US customary measures: friend or foe of British traditionalists?

In the early years of the twentieth century, both US customary (USC) and metric measures were seen by some in Britain as threats to the survival of the Imperial system. The end of Empire saw metric supplant Imperial, while USC has endured. Could it become the saviour of the few Imperial measures that survive in the UK, despite the differences between the two systems?

Some of those in Britain who wish to protect and promote traditional measures see the continuing dominance of customary measures in the US as a lifeline. Imperial, which a century ago was one of the most important measurement systems in the world, is now but a shadow of its former self, as this table illustrates:

measurement system               number of countries and                   population*                                                                                 dependent territories                    (approximate)

predominantly metric                                  209                                    6500 million

predominantly USC                                          6                                       318 million

other (eg. mix of Imperial and metric)        27                                       156 million

*Population information is taken from

Anyone looking across the Atlantic for support for the continuing use of traditional measures in Britain may try to minimise the differences between USC and Imperial. This deserves closer examination.

Agreed, for the past fifty years both the UK and the US have used the same definitions for mass (traditionally ‘weight’) and length: the pound (avoirdupois and US) is defined as 0.453 592 37 kg and the yard is defined as 0.914 4 metre. However, the long (Imperial) ton of 2240 pounds can be confused with the short (US) ton of 2000 pounds. There is also is no common definition of volume (capacity) – the Imperial gallon is defined as 4.546 09 litres, whereas the US liquid gallon is defined as 231 cubic inches (equivalent to 3.785 41 litres).

Even those who believe they are familiar with the differences between Imperial and USC may sometimes be caught out. For example:

Q. What is the capacity of a barrel?  A. Dry, cranberry, fluid or oil?

Q. What is the volume of a US quart?  A. Dry or liquid?

Q. How big is a cup?  A. There is no internationally-agreed standard definition of the cup, whose modern volume ranges between 200 and 284 millilitres.

(USC has separate dry and liquid volume measurements. Some larger volume measures such as pecks and bushels are only used for dry measure whereas some smaller volume measures such as fluid ounces and fluid drams are only used for liquid measure. In between, there are units such as the pint, quart and gallon that are used for both but represent different volumes, depending on whether they are used for dry or liquid measures. And yes, cranberries have a barrel all to themselves.)

These and other differences have for long provided pitfalls for the unwary on both sides of the Atlantic, and it would seem sensible in the UK to use Imperial and USC units only when this can not be avoided, in particular:

  1. When it is a legal requirement (for example on road traffic signs and for draught beer and cider).
  2. When subject to international treaties on transport (for example, feet for flight altitude and knots for flight speed).
  3. When an industry practice or standard is accepted worldwide (from musical organ building to car tyre descriptions).

Alas, procrastination by the Department for Transport and successive governments over the past forty years has taken the UK down a measurement cul-de-sac, a pleasant enough drive but leading nowhere. At the end of this road, there are two parking spaces for traditionalists to choose from, marked:

USC = foe”. Imperial measures that remain in use today but are not shared with the US, such as fuel consumption in mpg (Imperial ‘g’), the 568 mL pint, and “yards” on pedestrian tunnel safety signs, survive to take their place alongside tourist icons such as Beefeaters, red phone boxes and thatched cottages, thereby enhancing Britain’s Olde Worlde image. This might be called the “Go it alone” strategy.

USC = friend”. Imperial measures that do not accord with USC have to go, including measures of capacity, the long ton and the stone. But scope for confusion would diminish, and the idea, favoured by some traditionalists, of a transatlantic community of pound-foot users rivalling metric, might be easier to promote. This could be called “The special relationship” strategy.

Both have risks and potential problems. The solution is of course a single, simple measurement system used for all purposes, as many other Commonwealth countries concluded long ago.

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9 Responses to US customary measures: friend or foe of British traditionalists?

  1. John Steele says:

    I hope this is a humorous offer to the BWMA intended to lead them into a tiger trap when we (the US) finally complete metrication. If not, please don't adopt what we are trying to kill.

    I assume you have binned the US as the major player in "predominantly USC." We are admittedly more USC than you are Imperial. However, I would put us both at different ends of the "mixed" category. We have industries that are entirely metric internally, such as automotive, pharmaceutical, electronic (except screen sizes). We have others like Boeing, NASA, the American Petroleum Institute which are entirely USC internally. Both are forced into "duality" externally by net content legislation, and many companies use both USC and metric in their literature even when not required to. It is fair to say that the rate of conversion is in spite of, not because of, government effort.

    You probably don't need to learn our dry measure, as you appear to require punnets of fruit to be sold only by weight. We allow berry sized fruits, cherry tomatoes, mushrooms, etc to be sold by dry volume or weight in supermarkets, while larger produce must be sold by weight. At roadside stands and farmers' markets, dry measure may always be used and generally, nothing is weighed. In commercial contracts, bushels and barrels don't really exist, they are mythical units defined by weight; truckloads of produce are weighed on a scale and bushels or barrels calculated.

