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Swing in a British political context is a single figure used as an indication of the scale of voter change between two political parties. It originated as a mathematical calculation for comparing the results of two constituencies. The term "swing" has a different meaning in Australia, which has a different voting system. See Swing (Australian politics).

The original mathematical construct Butler Swing is defined as the average of the Conservative % gain and Labour % loss between two elections, with the percentages being calculated on the basis of the total number of votes (including those cast for candidates other than Conservative or Labour). There is an alternative version called Steed Swing which calculates the percentages on the basis of votes cast for Conservative and Labour only. It is possible for the same election to have a Butler Swing of one sign and a Steed Swing of the other.

As an example, if in the previous election Labour had 45%, the Conservatives 35% and the Liberal Democrats 20%, and in the new election the Conservatives had 45%, Labour had 40% and the Liberal Democrats 15%, then the Butler Swing would be the average of the Conservative gain (10%) and Labour loss (5%), which makes +7.5%.

Swing was originated by David Butler, a political science academic at Nuffield College, Oxford. In a contribution to 'The British General Election of 1945' he wrote "this measurement of 'swing', admittedly imperfect, does give us a broad idea of the movement of opinion from Conservative to Labour" and went on to compare the swings in each area of the country.

The concept became important in the general elections of the 1950s when it was found that there was a relatively uniform swing across all constituencies. This made it easy to predict the final outcomes of general elections when few actual results were known, as the swing in the first constituencies to declare could be applied to every seat.

Only a relatively small proportion of seats in most British General Elections are "marginal" and thus likely to change party. The swing enabled prediction of outcomes to be made even while "safe" seats were returning results whose victors were not in doubt. In several elections, such as 1970, the swing correctly predicted a majority for the then Opposition even while Government party victories seemed to predominate.

Taking the national vote shares in an opinion poll could also easily be translated into likely seat outcomes. Election night television programmes from 1955 have usually featured a device known as the 'swingometer' which consisted of a pendulum which could point to the swing nationally and illustrate the outcome.

During the post-war period British politics was characterised by a strong two-party system. Almost all voters who changed their preference from one election to another, swung between one of the two parties. Although the majority still do, there has a much greater variety in change since the re-emergence of three-party politics in the 1970s. The original calculation of swing did not make any allowance for other parties and when the votes for other parties rose, demands arose for a more sophisticated measurement. The continuation of the first-past-the-post electoral system, and the tendency for smaller parties to only run in some constituencies, made it increasingly difficult to use measures of swing to predict results.

The Liberals (and, later, Liberal Democrats) have been the main catalyst for this change, providing a centrist alternative to the two parties. The situation has also changed due to the success of the SNP in Scotland and Plaid Cymru in Wales, especially in elections to the Scottish Parliament and Welsh Assembly. Two other mass parties - the Green Party, which emerged in the 1980s, and UKIP, which emerged in the 1990s - have yet to win any seats in Parliament, but have had a significant effect on the swing in certain areas, most notably when the Greens took 22% of the vote in the Brighton Pavilion constituency in the 2005 general election.

Swing has also been complicated since the 1970s as the constituent areas of Britain have become increasingly fractured. The general sense of national unity that existed in the post-war era began to fall apart in the 1970s and broke, apparently irrevocably, during Margaret Thatcher's premiership. This has led to swings being very different in different areas - for instance, 1992 saw a swing to the Conservatives in Scotland, but a swing to Labour in the South East of England.

At the same time, other parties began to win significant levels of representation in the House of Commons. This has led to swing often becoming a measurement of the changes in votes of the two biggest parties in the constituency in question, rather than just Labour and the Conservatives.

Simply substituting the Liberal Party for the Labour Party in the calculation provides a measure of a 'Swing between Conservative and Liberal'. However election results showed that this was not a useful predictor in seats which were being fought by these parties. It came to be used as a measure of the significance of the change of the vote. Almost all published election results are derived from the Press Association results service which in recent years shows the swing as between the two parties that came first and second, rather than strictly between Conservative and Labour. For this reason, the direction of swing is explicitly stated, rather than simply indicated through the sign as applies to Butler Swing.

It should be noted that Butler Swings of over 10% in magnitude are very rare. Taking British politics after 1945 exclusively (as that election occurred ten years after its predecessor, and in a completely different political climate), only the 1997 general election had a national swing of more than 10% in magnitude, and that was -10.23%. The table below shows the national swing across Great Britain, and the number of individual constituencies out of more than 600 which had a swing of over 10% in magnitude.

Conventional swing is much more volatile, and many more constituencies have large conventional swings. In addition, the conventional swing in a constituency where the top two candidates are not Conservative and Labour cannot be meaningfully compared with the national or regional swing.

• The British General Election of 1945 by R.B. McCallum and Alison Readman (Oxford University Press, 1947) pages 263-5
• Political Change in Britain by David Butler and Donald Stokes (Macmillan, 1969) pages 140-51