# What is the purpose of the US customary system of measurement? **URL:** \ **Category:** Factual Questions\ **Created:** [May 31, 2013, 1:57pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784 "2013-05-31T13:57:03Z")\ **Posts on this page:** 20\ **Page:** 13
**Author:** ![lance\_strongarm](https://avatars.discourse-cdn.com/v4/letter/l/87869e/32.png) [@lance\_strongarm](https://boards.straightdope.com/u/lance_strongarm)\ **Post date:** [June 3, 2013, 2:20am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/241 "2013-06-03T02:20:40Z")
> [@DCnDC](#): > > #1 I didn’t make any mistakes. I think you did earlier when you said there were 1.6 miles in a km when it’s the other way around. > [@](#): > > #2 Once again, you’ve misinterpreted what was said. **Emtar KronJonDerSohn** ’s mistake was not a matter of the measurement system or arithmetic, it was a mix-up of units. Replace “gallons” and “feet” with “liters” and “meters” and his mistake would be identical. But that’s my point - it’s so much less likely that you’ll make such mistakes in metric. There are 1,000 liters in a cubic meter. Calculations of length related to volume are much easier, both with conversion and the math, so mistakes happen much less. ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [June 3, 2013, 2:44am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/242 "2013-06-03T02:44:17Z")
> [@lance\_strongarm](#): > > I think you did earlier when you said there were 1.6 miles in a km when it’s the other way around. Nope. The mistake was yours. Allow me to walk you through it: **AaronX** posted this question: > [@AaronX](#): > > Those used to the imperial system - can you convert, say, 2.567 km to miles, feet, inches? To which **Smapti** responded: > [@Smapti](#): > > I _could_ do that. > > Or I could say it’s 1.6 miles and leave it at that, because unless I’m trying to pass a middle-school math test I can’t fathom any reason why 1.6 miles wouldn’t be an acceptable solution. To which you responded: > [@lance\_strongarm](#): > > It’s not 1.6 miles though. You already made a mistake. Since you have to keep track of all these conversion factors, the mistakes add up. > > It’s actually 1.6 km in a mile, not the other way around. And the question was not how many miles in 1 km, it was how many in 2.567 km. Obviously you misinterpreted **Smapti** ’s response. Read it closely and see if you can figure out YOUR mistake. I even pointed this out to you: > [@DCnDC](#): > > Read it again. There was no mistake. 2.567 km IS 1.6 miles. **Smapti** also corrected you: > [@Smapti](#): > > 2.567 km = 1.5950599 miles. > > 1.5950599 miles is roundable to 1.6 miles. ---
**Author:** ![fumster](https://avatars.discourse-cdn.com/v4/letter/f/e9c0ed/32.png) [@fumster](https://boards.straightdope.com/u/fumster)\ **Post date:** [June 3, 2013, 3:23am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/243 "2013-06-03T03:23:04Z")
> [@DrDeth](#): > > Sure it is. But many other temps are not such round numbers. > > You can “instantly” know with F, too. In C you have to remember that 0 is freezing, in F 32 is freezing. In both cases you need to know a number. > > There is no superiority of C over F. I did not claim there was, I was disputing your assertion that F is superior to C. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [June 3, 2013, 3:34am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/244 "2013-06-03T03:34:48Z")
> [@fumster](#): > > I did not claim there was, I was disputing your assertion that F is superior to C. I’ve lost track. was he asserting that F is superior to C, or asserting that C is not necessarily superior to F? big difference. ---
**Author:** ![fumster](https://avatars.discourse-cdn.com/v4/letter/f/e9c0ed/32.png) [@fumster](https://boards.straightdope.com/u/fumster)\ **Post date:** [June 3, 2013, 3:41am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/245 "2013-06-03T03:41:01Z")
> [@jz78817](#): > > I’ve lost track. was he asserting that F is superior to C, or asserting that C is not necessarily superior to F? big difference. His claim was that F is superior to C because you don’t need to use negative numbers when describing the temperature outside. He apparently has not been to Minnesota. ---
**Author:** ![R3d\_Anonymous](https://avatars.discourse-cdn.com/v4/letter/r/aeb1de/32.png) [@R3d\_Anonymous](https://boards.straightdope.com/u/R3d_Anonymous)\ **Post date:** [June 3, 2013, 3:58am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/246 "2013-06-03T03:58:16Z")
I was the one who claimed that C is slightly better than F due to the freezing and boiling points marked with 0 and 100 which seem to make more sense than 32 and 212. But I agree that if you want to go truly non-arbitrary, you may want to take a look at Kelvin. The only problem with K is that we would constantly be in the 200-300’s. To avoid this, C is just K-273 so that you avoid the disadvantage of K’s really high values _and_ have the freezing and boiling points marked most sensibly. **jz78817** claims that C being less arbitrary is irrelevant, but I disagree because as you see, C is only K-273, and that is so that you avoid really high numbers. And the reason it is 273 is so that the freezing and boiling points come out nicely at 0 and 100. So it is not all that arbitrary and therefore, I claim that C is best for everyday life. ---
