# Why is -40 fahrenheit equal to -40 celsius? **URL:** \ **Category:** Factual Questions\ **Created:** [November 12, 2013, 4:30am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699 "2013-11-12T04:30:17Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [November 12, 2013, 4:29pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/21 "2013-11-12T16:29:47Z")
> [@Floater](#): > > Why Denmark? It sounds as arbitrary Yes, it is very arbitrary. Denmark because that’s where Fahrenheit lived. And you need a zero point. At the time, they had no clue what the actual lowest possible temperature was. So, they arbitrarily picked a value lower than the lowest recorded value in Denmark to avoid negative numbers. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 12, 2013, 4:33pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/22 "2013-11-12T16:33:35Z")
[quote=“CalMeacham, post:13, topic:673699”] It’s not that surprising. Fahrenheit chose a scale of 180 degrees between freezing and boiling. The Centigrade/Celsius scale chose 100 degrees of difference between the same two points. When both scales used nice whole numbers for both freezing and boiling points, it became likely that the intersection point between these two non-parallel lines was going to be , if not a whole number itself, then some simply fraction. [quote] Nope. It all has to do with the slopes of the two lines, and you can get any number of different slopes with “nice whole numbers”. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [November 12, 2013, 4:38pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/23 "2013-11-12T16:38:18Z")
[quote=“John\_Mace, post:22, topic:673699”] [quote=“CalMeacham, post:13, topic:673699”] It’s not that surprising. Fahrenheit chose a scale of 180 degrees between freezing and boiling. The Centigrade/Celsius scale chose 100 degrees of difference between the same two points. When both scales used nice whole numbers for both freezing and boiling points, it became likely that the intersection point between these two non-parallel lines was going to be , if not a whole number itself, then some simply fraction. > [@](#): > > Nope. It all has to do with the slopes of the two lines, and you can get any number of different slopes with “nice whole numbers”. Nope – you misunderstand my point. Please re-read more carefully. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [November 12, 2013, 6:40pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/24 "2013-11-12T18:40:07Z")
[quote=“CalMeacham, post:23, topic:673699”] [quote=“John\_Mace, post:22, topic:673699”] > [@CalMeacham](#): > > It’s not that surprising. Fahrenheit chose a scale of 180 degrees between freezing and boiling. The Centigrade/Celsius scale chose 100 degrees of difference between the same two points. When both scales used nice whole numbers for both freezing and boiling points, it became likely that the intersection point between these two non-parallel lines was going to be , if not a whole number itself, then some simply fraction. > > Nope – you misunderstand my point. Please re-read more carefully. Exactly. If the scale ( freezing to boiling) is defined as an exact number in both, then the ratio is such that any conversion will be a rational number. Since 1 in every 5 degrees C (integer) translates to an integer value of F, the odds are 1 in 5 that the point the lines cross is an integer in both scales. Actually, 1 in 4? (I.e if Freezing/boiling had been 33/213F, the crossover point would not have been integer. C= 5(F-32)/9; for C=F, this works when the offset works - let X be Fahrenheit freezing offset. C=5(F-x)/9 crosspoint is where C=F, so solve for x - F=5(F-x)/9, or 4F=-5x, F=-(5/4)x when F is the crossover. So crosover is an integer if 5x/4 is an integer, if x is divisible by 4, x being the offset between F and C in the scales. For example, if freezing in F was 28 degrees (boiling 208) then the conversion is C=5(F-28)/9 the common point is F=C, so F=5(F-28)/9 or 4F=-140 crossover F=-35 ---
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [November 12, 2013, 8:38pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/25 "2013-11-12T20:38:43Z")
> [@Xema](#): > > And he says something rather dubious: > > Everybody? > > There’s a lot of agreement that one scale would be better than two, and Celsius is the logical candidate (being more widely used). But the Fahrenheit scale has some advantages: The range 0 to 100 covers the typical range of climate experienced by the vast majority of humans. And a difference of one degree F is about the minimum increment that matters. > No - as others have noted, it’s exact. Nice to see there is one other person out there that thinks the F is not totally outclassed by C 🙂 How do you feel about the Electoral College? 😃 ---
