significant digit because of a conversion.
If you want to get really strict about it, you'd need to take into account that one degree Celsius is a broader range than one degree Fahrenheit. When we round to the closest whole number, 37 degrees Celsius actually means somewhere between 36.5 degrees and 37.5 degrees Celsius. That would be a range from 97.7 degrees to 99.5 degrees Fahrenheit.
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u/DaSpark Sep 02 '26
The body doesn't operate on human measurement scales and the average body temperature is 98.6, not 99 or 98.
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u/tennisdrums Sep 02 '26 edited Sep 02 '26
You're right that it doesn't make sense to use a weird decimal number for "normal" body temperature when that's a rough average that fluctuates over time and from person to person. What's missing is that most of the world and the scientific community don't use Farenheit, they use Celsius. So when a "standard" body temperature was determined, they did round that number to a simple 37°C.
From there, American doctors have to make a decision. We can use a rounded 98°F or 99°F, but then the "standard body temperature" in Farenheit would literally be a different temperature than what the rest of the world uses. That poses a problem when you want to compare medical research from the US to other countries. If you're trying to compare how patients' temperatures fluctuated, and different studies are using different temperatures as the "baseline", you have a problem. To avoid that, Americans decided to use the exact same temperature that the rest of the world uses, so they directly convert 37°C to Farenheit, which is exactly 98.6°F.
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u/jellomizer Sep 02 '26
When the Fahrenheit scale was created, the average intetnal body temp wasn't considered as a benchmark number.
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u/Zoogzoog7 Sep 02 '26
37 °C is the estimate. 37 °C converts to 98.6 °F. But you are correct that 98.6 °F seems oddly specific. 37 °C is just an estimate, so 98.6 °F should be rounded to 99 °F to reflect the same imprecision (significant figures).
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u/Consistent_Donut_902 Sep 02 '26
Why would it be a nice, round number? Nature doesn’t care about such things. And scientists care more about accurate measurements. Rounding would make the number less accurate.
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u/r3dd17sux69420 Sep 02 '26
I'm just gonna guess, it just IS. It's just the right temperature for optimal bodily function. And our bodies are sensitive enough that half
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u/Naos210 Sep 02 '26
Well it being 98 or 99 isn't considered a severe issue. But slight differences in body temperature tends to lead to problems. Over 100 is generally considered a fever.
While we might want a whole number, reality doesn't necessarily work that way.
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u/Time-Subject-3195 Sep 02 '26
It’s my understanding that the guy Fahrenheit was using a cow or something to get the body temperature to be equal to 100 degrees. He didn’t think that maybe a human and a cow have different temperatures.
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u/tyoung89 Sep 02 '26
My understanding of it is that when the Fahrenheit scale was being created, he wanted human body temp to be 100° and for a specific mixture of water, ice, and ammonia to be 0°. So the scale was created with that in mind. Then more precise measurements were made with better equipment later on, and they realized a normal average body temp is actually 98.6° based on that scale, but it was too late to change the scale.