A marginal seat is a constituency where the gap between the winning candidate and their nearest rival is relatively small. There is no single official percentage at which a seat becomes “marginal”.
Majority in votes
The simplest measure is the winner’s numerical majority: the number of votes separating first and second place. If the winner receives 20,100 votes and the runner-up 19,900, the majority is 200 votes.
Majority as a percentage
For comparisons between constituencies, analysts often express the majority as a percentage of votes cast. This helps compare seats with different turnout and electorate sizes.
How marginal was the 2024 election?
The House of Commons Library found that 115 constituencies were won by a margin of 5% or less in 2024. Of those, 46 were won by less than 2%.
The closest result was Hendon, where the winning margin was 15 votes, or 0.04%. Poole was decided by 18 votes.
Why marginal seats matter
Under First Past the Post, changing the winner of a constituency changes one Commons seat regardless of the size of the electorate or the previous majority. Parties, campaigners and analysts therefore pay close attention to seats where relatively small movements could change the result.
A marginal seat can change quickly
Past majority is useful context, but it is not a forecast. Boundary changes, candidates, tactical voting, turnout and changes in party support can all alter a constituency’s competitiveness between elections.
Sources and further reading
House of Commons Library: 2024 general election marginality ↗ · UKDecides election records