Linked by a common system of measures

There is a widely held view in the UK that we share our measurement system with the USA. True – but not in the way many believe.

During a broadcast on BBC Radio Scotland on 5 January 2009, viewable on the UKMetric YouTube channel, we heard again the suggestion that there is little need for the UK to continue with the metric changeover as imperial measures are similar to those used in the USA. If only this were true.

Practice in Britain and in North America had begun to diverge even before the Declaration of Independence in 1776. The governments of the thirteen colonies had their own physical standards of English measures, which seldom agreed precisely with those in Britain. And in 1790, when the newly-established federal government set about the creation of a unified system of measures for the United States, alignment with measures in Britain was low on its priorities. Later, a Royal Commission was appointed in the UK, then comprising Great Britain and Ireland, to consider the reform of weights and measures. The resulting Weights and Measures Act of 1824 retained many English measures, but not all of the unit names and with slightly different values, and it was this disparate collection that eventually acquired the title “imperial”.

So where does this leave us now?

The surviving imperial measure of liquid volume, the pint, differs significantly from its US counterpart. This is illustrated by the most popular size of supermarket fresh milk: the quart (0.95 litres) in the US, and 2 pints (1.13 litres) in the UK. The gallon, though obsolete in the UK, is still used to indicate fuel consumption in both countries – identical cars in the US achieve 17% less mpg due to the smaller US gallon. The imperial fluid ounce is now obsolete, so we need only remember that there are 16 (not 20) US fluid ounces to a US pint.

All of the imperial dry measures are also obsolete, so the confusing conversions between US and UK bushels, pecks, dry gallons and dry pints belong, happily, to history.

Problems arising during WW2 from differing standards for Canadian, UK and US equipment gave rise to a decision in 1948 to adopt a unified inch system of threads in all three countries. This led in 1959 to agreement on common definitions of the foot (0.3048 m) and the pound (0.453 592 37 kg). These definitions found their way into UK law in the Weights and Measures Act 1963.

However, although these definitions have been identical for fifty years, there remain many pitfalls for transatlantic travellers. The imperial ton of 2240 pounds is obsolete, replaced by the tonne. The US ton of 2000 pounds lives on. So we need to remember that one ton is about 0.907 tonnes. And of course, if you ask an American his weight, he will give it in pounds; here in the UK, the use of stones is being passed on to succeeding generations, to the bewilderment of the rest of the world.

Campaigners for the retention of imperial measures, confusingly also called ‘British measures’, make much of the importance of the ounce as well as the pound. Practice in the USA may disappoint them, as many supermarkets price random-weight pre-packs of meat and cheese in pounds and decimals, and weigh loose fruit and vegetables at the check-out likewise.

This preference in the US for decimals, instead of the varied relationships enjoyed by users of the imperial system, appears to extend to the highway. In the UK, measures of distance up to ½ mile have to be shown on road traffic signs in yards, not feet. Over this, distances must be shown in miles and fractions thereof. Feet and decimals of miles are prohibited. In the US, distances in feet are normal and distances shown in yards on signs very unusual. It is reassuring to know therefore that distance measures on UK road signs will confuse overseas visitors wherever they come from.

At the beginning of this article, it was suggested that the UK and the USA are linked by a common measurement system. In the light of the above, how can this be?

In their progress in adopting the metric system, the two countries have followed similar courses. In the early 1800’s, while Presidents Jefferson and John Q Adams were looking into the reform of US weights and measures, the UK appointed a Royal Commission with a similar task. In 1866, metric became legal in the USA for all purposes; late to the party, the UK followed in 1896. The USA was one of the earliest signatories to the Metre Convention in 1878. Late again to the party, the UK signed in 1884. However, in 1965, the UK was first to see that an increasing proportion of world trade would be carried out in metric units, and signalled a changeover from the units generally used in industry at that time. The USA followed in 1975. Both countries saw their hopes of a smooth, swift and effective transition dashed with a change of leadership in the early 1980’s. A large number of UK manufacturing companies did not make the switch and many of these have now gone out of business. Could it be that much of US manufacturing industry is now following the same path?

As the two countries work at a leisurely pace towards completing the metric changeover, there exist none of the differences that separate the imperial and the English/US Customary measurement systems. Both the UK and the USA, as signatories of the Metre Convention, share a commitment to adopting a truly international system of measures, and to participating together in its continuing development. So yes, the two countries are linked by a common measurement system, albeit an international one.

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13 Responses to Linked by a common system of measures

  1. John Steele says:

    That's a good summary of differences between Imperial and U.S. Customary. I would like to reinforce a couple of your points more strongly.