**Author:** ![Emtar\_KronJonDerSohn](https://avatars.discourse-cdn.com/v4/letter/e/58956e/32.png) [@Emtar\_KronJonDerSohn](https://boards.straightdope.com/u/Emtar_KronJonDerSohn)\ **Post date:** [June 3, 2013, 4:00am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/247 "2013-06-03T04:00:35Z")
> [@DCnDC](#): > > You got that the wrong way around. there are about 7.5 gallons in a cubic foot. There are 1728 cubic inches in a cubic foot, so a gallon is 231 cubic inches. Yeah, I realized that I had it backwards after I posted. I was shuffling around a lot of numbers in my head. The simpler solution which I have always encountered in the US is to buy aquariums by the gallon to begin with. ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [June 3, 2013, 4:57am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/248 "2013-06-03T04:57:15Z")
> [@septimus](#): > > BTW, Thailand kept its cake and ate it too. For example, originally there were 24 _niu_ (fingers) in a _sok_ (distance from fingertip to elbow) and 4 _sok_ in a _wa_ (distance from outstretched fingertip to opposite fingertip). The Thai finger size presumably differed from that of King Henry IV(?), but when they converted to metric, _sok_ and _wa_ continued in use, meaning 0.5 and 2 meters respectively. There are now 20 _niu_ to a _sok_ instead of 24, or, officially, 19.6850393701 _niu_ to a _sok_ since the _niu_ has changed to the _International Inch_. > > In rural Thailand, to measure a plot of land roughly(\*) in the shape of a rectangle ABCD, opposing sides are added and then multiplied: Area = (AB + CD) x (BC+ AD). You may think that a _divide-by-four_ is missing here, but no: the distances here are measured in _wa_ but the area in _square meters_. (My impression is that some Thais overlook this change of units and think the formula as stated is the correct formula for a rectangle’s area. :smack: ) > > (\* - If the shape is an exact rectangle, two of the measurements are unnecessary; if not, the formula is inexact anyway. Still, it gives a good-enough approximation for near-rectangles.) > > BTW; show of hands, please. How many knew the following > one _International Inch_ = 2.54000000 centimeters, but > one _Statutory Surveyor’s Inch_ = 2.54000508 centimeters? > Are the surveyor’s measures still in use in the U.S. for surveying? [Raises hand.] I knew that. Yes, the survey foot is used for surveying. No, the difference between an ordinary foot and a survey foot is not great; it only comes up for precise measurements across, say, half a county. BTW, surveyors don’t use inches much at all. If the survey is in ‘customary’ units, it will be in feet and decimal feet. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [June 3, 2013, 5:37am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/249 "2013-06-03T05:37:59Z")
The one instance in my life where metric proved far more _useful_ (as opposed to just arbitrarily picking one system or the other) was when I had to estimate how much a filled aquarium would weigh. Measure height, width and depth in centimeters, multiply to get total number of cubic centimeters, a cc of water weights just about a gram… easy peasy, though I think I converted the resulting weight in kilograms to pounds for the benefit of my mother, whose aquarium it was. The rest of the time, I use imperial for small distances, kilometers for longer ones, metric for volume, metric for small weights, imperial for larger weights… whatever seems more familiar and useful given the circumstances. ---
**Author:** ![sich\_hinaufwinden](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@sich\_hinaufwinden](https://boards.straightdope.com/u/sich_hinaufwinden)\ **Post date:** [June 3, 2013, 6:00am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/250 "2013-06-03T06:00:38Z")
> [@septimus](#): > > BTW; show of hands, please. How many knew the following > one _International Inch_ = 2.54000000 centimeters, but > one _Statutory Surveyor’s Inch_ = 2.54000508 centimeters? I knew. Actually, this is something I happened to think about while reading through the thread. > [@septimus](#): > > Are the surveyor’s measures still in use in the U.S. for surveying? Yes. My state is one of those that still uses the US Survey Foot for survey and civil engineering. > [@DrDeth](#): > > Let me introduce you to the concept of “SigFigs”. The difference between the two definitions can easily lead to non-trivial errors. As I noted above, the US Survey Foot is still commonly used in the US. > [@BigT](#): > > He’s saying no one knows it because it’s pretty much irrelevant as the difference is too small to be of any significance on most scales. Several years ago MicroStation (cad software) switched its coordinate storage system to metric. So internally, everything is actually stored in metric, despite what may be shown on the screen. Any non-metric coordinates, lengths, data input, etc. is actually a conversion based on the values in a configuration text file. Out of the box, the default definitions use the International Foot. To get the software to work correctly for those using US Survey Feet, the text file needs to be modified to use the correct definition. Where I am, the difference between the two definitions results in an error of about 15 feet for the state coordinate system. That is obviously a significant error when survey and design is generally done to 0.01 ft. And I’ve seen this error crop up numerous times over the years, both within my