**Author:** ![James\_Toothpaste](https://avatars.discourse-cdn.com/v4/letter/j/e56c9b/32.png) [@James\_Toothpaste](https://boards.straightdope.com/u/James_Toothpaste)\ **Post date:** [November 12, 2013, 10:11pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/26 "2013-11-12T22:11:59Z")
Because math. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 12, 2013, 10:51pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/27 "2013-11-12T22:51:01Z")
> [@](#): > > Quoth **Xema** : > > And a difference of one degree F is about the minimum increment that matters. Matters in what context? If you mean the smallest temperature difference that can readily be perceived by humans, that’s closer to a Celsius degree than a Fahrenheit one. Though it is a good point that Fahrenheit’s range of 0-100 is a more practically useful one for human experience. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 13, 2013, 12:20am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/28 "2013-11-13T00:20:31Z")
> [@CalMeacham](#): > > It’s not that surprising. Fahrenheit chose a scale of 180 degrees between freezing and boiling. The Centigrade/Celsius scale chose 100 degrees of difference between the same two points. When both scales used nice whole numbers for both freezing and boiling points, it became likely that the intersection point between these two non-parallel lines was going to be , if not a whole number itself, then some simply fraction. > > \ > > Nope – you misunderstand my point. Please re-read more carefully. Emphasis added. To be fair, one needs to read your typo correctly (“some simple fraction” and not “simply some fraction”) in order for it to make sense. So, if that’s what you meant by “read it more carefully”, one might wonder why you didn’t write it more carefully. Especially since most people on this MB aren’t going to know what a “simple fraction” is, and so wouldn’t know how to correct the typo. 😉 Besides, had the answer been -39.3125, do you really think that poster you responded to would have remarked about how strange it was? ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [November 13, 2013, 12:39am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/29 "2013-11-13T00:39:06Z")
> [@Chronos](#): > > Though it is a good point that Fahrenheit’s range of 0-100 is a more practically useful one for human experience. Hah! For _some_ human experience, I suppose. This old chestnut never fails to make me laugh. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [November 13, 2013, 1:38am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/30 "2013-11-13T01:38:54Z")
> [@Crafter\_Man](#): > > This is the correct and simplest answer. > > Since we’re on this topic, a little bit of trivia for ya: the freezing temperature of pure water is not exactly 0 °C. It is very close to 0 °C, but it is not exactly 0 °C. In addition, the boiling point of pure water at standard pressure is not exactly 100 °C. It is close to 100 °C, but it is not exactly 100 °C. If you want trivia, you can actually have negative absolute temperature (that is: negative degrees kelvin) using forbidden science magic I’m not even going to pretend to understand. Apparently because of the way entropy works, any arbitrary negative temperature is far, far hotter than any fixed positive temperature. ---
**Author:** ![Cugel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cugel/32/1199_2.png) [@Cugel](https://boards.straightdope.com/u/Cugel)\ **Post date:** [November 13, 2013, 3:43am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/31 "2013-11-13T03:43:54Z")
> [@Gorsnak](#): > > Hah! For _some_ human experience, I suppose. This old chestnut never fails to make me laugh. I hadn’t heard that one before, seems a close relative of “a third of an inch” ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [November 13, 2013, 2:26pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/32 "2013-11-13T14:26:03Z")
> [@John\_Mace](#): > > Emphasis added. > > To be fair, one needs to read your typo correctly (“some simple fraction” and not “simply some fraction”) in order for it to make sense. So, if that’s what you meant by “read it more carefully”, one might wonder why you didn’t write it more carefully. Especially since most people on this MB aren’t going to know what a “simple fraction” is, and so wouldn’t know how to correct the typo. 😉 > > Besides, had the answer been -39.3125, do you really think that poster you responded to would have remarked about how strange it was? 1381 – Which Tyler leads the Pedant’s Revolt. He is promptly forgotten by history ---
