Weights and Measures Act 1985

The Weights and Measures Act 1985 (c. 72) is an act of the Parliament of the United Kingdom that consolidated enactments related to weights and measures in Great Britain.^[1]

The act defines the four primary units of measurement as the metre or the yard (defined in terms of the metre) for length, and the kilogram or pound (defined in terms of the kilogram) for mass. The act also requires standard physical examples to be maintained (known as "United Kingdom primary standards") for each of the four primary units.^[2] (c. 72)

In addition, the definitions of units which are multiples or sub-multiples of the primary units are defined, in terms of the primary units, and given as: mile, foot, inch, kilometre, decimetre, centimetre, millimetre, acre, square yard, square foot, hectare, decare, are, square metre, square decimetre, square centimetre, square millimetre, cubic metre, cubic decimetre, cubic centimetre, hectolitre, litre, decilitre, centilitre, millilitre, gallon, quart, pint, gill, fluid ounce, pound, ounce, ounce troy, tonne, kilogram, hectogram, gram, carat (metric) and milligram.

As originally enacted, the act also defined, in the same way, units which could not be used for trade as: furlong, chain, square mile, rood, square inch, cubic yard, cubic foot, cubic inch, bushel, peck, fluid drachm, minim, ton, hundredweight, cental, quarter, stone, dram, grain, pennyweight, ounce apothecaries, drachm, scruple, metric ton and quintal.

As of May 2026^[update], following multiple amendments over the years since enactment, the metre, yard, kilogram and pound remain as the primary defined units and with the requirement to maintain the "United Kingdom primary standards" for them.

At the same time, all the imperial units, except pint and ounce troy (but including all of those which were originally defined as not to be used for trade) were reclassified as being available for use for trade as supplementary indications, namely: mile, furlong, chain, yard, foot, inch, square mile, acre, rood, square yard, square foot, square inch, cubic yard, cubic foot, cubic inch, bushel, peck, gallon, quart, gill, fluid ounce, fluid drachm, minim, ton, hundredweight, cental, quarter, stone, pound, ounce, dram, grain, pennyweight, ounce apothecaries, drachm, scruple and quintal. The tonne was also reclassified as being available for use for trade as a supplementary unit of measure,^[3]