company and with files from elsewhere. Just recently a new employee’s files weren’t ‘‘lining up’’ with mine and I realized that the IT guy hadn’t set up the new guy’s software correctly. > [@BigT](#): > > Hence it is a poor way to advocate for the superiority of the metric system; such advocation is the default assumption for any post in this thread that shows the complications of the customary system. I don’t have a huge preference for one or the other. Like everyone else, I used the metric system plenty going through school and was perfectly comfortable with it. Switching for me wouldn’t be a big deal at all, but I certainly see the side of the argument that there doesn’t seem to be much true benefit to changing. For the most part, it’s just a different set of numbers to be familiar with. I don’t buy that the ease of converting cubic meters to liters, etc. as any real incentive for the reason others have repeatedly pointed out. I also don’t consider either system to be all that more intuitive than the other. Like others have noted, it’s just whatever you get used to. Realistically, I think that without meaningful advantage, inertia will tend to win out. And that’s what we see: where it makes economic sense to switch it’s already been done and where it doesn’t, switching probably will not happen. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [June 3, 2013, 7:23am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/251 "2013-06-03T07:23:30Z")
> [@drewtwo99](#): > > You absolutely weren’t wrong that an ounce and a kilogram do not measure the same quantity. Then is the NIST wrong? Units of measurement are standards. Standards come from authorities. The NIST is an authority. You are not. Therefore, in some contexts, the ounce and the kilogram do indeed measure the same thing. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [June 3, 2013, 4:37pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/252 "2013-06-03T16:37:01Z")
> [@sich\_hinaufwinden](#): > > I knew. Actually, this is something I happened to think about while reading through the thread. > > Yes. My state is one of those that still uses the US Survey Foot for survey and civil engineering. > > The difference between the two definitions can easily lead to non-trivial errors. As I noted above, the US Survey Foot is still commonly used in the US. > > . But you see, no one uses the “International Foot”. In the US we always use the Survey Foot. They can’t usually measure close enuf that 2 parts per million makes a difference is a house or a street. And the difference is NOT significant: _National Geodetic Survey also states, “these two conversion factors produce results that differ by 2 parts per million; hence for most practical work it does not make any difference to the average surveyor which one is used since [surveyors rarely] encounter distances [large enough for this to be a factor]. Converting a distance of 304,800 meters to feet using the two conversion factors, these are the results: 304,800 meters = 999,998 U.S. Survey Feet and 304,800 meters = 1,000,000 International Feet. A difference of 2 feet in one million feet.” [Editor’s Note: A million feet is approximately 189 miles.]_ ---
**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:** [June 3, 2013, 4:43pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/253 "2013-06-03T16:43:38Z")
And if you’re measuring from a reference benchmark at the corner of a state, that could indeed make a difference of several feet at the other end of the state. Which, yes, can be quite significant. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [June 3, 2013, 4:46pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/254 "2013-06-03T16:46:34Z")
> [@septimus](#): > > ??? > > ETA: Do you think international and survey inches are\* identical\* ? So, yes, you really are unclear on what a Sig-Fig is. It doesnt matter if they are identical or not- if the difference is not significant or will be less than the fuzziness brought in when measuring on that scale. When you’re surveying a road, a difference of 2 feet in one million feet doesn’t mean much. Even for the longest road in the world, the difference is less than an inch. > **[Significant figures](https://en.wikipedia.org/wiki/Sig-figs)** > > Significant figures, also referred to as significant digits or sig figs, are specific digits within a number written in positional notation that carry both reliability and necessity in conveying a particular quantity. When presenting the outcome of a measurement (such as length, pressure, volume, or mass), if the number of digits exceeds what the measurement instrument can resolve, only the number of digits within the resolution's capability are dependable and therefore considered significant. > ... ---
**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:** [June 3, 2013, 5:16pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/255 "2013-06-03T17:16:52Z")
> [@DrDeth](#): > > So, yes, you really are unclear on what a Sig-Fig is. > … > Even for the longest road in the world, the difference is less than an inch. We can agree to disagree on whether I know what a Sig-Fig is. Apparently we also disagree on whether the longest road in the world exceeds 8 miles or not. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [June 3, 2013, 6:31pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/256 "2013-06-03T18:31:20Z")