**Author:** ![runningdude](https://avatars.discourse-cdn.com/v4/letter/r/e9c0ed/32.png) [@runningdude](https://boards.straightdope.com/u/runningdude)\ **Post date:** [November 13, 2013, 6:17pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/33 "2013-11-13T18:17:02Z")
At some point, Fahrenheit became tied to Celsius. They kept the traditional 32 degree freezing point on Fahrenheit, and then defined the boiling point using the not-quite arbitrary value of 32+180 = 212, which roughly aligned with Fahrenheit’s original scheme. I assume this definition was made in part to ensure an exact conversion between the two scales of temperature (similar to an inch being redefined as exactly 2.54 cm.). Fahrenheit has the advantage of 0 being approximately how cold it gets and 100 being approximately how hot its gets in many parts of the world (New England in particular). It has a bit of a “false decimal” feel to it, even though it is awkward for scientific use. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [November 13, 2013, 7:25pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/34 "2013-11-13T19:25:14Z")
> [@Cugel](#): > > I hadn’t heard that one before, seems a close relative of “a third of an inch” Which IIRC is (very) roughly a centimeter. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [November 13, 2013, 11:57pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/35 "2013-11-13T23:57:05Z")
Strictly speaking, the Celsius scale shouldn’t be called that at all. Celsius’ scale was reversed, with 100 degrees being the freezing point of water and 0 degrees being the freezing point. Linnaeus flipped the scale, and it became known as the Centigrade scale (and was later re-renamed the Celsius scale because centigrade was already in use in certain languages.) ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [November 14, 2013, 12:19am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/36 "2013-11-14T00:19:55Z")
> [@Really\_Not\_All\_That\_Bright](#): > > Strictly speaking, the Celsius scale shouldn’t be called that at all. Celsius’ scale was reversed, with 100 degrees being the freezing point of water and 0 degrees being the freezing point. Linnaeus flipped the scale, and it became known as the Centigrade scale (and was later re-renamed the Celsius scale because centigrade was already in use in certain languages.) Man, it would be a weird world where you can’t go to 373.15C but can freely go to an arbitrary negative number. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [November 14, 2013, 3:08pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/37 "2013-11-14T15:08:54Z")
> [@Chronos](#): > > If you mean the smallest temperature difference that can readily be perceived by humans, that’s closer to a Celsius degree than a Fahrenheit one. I believe it’s easy to perceive the difference between water at 97 and 98 degrees F. Between 40 and 39 degrees F, probably not so much. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 14, 2013, 3:26pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/38 "2013-11-14T15:26:16Z")
Water, maybe, but the scale is used far more often for air temperature. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 14, 2013, 6:59pm UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/39 "2013-11-14T18:59:07Z")
> [@Xema](#): > > I believe it’s easy to perceive the difference between water at 97 and 98 degrees F. Between 40 and 39 degrees F, probably not so much. I support this Pitting. The Celsius degree is too large. I am reminded of this constantly by air-con settings which do not allow fractional degrees. In fact I was reminded just a moment ago: Mrs. Septimus said “Cooler, please. Just one degree.” I clicked the setting from 27° to 26°. After a very short time: “Warmer please.” (Mrs. Septimus does have her idiosyncrasies, but I experience the same inability to fine-tune that setting myself.) ---
**Author:** ![TBG](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@TBG](https://boards.straightdope.com/u/TBG)\ **Post date:** [November 15, 2013, 12:52am UTC](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699/40 "2013-11-15T00:52:34Z")
I get why having boiling point of water being at 100 might be great for scientific applications, but most people aren’t doing scientific experiments when they use temperature, so for weather and the like it’s more user friendly to fit the normal, non-extreme weather into 0-100 degrees. Not needing decimals on the thermostat is an added bonus. [Previous page](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699.md?page=1) [Next page](https://boards.straightdope.com/t/why-is-40-fahrenheit-equal-to-40-celsius/673699.md?page=3)