    In the spirit of your proposal, I would argue the cup is or should be 236.6 mL. The USC cup is 1/16 of the US gallon, and the cup most encountered in recipes. (it is based on the liquid gallon, even for flour). Against its defining gallon, it is 0.5 pint or 2 gills (we NEVER use the gill, however). I would guess the UK uses a cup similarly derived from the Imperial gallon. Australia and Canada appear to use a 250 mL "metric cup." I think most countries that are "really metric" just use deciliters or centiliters in recipes. Note: Some US literature approximates the cup as 240 mL; this metric value allows integer subdivisions down to the teaspoon (5 mL) and is accurate enough for cooking.

  2. John Frewen-Lord says:

    Canada is a bit confused when it comes to mixing measurements. Yes, the country is officially metric, and the road signs (perhaps the most obvious sign - sorry!) confirm this. But Canada used to use the imperial gallon, not the US gallon. The problem is, today, while Canadian-sourced fuel consumption is described as L/100 km (and is what almost all Canadians today relate to), when you see a story quoting mpg, you don't know whether they are US or imperial gallons, especially if the story has an original US source but is written in a Canadian publication.

    One thing Canada has done is always use the 2000 lb ton, not the 2240 lb ton. Quite why I don't know. In regards to recipes, Canadian recipes did become metricated (and using, as John Steele mentioned, the 250 mL cup), but in my opinion an opportunity was missed in not converting to weight-based quantities at the same time. Too many products (more than there used to be) are soft-converted US quantities - e.g. windscreen washer fluid, which you used to be able to buy in 4 L containers, but are now 3.8 L.

    Although not strictly a metric issue, date formats in Canada are a major problem today, thanks to the US. Canada always adhered to the dd-mm-yyy format found in most parts of the world, but officially changed to yyyy-mm-dd format in the early 1980s during metrication. Then Microsoft came along, and shipped Windows, Office, etc. already defaulted to the US mm-dd-yyyy format. Most Canadians didn't bother to change the format on their computers (or didn't even know about Canada's preferred format, especially if you were a 'new' Canadian), with the result that dates are all over the map. My Canadian HSBC bank account (run from the US) uses all three formats on the website! At least on my Canadian cheques it specifically requires me to use the dd-mm-yyyy format. Scotiabank has the same rule.

    It all goes back to what has been termed US exceptionalism (also referred to as US intransigence) - being different for the sake of being different (combined with a dose of NIH - Not Invented Here), even if the differences mean accepting, or producing, something inferior to how others do things. The continuing use of outdated, timewasting and confusing measurement units is the major example. It is an example the UK must do without.

  3. Ed the yank says:

    Hi everyone,
    When I have to work in the kitchen I use the 250 mL for liquid measure and 237 mL for weighing flour and other dry ingredients.
    When writing cheques i need to use mm-dd-yyyy, for letter writing the dd-mm-yyyy where the month is spelt, and for documents, what i call the passport format, yyyy-mm-dd. To me this is part of the transition to being in a global economy and communication.

    Be well.

  4. Bob says:

    Quote: "When subject to international treaties on transport (for example, feet for flight altitude and knots for flight speed)."

    This is a popular misconception. The relevant treaty is the ICAO Chicago Convention. It specifies SI and merely permits non-metric alternatives. The UK could, as other states do, use the metre for altitude. See:
    "meters is the Primary unit for altitude and elevation while feet is Non-SI alternative"

  5. derekp says:

    Further to Bob's comment, Kilopascal draws our attention to a report to the International Civil Aviation Organisation (ICAO) from 2007:

    Although the report relates to the Flight Level Allocation Scheme (FLAS) in China, it indicates that the world's current four FLAS will reduce to two, presumably one metric (China and Russia) and one USC/Imperial.

  6. Bob says:

    derekp wrote: "... the world’s current four FLAS will reduce to two, presumably one metric (China and Russia) and one USC/Imperial."

    Almost but not quite. Russia adopted feet for RSVM altitudes.

  7. John Steele says:

    And China assigns the Reduced Vertical Separation Minimum (RVSM) altitude in meters, but the pilot must use a conversion table to feet and fly it with a foot-based altimeter. The use of a metric altimeter is not allowed in China's RVSM space, but is permitted below it.

  8. BrianAC says:

    It seems to me the air traffic regulations are as screwed up as the UK road regulations. Makes you think a big disaster is never far away.

  9. michduncg says:

    Saw this episode of the 'Big Bang Theory' the other day and thought of us all!

    This seemed an appropriate place to post this. I do regret the US & UK sharing a common language - we are inundated with USC every time we watch any US TV. Although in the 'Big Bang Theory' most of the scientific reference seem to be metric!



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