    Yard: Only used in football. The 2009 MUTCD does not contain the word "yard" or the symbol "yd." An obsolete sign that was legal when erected is "grandfathered" to end of life, so there might be lingering "yard" signs but new ones are not legal (and I've never seen one). Sign distances must be feet or miles (inches accompany feet on height and width restrictions). Many fewer warning signs reference a distance. We use Perception-Response Time and travel time (headway) at the posted speed limit to place signs at the point where the warning is needed.

    Pound: Random weight packages over 1 lb MUST be weighed in decimal pounds (max three decimal digits). Under 1 lb may be weighed in decimal pounds or ounces. I have never seen the ounce option used, and most deli clerks would not understand a request in ounces - use decimal pounds or fractions thereof. (the law part of the above comes from FTC rules in support of FPLA). Standard weight packages must be dual marked, metric and Customary.

    As you noted, the stone is unknown here. Hence the hundredweight and ton are 100 lb and 2000 lb respectively. Our gallon and bushel are obsolete British measure. The gallon is the Queen Anne or wine gallon of 231 in³, and the bushel, the Winchester or corn bushel of 2150.42 in³. My understanding is that both were redefined in 1707 from cylindrical to cubic measure by Parliament. The definitions above and the 1959 metric inch of 25.4 mm convert both to liters (oh, yes, there are spelling differences too).

  2. Alex Bailey says:

    If only some of the so-called obsolete measures here in the UK actually were.

    Despite the fl oz being obsoleted by various updates to W&M legislation I've lost track of the number of bars and restaurants who still display soft drinks on their menus and receipts in oz with no metric indication whatsoever. In fact, I recently learned that the "Venti" size used by Starbucks refers to the Italian word for Twenty and indicates a size of 20 fl oz... although most likely they're US fl oz even here in the UK!

    Despite the ton being obsoleted there are still many road signs which use this as a weight limit, I know of one in Cambridgeshire which actually contains the words "ton"and "tonne" on the same sign!

    The difference between the US and UK pint and gallon never fails to amuse when speaking to Brits who have been in the USA about beer strength or fuel consumption but the best one of all was an otherwise very intelligent American colleague who was seriously considering importing a Volvo from Europe because the fuel consumption figures on the British website were better than the same model on the US version!

  3. David Brown says:

    The UK has a choice about its units of measure. It needs either to embrace the International System (metric) and obsolete all Imperial unit usage, or to migrate entirely onto the American system (and again obsolete the Imperial system entirely). It is simply too confusing for a small island inhabited by about 1% of the world’s population to have its own, unique system of measurement that differs from the rest of the world. As globalisation makes the world a smaller place, we need to decide whether we are the 51st state of the USA, or whether we are an international country that looks to the whole world as its neighbours.

    (Incidentally, beer drinkers who may worry that a “half litre” is smaller than a pint should consider the fact that a US pint is smaller still!)

  4. Martin Vlietstra says:

    Churchill, or was it G B Shaw, said "England and America are two countries divided by a common language". He could also have added "and by common units of measurement".

  5. Jeremiah says:

    In my experience, Americans don't buy many British products. I can't recall the last time I saw a British consumer product for sale. Foreign consumer products that I have seen are mostly, but not always packaged in rounded metric amounts with USC in parenthesis. In some cases the USC is primary and metric is in parenthesis even though the metric is a round number. Example: 500 g (1.1 lb) or 1.1 lb (500 g).

    British industrial products are equally rare. We buy products that are mostly from Europe. Siemens, ABB, AEG, Weidmüller, Wieland, Demag, IDEC, etc are very common names in industry in the US and they produce only in metric. I can't think of one British company that I have dealt with in the past 10 years or so. Some catalogs offer conversions between millimetres and inches, but the millimetres are rounded and the inches aren't.

    Even some "American" products are metric, either because that is how they make them so they can be sold everywhere or because the American company is reselling under their own name a product made elsewhere. Products made to conform to ISO or IEC standards will be metric.

    To claim there is an advantage to selling inch products in the US is false. In fact if someone does try and market an inch product it may not interface well if it is intended to be a part of a bigger assembly that contains all metric parts.

    If a British company tried to sell me a product based on it being in inches, I would decline their offer to purchase telling them I don't do inches. I would tell them to come back and see me when they become modern.

