At their irreducible core, general elections are about mathematics.

It matters not a jot in which constituency Boris Johnson decides to stand if all he does is replace one Tory MP (i.e. John Randall) with another (himself). He hasn’t helped his party add to its 2010 total of seats; he has only helped himself into parliament.

Equally it doesn’t matter in the slightest if Labour manages to see off the Tory threat to each and every one of its marginal seats, but then fails to win anything off them in return. Swings to Labour are worthless in Labour seats. Swings in Tory seats are the only ones that count.

The outcome of the 2015 election will come down to numbers. Here are six of them.

68

The number of seats Labour needs to win in addition to those it won in 2010, in order to reach 326: an absolute majority of one in the House of Commons. This includes retaking Bradford West, which it lost in a by-election to George Galloway in 2012.

20

The number of seats the Tories need to win in addition to those they won in 2010, in order to reach the same magic figure of 326. This includes retaking Corby, which they lost to Labour in a by-election in 2012. It doesn’t include the seat of Tory MP John Bercow, the Speaker of the House of Commons.

5

The number of MPs won by Sinn Fein in 2010. If the party wins all these seats again in 2015, it reduces from 326 to 323 the number needed for a working majority in the House of Commons. This is because Sinn Fein refuses to take its seats in parliament. The absence of these five MPs could prove crucial in calculations to form a working coalition or if the Tories or Labour decide to try and run a minority government.

5 is also the size of percentage swing Labour needs to gain its additional 68 seats to form a government.

34.9

The percentage of votes cast in 2010 for parties other than Labour and the Conservatives. This was the highest figure at any general election since the war. The decline of two-party politics is one of the most persuasive trends in modern political history. Will it be reversed in 2015? Current opinion polls suggest that it might.

Incidentally, Labour would be able to win the election with a share as low as 34.9%. They could do this if the Tories got 32%, the Lib Dems got 15% and the others got 18.1%. On a uniform nationwide swing, these shares would produce a result of Labour: 334 seats; Conservatives 250; Lib Dems 39; Others 27 (with zero seats for Ukip).

600

This would have been the number of constituencies in the UK at the 2015 general election, had the Parliamentary Voting System and Constituencies Act of 2011 not been amended by a vote in the House of Lords in 2013 to delay until 2018 the implementation of proposed boundary changes.

A reduction in constituencies from 650 to 600 was widely expected to have hurt the Tories the least and Labour the most, and made it slightly easier for the Conservatives to win a majority in 2015.  Instead the election will be fought on the existing boundaries, which are perceived – though there is debate about this – to benefit Labour’s tendency to win more seats than other parties on low turnouts. If the 2015 election leads to another hung parliament, many Conservatives will wonder how things might have transpired had 600 constituencies been at play instead of the present number.

0

The number of seats Ukip is defending at the election. Party leader Nigel Farage has said he will resign if this number is unchanged after polling day.