# 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:** 14
**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 4, 2013, 2:40am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/261 "2013-06-04T02:40:36Z")
Measurements will be imprecise to varying degrees. Definitions, however, have perfect precision. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 6, 2013, 10:37pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/262 "2013-06-06T22:37:26Z")
> [@DrDeth](#): > > But you see, no one uses the “International Foot”. In the US we always use the Survey Foot. Wrong. We only use the Survey Foot in real estate. In construction, we measure and cut boards using the international foot. Construction blueprints, as far as I know, are also in international feet. And then we measure the area of the house we just built in Square Survey Feet. Go figure. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 6, 2013, 10:55pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/263 "2013-06-06T22:55:25Z")
> [@tracer](#): > > In construction, we measure and cut boards using the international foot. Construction blueprints, as far as I know, are also in international feet. Actually … I take that back. Sort of. The typical steel measuring tapes used in construction work probably undergo enough thermal expansion and contraction for the difference between Survey Feet and International Feet not to matter. ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [June 7, 2013, 2:45am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/264 "2013-06-07T02:45:36Z")
> [@RobDog](#): > > Actually, the imperial unit of sound is the Blessed. This demonstrates why the subdivisions of a decimal system are so handy. Few sounds are loud enough to be measured in full Blesseds, just things like a MOAB or the creation of the universe. Normal sounds are measured in picoblesseds. > [@Critical1](#): > > Related to the first quote, we are throwing away millions of dollars every year we continue to use the standard system. It takes ages to do calculations with the standard system compared to the metric. There are plenty of machine shops who have effectively converted to metric simply by converting every single measurement into thousandths of an inch so they can avoid the insanity of a system with no basis in logic what so ever. Arrant nonsense. It takes no longer to calculate in decimal inches than millimeters. And no shop I ever worked in changed to metric to make calculations easier because I was the guy doing the calculating, and who cared if I had to do some extra work? Nobody, that’s who. And one company had changed to metric some years before I started putting their drawings into the CAD system, so I found I had to convert from metric to decimal inches, round it to the closest fractional inches, then go back to decimal inches and then to metric, using the most decimal places I could (not scientific notation–too inaccurate), not because the part needed that degree of accuracy but because the CAM software did, and being off by a billionth of a millimeter would cause it to fail. ETA: Sometimes a rounding error in the software down around a trillionth of a millimeter would crash it. That was a lot of zeros before it broke. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 7, 2013, 6:35pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/265 "2013-06-07T18:35:18Z")
> [@dropzone](#): > > And one company had changed to metric some years before I started putting their drawings into the CAD system, so I found I had to convert from metric to decimal inches, round it to the closest fractional inches, then go back to decimal inches and then to metric, using the most decimal places I could (not scientific notation–too inaccurate), not because the part needed that degree of accuracy but because the CAM software did, and being off by a billionth of a millimeter would cause it to fail. So … this CAD software system could only handle fractional inches natively? It had no native support for millimeters? That’s … _ugh._ ---
**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 7, 2013, 7:08pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/266 "2013-06-07T19:08:55Z")
> [@tracer](#): > > Actually … I take that back. Sort of. > > The typical steel measuring tapes used in construction work probably undergo enough thermal expansion and contraction for the difference between Survey Feet and International Feet not to matter. Or the error could come from your vision. Or the angle from which you make a mark. Or the width of the pencil mark. Or the kerf of the saw if you’re cutting. ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [June 7, 2013, 11:15pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/267 "2013-06-07T23:15:22Z")
> [@tracer](#): > > So … this CAD software system could only handle fractional inches natively? It had no native support for millimeters? > > That’s … _ugh._ No, it could only deal with decimal units. The units could be anything, even light years. Fractional inches are easily-memorized rational numbers–nothing goes beyond the sixth decimal place–so I would look at the length in units in the drawing, find the closest fractional inch, and reengineer it from there. No one told me to do that, but after a few dimensions it became obvious that the last guy had converted a fractional-inch drawing to metric by erasing the old numbers and writing in the new ones. The metric dimensions were only shown to one or two decimal places because that was close enough for the real world, but it wasn’t close enough for the computer world. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 8, 2013, 1:57am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/268 "2013-06-08T01:57:19Z")
