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.