How many UK voters make a difference in British General elections?

People often speculate about the number of votes that actually made the difference in any given election. In other words, the minimum number of net votes that needed to be cast for the opposition in sufficient seats to produce a different result. This number, in comparison to the total electorate, is quite small.

For each election, apart from February 1974, the Key Votes are calculated as the summation of half the majority, rounded up to the next whole integer, in the smallest number of the most marginal seats that would have made the difference. Comparisons between elections don't make much sense because of complications such as boundary changes. The calculation of the government majority excludes the Speaker.

Year	Govt	Majority	No of Seats to reverse the result	Key Votes	Electorate	% of Electorate
2005	Lab	71 ¶	36	16,434	44,245,939	0.0371
2001	Lab	166	84	138,720	44,401,238	0.3124
1997	Lab	178	90	166,067	43,881,939	0.3784
1992	Con	21	11	1,241	43,249,721	0.0028
1987	Con	101	51	46,668	43,181,321	0.1081
1983	Con	144	73	80,368	42,197,832	0.1905
1979	Con	44	23	7,899	41,091,260	0.0192
1974 Oct	Lab	4	3	306	40,101,265	0.0008
1974 Feb	Lab	-32	17>	3,677	39,769,321	0.0092	§
1970	Con	31	16	3,286	39,398,518	0.0083
1966	Lab	97	49	37,589	35,964,998	0.1045
1964	Lab	5	3	42	35,882,527	0.0001
1959	Con	100	51	39,153	35,388,579	0.1106

§ votes in non-Labour marginal seats required to give Labour a majority

¶ This figure, unlike that for previous years, assumes that the Speaker and three Deputy Speakers (who do not normally vote) are excluded, along with the 5 Sinn Féin MPs (who did not take up their seats)

Votes that determine elections