> [@dropzone](#): > > No, it could only deal with decimal units. The units could be anything, even light years. Fractional inches are easily-memorized rational numbers–nothing goes beyond the sixth decimal place–so I would look at the length in units in the drawing, find the closest fractional inch, and reengineer it from there. No one told me to do that, but after a few dimensions it became obvious that the last guy had converted a fractional-inch drawing to metric by erasing the old numbers and writing in the new ones. The metric dimensions were only shown to one or two decimal places because that was close enough for the real world, but it wasn’t close enough for the computer world. Ah … so the issue here wasn’t the limitations of the CAD software, it was that the _original drawings_ were labelled in fractional inches. Since the shop you were working in had “gone metric”, I take it these drawings had been supplied by an outside customer, or were “legacy” drawings or something? ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [June 8, 2013, 2:51am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/269 "2013-06-08T02:51:15Z")
We had drawings that went back to the 1890s. They weren’t still in use, but yeah, there were some legacy drawings still in use from ten years before I worked there, which would put them before the metrication of the US. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [June 8, 2013, 3:19am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/270 "2013-06-08T03:19:46Z")
> [@jz78817](#): > > and yet neither you nor anyone on your “side” have demonstrated why I should care. as I already said, real life is not a _Professor Layton_ game. I’m not going to go into a shop and have the shopkeeper make me solve a story problem before he’ll sell me anything. If I need to know something like this, I know the conversion factor(s.) Exactly. I think the only thing I ever had to do on that list **Chronos** made was scale a recipe. On the other hand, when doing some minor carpentry I came to appreciate the convenience of having feet divide twelve times into inches. Twelve has a lot of factors, and dividing by three is very easy. Do that with a decimal system and, well, your in decimal city. Meanwhile, I could divide by three and then by two all in my head getting whole numbers with each answer. It’s really quite elegant. ---
**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 9, 2013, 12:32am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/271 "2013-06-09T00:32:00Z")
Heh. To those saying that my examples have very little practical application, there’s [another thread that was just posted](http://boards.straightdope.com/sdmb/showthread.php?t=693228) that’s basically the same problem as one of them (find volume in gallons, given linear measurements). Admittedly, that question was probably also based on not knowing how to calculate the volume of a cylinder, but the unit conversions are still necessary for it. ---
**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 9, 2013, 2:05am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/272 "2013-06-09T02:05:49Z")
> [@Chronos](#): > > Heh. To those saying that my examples have very little practical application, there’s [another thread that was just posted](http://boards.straightdope.com/sdmb/showthread.php?t=693228) that’s basically the same problem as one of them (find volume in gallons, given linear measurements). yes, this example does not support your argument in the slightest since irrational numbers are involved regardless of whether the pool’s dimensions are measured in feet or meters. > [@](#): > > Admittedly, that question was probably also based on not knowing how to calculate the volume of a cylinder, but the unit conversions are still necessary for it. so fucking what? ---
**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 9, 2013, 5:28pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/273 "2013-06-09T17:28:20Z")
It does support my argument, since being able to solve the problem requires knowledge of the conversion factors, and almost nobody knows the relevant conversion factors for the US system without looking them up, and a significantly larger set of people do know the relevant metric conversion factors. I fail to see how the presence of an irrational number in any way changes this. Especially since nobody ever actually uses irrational numbers in calculations, just rational approximations to them. You could just say “pi is about 3”, and get a reasonable answer to that question. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 10, 2013, 5:41pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/274 "2013-06-10T17:41:24Z")