  6. The UK has a choice about its units of measure. It needs either to embrace the International System (metric) and obsolete all Imperial unit usage, or to migrate entirely onto the American system (and again obsolete the Imperial system entirely). It is simply too confusing for a small island inhabited by about 1% of the world’s population to have its own, unique system of measurement that differs from the rest of the world. As globalisation makes the world a smaller place, we need to decide whether we are the 51st state of the USA, or whether we are an international country that looks to the whole world as its neighbours.

  7. Mike says:

    I found this interesting -

    "In the UK, measures of distance up to ½ mile have to be shown on road traffic signs in yards, not feet. Over this, distances must be shown in miles and fractions thereof."

    There is a road sign near me recently erected that warns of no U-turns for '1110 yards'. Is this technically illegal?

  8. philh says:

    We in Britain no longer have an empire (nor should we) and we don't need Imperial measures to assert it.

    We, like every other country in the world, need a single system of measurement that everyone can understand and use. We are also part of a global community so why not use the one sensible international system adopted by them.

    We also need, again like everyone else, to legally enforce units for trade purposes. All civilised countries do this including the US.

    As far as our "greatness" is concerned we'd much rather that come from a reputation of being a humane intelligent society that tries to be an influence for peace and prosperity in the world, not a bunch of eccentrics with a quirky "system" of measurement.

  9. derekp says:

    England late as usual for the party.
    I was amused by this comment in Radio Times:

    "World Cup Final
    The inaugural World Cup in 1930 was hosted by Uruguay, who duly won it. Thirteen teams took part, the majority from South America - at first no European team entered until Fifa president Jules Rimet persuaded four to come to the party. England were absent - they were not in Fifa and thought they had better things to do. They belatedly made their World Cup debut in Brazil in 1950."

  10. Ed the Yank says:

    I thought here in the States the Imperial or Long Ton is still used in shipping of cargo.
    The UK like the US still keep the adoption of the SI units at arms length by calling the system "metric" as if the world is still debating the cgs over the mkgs systems.

  11. eric burns says:

    HONG KONG—The finance leasing arm of Industrial & Commercial Bank of China Ltd. has made 45 firm orders for China's first home-grown commercial jetliner, bringing total orders for the C919 jets to 145, an ICBC executive said Thursday.


    This could be a game changer for Airbus and especially for Boeing. With the duopoly broken two major plane makers will design and manufacture planes in metric units. Right now, both Airbus and Comac have to buy inch products because it is not yet worthwhile for metric manufacturers to produce particular components for Airbus only. If Comac is successful, combined they will in time generate enough orders to support metric companies producing aircraft components in metric. It may also induce some US component suppliers to switch to metric?? Should that happen, Boeing will be left out in the cold with having either to pay exorbitant prices to get inch based parts, or switch to metric products. Well in a way that would be the long overdue poetic justice for forcing the world to fly in medieval feet and having to use that anachronism in everything pertaining to aviation. Another solution for Boeing and its suppliers would be to make the intelligent move and go metric. Failing that, it will be at competitive disadvantage that it cannot afford.

  12. eric burns says:

    Re-kilopascal: "Your comment on the UKMA blog is somewhat in error. From what I have been told Airbus and all other non-US airplane companies do design their planes only in metric. However, for practical reasons, they do use inch based fasteners. This is because the US industry set the standard and to introduce a metric series could create a big muddle. You would have technicians who service all brands of planes having to stock both metric and inch hardware. Because of this you can end up with safety issues if two similar bolts ever got crossed. You would not want to be flying in a plane when part of the plane shears away because the technician earlier forced an inch fastener into a metric thread or vice versa. It is possible the Chinese may in fact decide to use metric fasteners and can in fact effect a world-wide change".

    Your observation about inch based aircraft bolts and other parts has some merit, but not with the Chinese. It has home designed and built military aircraft not using inch bolts and other inch gear for one good reason, it would be dependent on the supplier, which in this case would be the US. It is somewhat different with civilian aircraft that have to go through rigorous certification processes in America and Europe. Yet, I am pretty sure it won't be all that long before China develops home based metric standards and have them accepted in the certification process. Airbus should use that opportunity and its know how to team up with China to set those metric standards for civilian aircraft. Agreed, it would be a miracle if that happens.

  13. Ronnie Cohen says:

    Many point to the fact that both the UK and US use the imperial system but fail to acknowledge that the US uses its own version of the imperial system. The reforms that were implemented in the 1824 Weights and Measures Act, which introduced the British imperial system of measurement across the British Empire, were never adopted by the US. By that time, the American colonies were already independent and continued using the older English measurements and ignored the reforms that were taking place in the UK.


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