I know what a significant figure is, I just can’t puzzle out the math in this “longest road” dispute. Yonge Street was once cited as the longest road, at 1178 miles, or 6.220 million feet, or 7.464 x 10[sup]7[/sup] inches. But I can’t tell if that’s relevant. ---
**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:** [June 3, 2013, 9:29pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/257 "2013-06-03T21:29:02Z")
Well, _someone_ sure doesn’t know what significant figures means. The international inch and the surveyor’s inch are both defined precisely in terms of metric units, and those definitions are different. There are precisely 2.54000508 centimeters in one surveyor’s inch. There is no chance that a surveyor’s inch might actually be 2.54000507 or 2.54000509 cm. It’s always 2.54000508 precisely. Therefore, all of those digits really are significant. Even if the official definition of the surveyor’s inch went out to 20 decimal places, or 50, or 100, those digits would _still_ be significant digits, because that would then be the definition. It matters not that the Universe is not large enough to tell the difference: In any actual calculation, the precision would be much less than that, but that’s a limitation of the measurements, not of the definition of the unit itself. ---
**Author:** ![elfkin477](https://avatars.discourse-cdn.com/v4/letter/e/a9a28c/32.png) [@elfkin477](https://boards.straightdope.com/u/elfkin477)\ **Post date:** [June 3, 2013, 10:26pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/258 "2013-06-03T22:26:09Z")
> [@kenobi\_65](#): > > I don’t know how young / old you are, but you may be too young to remember the U.S.'s failed attempt to get people to switch to the metric system in the 1970s. There was a law (the [Metric Conversion Act](http://en.wikipedia.org/wiki/Metric_Conversion_Act)) passed in 1975, declaring the metric system “the preferred system of weights and measures for United States trade and commerce”. I’m almost 3 times as old as the OP and I wasn’t even born yet when this went on… ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [June 3, 2013, 10:31pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/259 "2013-06-03T22:31:58Z")
> [@Chronos](#): > > Well, _someone_ sure doesn’t know what significant figures means. The international inch and the surveyor’s inch are both defined precisely in terms of metric units, and those definitions are different. There are precisely 2.54000508 centimeters in one surveyor’s inch. There is no chance that a surveyor’s inch might actually be 2.54000507 or 2.54000509 cm. It’s always 2.54000508 precisely. Therefore, all of those digits really are significant. > > Even if the official definition of the surveyor’s inch went out to 20 decimal places, or 50, or 100, those digits would _still_ be significant digits, because that would then be the definition. It matters not that the Universe is not large enough to tell the difference: In any actual calculation, the precision would be much less than that, but that’s a limitation of the measurements, not of the definition of the unit itself. From wiki: _In various technical fields, “accuracy” refers to the closeness of a given measurement to its true value; “precision” refers to the stability of that measurement when repeated many times. The number of significant figures roughly corresponds to precision, not accuracy. The smaller digits are not significant because they effectively random noise generated by the measurement process having little to do with the true value; they, they can be omitted for some reporting purposes. (This approach ignores techniques such as averaging to produce a higher-precision result.)_ In other words, if the tools being used or the work that is being done can do or use that level of accuracy, it is not SigFig. And yes, I made a math error about the road. ---
**Author:** ![fumster](https://avatars.discourse-cdn.com/v4/letter/f/e9c0ed/32.png) [@fumster](https://boards.straightdope.com/u/fumster)\ **Post date:** [June 3, 2013, 11:03pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/260 "2013-06-03T23:03:27Z")
> [@DrDeth](#): > > From wiki: > _In various technical fields, “accuracy” refers to the closeness of a given measurement to its true value; “precision” refers to the stability of that measurement when repeated many times. The number of significant figures roughly corresponds to precision, not accuracy. The smaller digits are not significant because they effectively random noise generated by the measurement process having little to do with the true value; they, they can be omitted for some reporting purposes. (This approach ignores techniques such as averaging to produce a higher-precision result.)_ > > In other words, if the tools being used or the work that is being done can do or use that level of accuracy, it is not SigFig. Here is a story that illustrates the concept. A guard is giving a tour of the natural History Museum. He comes to a skeleton of a dinosaur and says “This dinosaur lived 250 million and 14 years ago”. Someone in the tour group asks “How can you possibly know it’s exact age?.” The guard replies “All I know is that when I started here they told me it was 250 million years old; and that was 14 years ago”. [Previous page](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784.md?page=12) [Next page](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784.md?page=14)