Supplementary Vote

With the Supplementary Vote, candidates have to campaign to get a broader base of support.

Supplementary Vote

The Supplementary Vote (SV) is used for electing Mayors and Police and Crime Commissioners in the UK.

It part of a broad group of ‘preferential’ voting systems, which include the Alternative Vote used in Australia and the Contingent Vote used to elect the Sri Lankan President.

How to vote

There are two columns of boxes alongside the candidates’ names on the ballot paper. One column of boxes is for voters to mark their favourite candidate with an X and one in which to mark a second favourite with an X. Voters don’t have to mark a second favourite if they do not have one. Voters can put a X in both boxes for one candidate but this is effectively the same as just marking your favourite and no additional benefit comes from this.

How is it counted

All the votes for favourite candidates are counted and if one candidate gets over 50% they are elected.

If no candidate gets over 50%, the top two candidates continue to a run-off round and all other candidates are eliminated. The votes of everyone whose favourite candidate has been eliminated are moved to their second favourite.

Votes from people who had their favourite candidate eliminated and their second favourite candidate is in the run-off, are added to their first-round totals for the two run-off candidates. The candidate with the most votes at this stage is declared the winner.


As voters mark two Xs rather than writing a number to show their first and second choice, it is simpler for the voter when they are also using other electoral systems that use Xs, such the Additional Member System for the London Assembly or local councils using First Past the Post.

The Supplementary Vote means politicians need a wider base of support than First Past the Post, but they are much less likely to get half the vote than with the Alternative Vote. SV is viewed as encouraging a more positive style of campaigning as candidates desire the second preferences of third parties.

Supporters argue that first and second choices are more firmly held than say the difference between a fifth and sixth choice.

"The Supplementary Vote stops candidates winning on low levels of support, but doesn't need them to get half the vote"

Electoral Reform Society

As the Supplementary Vote only lets voters express two choices, it is possible for a high number of voters’ first choices to be excluded in round one and for their second choice to not be in round two. In almost two decades of Supplementary Vote elections for the Mayor of London, only in 2016 has a mayor won more than 50% of the total ballots once these ‘non-transferable’ ballots are included.

The 2016 London Mayoral Election

As the top two in the first count (as seen on the left), Zach Goldsmith and Sadiq Khan go through to the second round (on the right). The remaining candidates are excluded and people who voted for them have their votes moved to their second choice. If their second choice was Zach or Sadiq these votes are added to the totals to find the winner.

Supplementary Vote then encourages a form of strategic voting where the most logical thing to do is to cast your first choice for your genuine favourite candidate, and your second choice for your preferred candidate that you think will reach the top two in the second round.

But this becomes more difficult as the number of candidates increases, in particular in races where more than two candidates stand a chance of being in the top two and entering the second round.

Although deciding on just a first and second choice is simpler for the voter than putting all the candidates in order (as you could with the Alternative Vote), some consider that this tactical element makes SV more confusing overall.