> [@WarmNPrickly](#): > > On the other hand, when doing some minor carpentry I came to appreciate the convenience of having feet divide twelve times into inches. Twelve has a lot of factors, and dividing by three is very easy. Do that with a decimal system and, well, your in decimal city. Meanwhile, I could divide by three and then by two all in my head getting whole numbers with each answer. It’s really quite elegant. I’ve seen this particular argument on more than one anti-metric webpage. Feet can be divided easily into halves, thirds, and quarters, whereas meters can only be conveniently divided into halves and fifths – and there are not many practical situations where you need to divide something into fifths. However, I’d think it would be quite rare indeed in carpentry when you’d find yourself faced with something that was _exactly_ a foot long, and you needed to space something 1/3 of the way along. I’d think that more often, the thing you’d need to divide into thrids would be, say, 14 and 5/8 inches long – which is no easier to divide into thirds than 37.1 centimeters. ---
**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 10, 2013, 5:45pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/275 "2013-06-10T17:45:14Z")
> [@tracer](#): > > I’ve seen this particular argument on more than one anti-metric webpage. Feet can be divided easily into halves, thirds, and quarters, whereas meters can only be conveniently divided into halves and fifths – and there are not many practical situations where you need to divide something into fifths. But the math to do that sucks. I don’t want to have to deal with various fractions, and mess with inches. I can use decimals to do anything. ---
**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 10, 2013, 5:52pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/276 "2013-06-10T17:52:04Z")
> [@tracer](#): > > I’d think that more often, the thing you’d need to divide into thrids would be, say, 14 and 5/8 inches long – which is no easier to divide into thirds than 37.1 centimeters. It doesn’t refute your argument, but you did pick a poor example. A third of 14 and 5/8 inches is exactly 4 and 7/8 inches, while a third of 37.1 cm is 12.3666666… cm. 😃 It seems interesting that Brits often did divide their old-style money into thirds (and presumably had the amounts memorized); a “mark” was 13 shillings and 4 pence. William Shakespeare left 26 shillings and 8 pence (2 marks) to each of three fellow players. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 10, 2013, 6:11pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/277 "2013-06-10T18:11:49Z")
> [@septimus](#): > > It doesn’t refute your argument, but you did pick a poor example. > A third of 14 and 5/8 inches is exactly 4 and 7/8 inches, while a third of 37.1 cm is 12.3666666… cm. 😃 D’oh! :smack: > [@](#): > > It seems interesting that Brits often did divide their old-style money into thirds (and presumably had the amounts memorized); a “mark” was 13 shillings and 4 pence. William Shakespeare left 26 shillings and 8 pence (2 marks) to each of three fellow players. It’s interesting, though, that feet and shillings were divided into twelfths, but pints and pounds weren’t. It’s even more interesting (to me, anyway) that while the U.S. liquid pint was divided into 16 fluid ounces, the Imperial pint was divided into 20 fluid ounces. I love to mention that when anybody recites “A pint’s a pound the world around” – apparently the world does not include the U.K.. 😉 ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 10, 2013, 6:16pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/278 "2013-06-10T18:16:20Z")
> [@septimus](#): > > It seems interesting that Brits often did divide their old-style money into thirds (and presumably had the amounts memorized); a “mark” was 13 shillings and 4 pence. William Shakespeare left 26 shillings and 8 pence (2 marks) to each of three fellow players. Well that was a little before my time, but when I was at school we had pounds shillings and pence. Twelve pence to the shilling and twenty shillings to the pound. Now it is certainly true that a third of a pound was easy at 6s and 8d (normally written as 6/8) but that did not make up for the complexity of adding up columns of figures. Try it? ## £2 12/6 £4 9/4 £3 17/9 6+4+9=19, so 7d and carry 1s 12+9+17+1=39, so 19s and carry £1 2+4+3+1=10 The answer is ten pounds nineteen shillings and seven pence. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 10, 2013, 9:00pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/279 "2013-06-10T21:00:12Z")
> [@bob\_2](#): > > Now it is certainly true that a third of a pound was easy at 6s and 8d (normally written as 6/8) but that did not make up for the complexity of adding up columns of figures. Try it? > > ## £2 12/6 £4 9/4 £3 17/9 > > 6+4+9=19, so 7d and carry 1s > 12+9+17+1=39, so 19s and carry £1 > 2+4+3+1=10 > The answer is ten pounds nineteen shillings and seven pence. Thank goodness none of those figures was in Guineas. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 10, 2013, 10:00pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784/280 "2013-06-10T22:00:56Z")
In my lifetime, guineas were only ever used for buying and selling racehorses. Curiously, and for a long time in the 60s, a pound was worth $2.40. This is the same as the number of pennies in a Pound, so a US cent and an English penny were the same value. [Previous page](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784.md?page=13) [Next page](https://boards.straightdope.com/t/what-is-the-purpose-of-the-us-customary-system-of-measurement/659784.md?page=15)