When Parliament burned in 1834, Britain lost its official yard and pound
The 1824 law creating imperial units planned for disaster. If the master yard were destroyed, it could be rebuilt from the length of a pendulum beating seconds at Greenwich. Ten years later, fire at the Houses of Parliament destroyed the prototypes, and the backup recipe proved useless. Britain had to rebuild from copies instead.
The imperial system grew out of older English units, replacing the Winchester Standards that had held since 1588, and came into force across the British Empire in 1826 after a delay of several months. Its founding act swept away the confusing array of gallons then in use, substituting a single statute gallon close in size to the old ale gallon and defined by the volume of ten pounds of distilled water. Later acts refined that definition until, in 1985, the gallon was set at exactly 4.54609 litres. American customary units descend from the same English roots but diverged, which is why the two gallons differ.
Weight was messier. For a time the troy pound was the primary unit of mass, until an 1855 act handed that role to the avoirdupois pound; troy weight survived only as the ounce used for precious metals. Medicine kept its own apothecaries' measures, regulated by colleges of physicians in London, Dublin and Edinburgh, until Britain switched pharmacy to metric at the start of 1971.
As of 2026, Britain remains stubbornly bilingual. Road signs give miles and speed limits in miles per hour, draught beer and cider must be served in pints or fractions of them, and since September 2024 wine may be sold in pint bottles. Cricket pitches measure 22 yards, horses are measured in hands, and many adults quote their weight in stones. A 2015 YouGov survey found a generational split, with under-40s preferring metric. Many packaged foods remain imperial in disguise: a tin of Lyle's Golden Syrup holds exactly a pound but is labelled only as 454 grams.
Elsewhere the old units fade more quietly. India adopted metric by law in 1956, making it exclusive by 1962, though homes are still sized in square feet. Hong Kong juggles metric, imperial and traditional Chinese units, writing imperial ones with an extra mouth radical to tell them apart.
Source: Imperial units
