There are two forms of legislation relating to metrication:

  • UK legislation: laws debated and passed by Parliament. Some of these laws implement European Union directives as part of the UK’s obligation under European Union treaties. Other laws have been enacted as a result of Britain’s own local initiative.
  • European Union directives: these are agreed by the European Parliament and the Council of Ministers, where the UK is fully represented and participates with voting rights as a member state.

UK legislation

UK legislation is termed either “primary” or “secondary”.

Primary legislation consists of Acts of Parliament that have been fully debated in and approved by both Houses of Parliament and given the Royal Assent.

Secondary legislation (also known as “delegated legislation”) consists of various Orders, Regulations and Directions made by Ministers and then laid before both Houses for approval. They can be challenged by any MP or peer, in which case there has to be a debate and a formal vote.

It is for the courts to interpret legislation, and they sometimes establish new “case law”, which must then be followed (unless subsequently overturned on appeal by a higher court – or amended by Parliament).

Consumer legislation

The basic law affecting measurement units is the Weights and Measures Act 1985 (as amended).

This Act sets out the units of measurement that are “legal for trade” – that is, the legal units that MUST be used when weighing or measuring goods at the point of sale or letting. With one or two exceptions (draught beer and cider, milk in returnable bottles) these are exclusively metric. Further detail is contained in the Units of Measurement Regulations 1986 (SI 1986, No 1082) as amended. Contrary to misleading advice from Government websites, this rule does apply to services as well as to goods – for example, carpet fitting or the letting of office floorspace.

The Price Marking Order 2004 (SI 2004, No 102) is designed to make sure that the pricing of goods in labelling and advertising enables the consumer to compare value for money easily. The Order requires that, for “loose goods” sold from bulk, all traders must display the “unit price” – that is, the price per legal, metric unit. Larger shops must also give the unit price (per metric unit) of packaged goods so that consumers can compare the value for money of packages of different sizes.  The Order also permits traders and packers to display a “supplementary indication” (imperial conversion) provided that it is not more prominent than the legal, metric unit.

It was originally intended that supplementary indications would be phased out after 2009. However, in September 2007, the European Commission proposed that they should be authorised indefinitely and this was agreed by the European Parliament and the Council of Ministers. (Note that this does NOT mean that pounds and ounces may be used for weighing at the point of sale or as a primary measure on price labels.)

The Price Marking Order also provides that if an advertisement includes a price, it must conform to the same rules. However, some major retailers have claimed that the Order is ambiguous and does not apply to some forms of in-store advertising. They have taken advantage of this alleged ambiguity (and the reluctance of authorities to prosecute) to advertise in misleading ways (e.g. giving prices per lb alongside price reductions per kg). UKMA does not believe the rules are genuinely ambiguous and considers that this is irresponsible and unfair to customers. The purpose of the Order is clear and all traders should comply with it in both the short and longer term interest of people shopping for food in British stores.

In 2001 a number of traders, supported by the UK Independence Party, attempted to challenge the Price Marking Order, claiming that it was unconstitutional and infringed their human rights. All their arguments were rejected by the UK Courts and by the European Court of Human Rights (to which they appealed).

Enforcement of trading standards legislation (including the Price Marking Order) is mainly the responsibility of local authorities. They act under guidance provided by Local Government Regulation (formerly known as the Local Authorities Coordinators of Regulatory Services – LACORS). Although this guidance is primarily intended for Trading Standards Officers, it may also be helpful to the general public if they wish to make a complaint and to traders who wish to comply with the law.

Unfortunately, many Trading Standards Authorities give low priority to enforcement of this legislation, with the result that the law is routinely flouted, especially in street markets.

A full list of legislation relating to Weights and Measures can be seen at this website.

Department for Transport

Currently, the UK is the only major country in the world whose road sign regulations do not include metric units as options for distances and speed limits. This is despite general use of metres in the Highway Code and roads having been designed and built in metric for decades. Ireland changed its distance signs over a period of years and subsequently changed its speed limits to metric in January 2005. Even the USA permits metric-only signs.

