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 http://en.wikipedia.org/wiki/List_of_countries_by_population

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:

When it is a legal requirement (for example on road traffic signs and for draught beer and cider). When subject to international treaties on transport (for example, feet for flight altitude and knots for flight speed). 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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