Decimal Day () in the United Kingdom and in Ireland was Monday 15 February 1971, the day on which each country decimalised its respective £sd currency of pounds, shillings, and pence.
Before this date, the British pound sterling (symbol "£") was subdivided into 20 shillings, each of 12 (old) pence, a total of 240 pence. With decimalisation, the pound kept its old value and name, but the shilling was abolished, and the penny was revalued, such that the pound was subdivided into 100 of what were originally called "new pence" ("NP"), and later just pence ("p") when confusion was no longer likely. Each new penny was worth 2.4 old pence (abbreviated "d.").
A coin of half a new penny, a halfpenny, was introduced to maintain the approximate granularity of the old penny, but was dropped in 1984 as inflation reduced its value. An old value of 7 pounds, 10 shillings, and sixpence, abbreviated £7-10-6 or became . Amounts with a number of old pence which was not 0 or 6 did not convert into a round number of new pence.
The Irish pound had the same £sd currency structure, and the same decimalisation was carried out.
United Kingdom
Background
The Russian ruble was the first decimal currency to be used in Europe, dating to 1704. Elsewhere, the Coinage Act of 1792 introduced decimal currency to the United States, the first English-speaking country to adopt a decimalised currency. In France, the decimal French franc was introduced in 1795.
Before the 1970s, earlier efforts in the United Kingdom to introduce decimalised currency had failed; in 1824, the United Kingdom Parliament rejected Sir John Wrottesley's proposals to decimalise sterling, which were prompted by the introduction of the French franc three decades earlier. Following this, little progress towards decimalisation was made in the United Kingdom for over a century, with the exception of the two shilling silver florin, first issued on 1849, worth of a pound. A double florin or four shilling piece, introduced in 1887, was a further step towards decimalisation, but failed to gain acceptance and was struck only between 1887 and 1890.
Though little further progress was made, The Decimal Association, founded in 1841 to promote decimalisation and metrication, saw interest in both causes boosted by a growing national realisation of the importance of ease in international trade, following the 1851 Great Exhibition; it was as a result of the growing interest in decimalisation that the florin was issued. In a preliminary report issued in 1857 by the Royal Commission on Decimal Coinage, the benefits and drawbacks of decimalisation were considered, but the report failed to draw any conclusions on the adoption of a change in currency. A final report in 1859 from the two remaining commissioners, Lord Overstone and Governor of the Bank of England John Hubbard, came out against the idea, claiming that it had "few merits".
In 1862, the Select committee on Weights and Measures favoured the introduction of decimalisation to accompany the introduction of metric weights and measures.
The Royal Commission on Decimal Coinage (1918–1920), chaired by Lord Emmott, reported in 1920 that the only feasible scheme was to divide the pound into 1,000 mills (the pound and mill system, first proposed in 1824), but that it would be too inconvenient to introduce. A minority of four members said that the disruption would be worthwhile. A further three members recommended that the pound should be replaced by the royal, consisting of 100 halfpennies, with there then being 4.8 royals to the former pound.
In 1960, a report prepared jointly by the British Association for the Advancement of Science and the Association of British Chambers of Commerce, followed by the success of decimalisation in South Africa, prompted the Government to set up the Committee of the Inquiry on Decimal Currency (Halsbury Committee) in 1961, which reported in 1963. The adoption of the changes suggested in the report was announced on 1 March 1966. The Decimal Currency Board (DCB) was created to manage the transition, but the plans were only approved by Parliament with the Decimal Currency Act 1967. The former Greater London Council leader Bill Fiske was named as the Chairman of the Decimal Currency Board.