When can I use non-metric (ie imperial) units?

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Published on LexisPSL on 11/03/2014

The following Commercial Q&A provides comprehensive and up to date legal information covering:

  • When can I use non-metric (ie imperial) units?
  • When can I use imperial units?
  • Supplementary indications
  • Can I use imperial units in advertising?
  • Will imperial units eventually be phased out?

Since 1965, the United Kingdom has gradually changed to the international system of metric weights and measures (the International System of Units or 'SI' system); although, despite having had almost 50 years to do so, the process is not yet complete. A limited amount of imperial (ie non-metric) units must still be used in certain limited circumstances.

It is clear however that the United Kingdom is currently in a bit of a conceptual muddle given that these two systems are concurrently in use, both informally and formally; albeit to a limited degree in the latter case. A good example is the Department for Transport's (DoT's) Traffic Signs Manual, which refers to 'road works' signs with a '200yds' plate. However, it goes on to state in this guidance that such signs must be placed 200 metres (and not 200 yards) in advance of the road works: a discrepancy of some 17.12 metres (or about 18 yards and 2 feet). Mistakes like this are quite common.

Indeed, there is still much debate in this area on what systems ought to be used. Lord Howe of Aberavon, called the use of two systems a 'wasteful, untidy mess' in the UK Metric Association's (UKMA) 2004 report 'A Very British Mess' which called for full metrification in the United Kingdom. On the other hand, the British Weights and Measures Association opposes

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