The current legislation governing speed limits and road signs in England and Wales is:

  • The Road Traffic Regulation Act 1984 (especially Parts V and VI) (also applies to Scotland)
  • The Traffic Signs Regulations and General Directions 2016 (TSRGD) (SI 2016 No 362) (also applies to Scotland)
  • The Town and Country Planning (Control of Advertisements) Regulations 1992 (SI 1992, No 666). (Scotland has its own Regulations)

Legislation in Scotland and Northern Ireland (where devolved) closely follows the England and Wales model.

It is the view of UKMA that the TSRGD are inconsistent with the Units of Measurement Regulations (UMR) in a number of respects. In particular:

  • The TSRGD permit mile to be represented by “m” while the UMR reserve  “m” as the symbol for metre (“mile” should be written in full).
  • The UMR permit imperial units only for distances and speeds, whereas the TSRGD incorrectly prescribe feet and inches for vehicle dimensions.
  • The TSRGD require that the symbol for feet and inches be single and double quotation marks respectively while the UMR require that “ft” and “in” be used since the single and double quotation marks are used as the symbols for minutes and seconds of arc respectively.
  • The UMR specify “yd” as the symbol for yards (and yard). The TSRGD permit “yds”.

As far as the UKMA is aware these inconsistencies have not yet been challenged in a court of law.

The rules for speed limits are laid down in Part VI of the Road Traffic Regulation Act. Clause 81 specifies the default limit for urban areas (30 miles per hour) but this may be varied by secondary legislation. Other speed limits are determined by traffic authorities under rules made by the Secretary for Transport.

European Union directives

EU Directives are not laws in themselves. Rather they are instructions to the Member States, requiring them to amend their domestic legislation to bring it into conformity with the provisions of the Directive. The English language version of the EU requirements for metric units of measurement are in Directive 80/181/EEC. This Directive effectively implements the recommendations of the International Bureau for Weights and Measures (BIPM).

The Directive contains a derogation authorising the UK to use miles, yards, feet and inches for “road signs, distance and speed measurement”. Previously, there was a requirement that these imperial measures should be phased out by “a date to be fixed by the Member State”. However, the 2009 amendment to the Directive removed this requirement.

The United Kingdom implemented this Directive in 1994 by means of The Units of Measurement Regulations 1995 (SI 1995:1804). The EU directive also meant a number of changes in United Kingdom law, which were implemented by Statutory Instruments. Some of these are listed below along with relevant SIs from previous years.

  • SI 1992:36 – The Clean Air (Units of Measurement) Regulations 1992
  • SI 1992:450 – The Gas (Metrication) Regulations 1992
  • SI 1992:1751 – The Gas (Modification of Therm Limits) Order 1992
  • SI 1992:1918 – The Shipbuilders’ Relief (Specified Structures) (Amendment) Order 1992
  • SI 1994:1852 – The Weights and Measures (Packaged Goods and Quantity Marking and Abbreviations of Units) (Amendment) Regulations 1994
  • SI 1994:2866 – The Weights and Measures Act 1985 (Metrication) (Amendment) Order 1994
  • SI 1994:2867 – The Units of Measurement Regulations 1994
  • SI 1994:2868 – The Weights and Measures (Metrication) (Miscellaneous Goods) (Amendment) Order 1994
  • SI 2000:3236 – The Non-automatic Weighing Instruments Regulations 2000
  • SI 2001:85 – The Weights and Measures (Metrication Amendments) Regulations 2001

Finally, The Passenger Car (Fuel Consumption and CO2 Emissions Information) Regulations 2001 (SI 2001:3523) contain the interesting clause “Fuel consumption shall be expressed either in litres per 100 kilometres (L/100km) or kilometres per litre (km/L), and quoted to one decimal place, or, to the extent compatible with the provisions of Council Directive 80/181/EEC, in miles per gallon.” UKMA’s interpretation of the Directive is that, since gallons ceased to be authorised by the Directive in 1995, “miles per gallon” may only be used as a supplementary indication alongside a legal (metric) unit.