Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why the USA does not use the metric system

  1. Aug 2, 2015 #1


    User Avatar

    Staff: Mentor

  2. jcsd
  3. Aug 2, 2015 #2
    Lame, we need to go metric!
  4. Aug 2, 2015 #3


    User Avatar

    Staff: Mentor

    We must have been editing my post simultaneously. I originally went on autopilot and used HTML 'a href' tags for the link because I had just been editing some of my own web pages. Then I realized what I had done and tried to fix it, but it came out differently from what I intended! Your version works, so I'll leave it alone. :-p
  5. Aug 2, 2015 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    These types of stories just serve as Click-Bait for CNN. :mad:

    The metric system gets plenty of workout in the US in science and technology, especially manufacturing. Just about every car and truck on the road now is designed using the metric system, not only those models made by the Big Three domestic car makers, but also all the European and Asian brands which have set up manufacturing facilities in the US, a lot of their output which is exported back into their home markets.

    I'm an engineer by profession, so while I do most of my work in the USCS system, I do have to understand SI as well. Even though I was not raised to think metric, I believe knowing two systems has been beneficial to me in my work because I can check a metric result to see if it is reasonable.

    On the pages here at PF, I have seen students raised wholly in the metric system who sometimes calculate fantastically wrong results in their native measurement system and not realize what has occurred. For whatever reason, they're not sure if a cubic meter is bigger or smaller than a litre, and the welter of different SI, MKS, and CGS units can often give the unwary fits.

    Face it, measurement systems are conventions, where a group of people agree that this unit of measurement (the foot, the meter, the pound, the kilogram, etc.) will be the standard used in trade or science. No one convention, all such being human constructs, will be perfect or will satisfy all who use it.
  6. Aug 2, 2015 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    ... and, you've had to develop the mental agility to use both. Hear-hear.
  7. Aug 2, 2015 #6
    The big question is, 'is the US gallon a superior measure of volume to the British Imperial gallon?'
    Never mind them weird foreign liters and stuff.


    I once heard (can't recall source), that an early Mars probe missed the planet because of a part designed in one system of units was mistakenly made with a different system.
    Dunno if that's true.
    Last edited: Aug 2, 2015
  8. Aug 2, 2015 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    IMO, it's just different.

    The Imp. gallon was sized so that 1 gallon of fresh water at 62 F weighed exactly 10 pounds. This makes it a snap to figure of the weight of so many Imp. gallons of water, and if you know the specific gravity of a different liquid, you can quickly figure its weight as well.


    Apparently, the US gallon is a holdover unit from the reign of Queen Anne (d. 1714), when it was used for measuring wine.

    Both gallons are divided into quarts, pints, and so many fluid ounces, such that the UK pint is about 1.2 times the size of a US pint. There are 128 fluid ounces in a US gallon, but 160 ounces in 1 Imp. gallon.
  9. Aug 2, 2015 #8
    What! .. decimalisation!
    What's wrong with dozens, scores, and bales and reams!
  10. Aug 2, 2015 #9
    You're probably thinking of the Mars Climate Orbiter. Great fodder for illustrating the importance of labeling units to students.

    Being adept at working in different unit systems is certainly a skill worth having. It seems like there are still many units in use that are very field specific. The bolt (fabric length unit) and brix (sugar concentration) come to mind. I wonder how many different unit standards there are today. Apparently toward the end of the 18th century in France there were more than 250,000 different units in use because each town had their own system (source) - hence the push for standardization.
  11. Aug 2, 2015 #10
    Yes that would be it, but I was wrong about it missing Mars.
    It was meant to go into orbit but instead it crashed into the atmosphere and the problem wasn't to do with the manufacture of a part, it was to do with software that used a different measurement system to the hardware.
  12. Aug 2, 2015 #11
    Britons use Imperial in every day life - pints, miles, feet, inches, stones. Most people my age or older still mentally convert (double it and add thirty) centigrade to fahrenheit - 40 is chilly, 50 is cool, 60 is warm 70 is perfect, 80 is hot, 90 is a heat wave coming on. 23°C - not so much of a mental picture there. I have no mental picture of what somebody 167cm tall and 74kg looks like. But 5'10 and 9 stone - I can see that. I can imagine a hike up a mountain 4000ft. 1300 meters not so much.

    But, for most calculations, Imperial (and the strange units) are just way too clumsy.
  13. Aug 2, 2015 #12
    Hmm, That would be quite a noticeably thin person but not extemely thin, could probably benefit from eating more and daily exercise.
  14. Aug 2, 2015 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    There's still plenty of those. There's 8 stone to a hundredweight (cwt), and 20 cwts. to the ton (2240 lbs), which is a couple of percent larger than a metric ton (1000 kg).

    The world still prices some commodities using non-metric units, like oil is priced at so many US dollars for a barrel of 42 US gallons. You can't actually buy a physical container with a capacity of 42 gallons any more, as most steel drums measure 55 US gallons.



    Beer is also sold by the barrel, but these barrels measure 31 gallons in the US.
  15. Aug 3, 2015 #14
    I've spent my career using non-metric units (what are they called, "Imperial." "US," "customary"??) in other words: feet, pounds, gallons and so on. I don't understand the fuss people make over which system to use. My approach? Use the system your co-workers use.

    On a whim I checked the pipe standards, to compare the ANSI sizes to the DIN sizes. Using 6-inch pipe as an example, the ANSI pipe is 6.625 inch OD. The DIN 150mm pipe OD 168.3 mm. The ANSI schedule 80 pipe wall thickness is 0.432 inch; the DIN pipe has 11.0 mm wall. Now for the surprise: the two pipes are identical - 6.625 inches is 168.3 mm; and 0.432 inches is 11.0 mm. So, are we not using metric pipe? Or are the Europeans not using ANSI pipe? Actually I was surprised; I had always assumed that the DIN sizes would be maddeningly different. Cooler heads obviously prevailed when the "new" standard was created...

    If there is anything I really like about SI, it is the separation between mass & weight, to avoid the endless confusion we have here in the US (pounds mass, pounds force, do I divide by 32.2 here or not...). For the SI aficionados: anyone report their body weight in newtons? Or are you all still using kg-force? To me, that's just leaving the best part off.
  16. Aug 3, 2015 #15
    Mass in Newtons?
  17. Aug 3, 2015 #16
    Weight in kilograms??
  18. Aug 3, 2015 #17

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    How many scruples are in a hogshead again?
  19. Aug 3, 2015 #18
  20. Aug 3, 2015 #19
    11 imperial gallons is almost exactly 50 liters in the metric system.
    Have I discovered a new universal constant?
  21. Aug 3, 2015 #20


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Or how many ergs are there in a poundal?

    Every measurement system comes with bagfuls of obsolete and obscure units, even the (one of many) metric systems. How many people know what a sthene is?
  22. Aug 12, 2015 #21

    Jonathan Scott

    User Avatar
    Gold Member

    Apart from an exact factor of 10, this is very closely related to 2.2 pounds being about a kilogram. An imperial gallon of water is 10 pounds and a liter is a kilogram.
  23. Aug 15, 2015 #22


    User Avatar
    Gold Member

    Did you say, "How many eggs in a dozen?":biggrin:

    I still buy slices of meat by " that thick" even if we supposedly metric in the northern country.

    Funny thing is after the switch, a pound of butter became 454 g, for example, as who in the manufacturing, production industry was going to switch their tooling and assembly over to be absolutely metric overnight. Years later it is still 454 g. Just change the paperwork instead. Milk and juice is by the liter though by the carton. Buy 4 liters of milk by but in 3 bags. Hmmm.

    Still can't figure out if when reading a metric or English recipe if a "add a dash" of salt is different.
  24. Aug 15, 2015 #23
    Having lived through the metrification of Canada in the mid 70s and continuing... I can say that the process will not be without the expected user confusion compounded by businesses taking advantage of the confusion.

    If US does change to metric be prepared for the following... This is what they don't tell the public is in store for them.

    - Excruciatingly gradual adoption, making the entire process take at least one if not more generations to adopt. For example weather reports always given in both F and C. This keeps people lazy waiting for just their favorite units.
    - Products sold in oddball metric numbers that reflect the imperial equivalents rather than retooling the sizes to make nice numbers. I think we'll have to live with that forever.
    - Product dimensions given in millimetres... thousands of them, rather than the more utilitarian centimetres and decimals if needed. Looks like the people in charge never grew up in metric countries. So now numbers are stupidly large.
    - Where both Imperial and metric units are shown, it's often clear that the person had no idea about significant digits. We now magically gain a precision down to 1/10th of a unit or better, because someone's calculator said so.
    - Gasoline prices which used to fluctuate by several cents per gallon now fluctuate by several cents per liter. No one seems to notice that now things go up and down hugely over the course of a day... no one minds. In Canada we had used imperial gallons which are 4.55 liters. There would have been riots if gasoline jumped haphazardly by 5 to 10 cents per gallon on one day.
    - Gasoline costs seem to always be pegged at xxx.9 cents per liter. Similar as before when they were xxx.9 cents per gallon. Funny how everyone casually accepts this bogus rounding even though +/- one cent is now the equivalent of 3.79 (US gallon) previous cents.
    - Car's gasoline consumption changed from miles-per-gallon to liters-per-100km. So not only are two units changed but also the reciprocal is used. Then factored by 100 for good measure. Dealers confuse and make the evaluation of new cars compared to old extra tricky.
    - One used to be ticketed for exceeding the speed limit by amounts of 10 miles-per-hour or multiples. Now it's magically 10 kilometers-per-hour. As if all radar guns had suddenly gained precision by a factor of 1.6x and the fines allotted accordingly. No one but me noticed. Oops, I've said too much.

    There are likely more examples but that's all I can think of for now.

    And here are some areas where the conversion is far from absolute.
    - Air pressure for tires is shown in kilopascals at the gas station air pump. No one in their right mind uses this scale. Everyone still has a 'feel' for pounds-per-square-inch. Pounds are what's printed on the tires as recommendation. Who is going to multiply by 6.9 in their heads?
    - Weight - I still see and understand pounds on my scale.
    - Bridge or overpass heights and garage clearances, given in meters and decimals now. Can I trust the conversion? Should I reverse engineer the likely original imperial numbers that were in fact the architect's design?
    - A McDonald's Quarter Pounder will now be a 113.3981 grammer? I bet that's the first product that will be rounded down to a nice round number such as 100. You know, for the customer's convenience.
  25. Aug 15, 2015 #24


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    All of our products in the US are being or have been re-labeled in Spanish, so there's not much money left over to convert everything to metric just yet.

    It's funny how the words for so many everyday items are the same in Spanish and English, yet labels must be printed in two languages. Since there's only so much room to print all this extra verbiage, the type size gets smaller to cram it all in. Talk about reading the fine print!

    Still, like a lot of countries which have switched measurement systems, there is going to be, for a number of years, constant reminders of the old measurement system still about. For instance, houses are built using wood measured in imperial sizes, with things like wall studs and such on 16-inch centers. Trying to replace a 2x4 with a new 50x100 piece of wood will either leave unsightly gaps or require a little extra force to cram into the available space.

    Mechanics tool makers have furnished tool sets in dual measurements for years. You need these because you never know if something has been assembled using metric or imperial fasteners, and often you will find a mixture of both types on the same unit.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook