Ranked choice voting in multi-winner elections (also commonly known as "single transferable vote" or simply "choice voting") maximizes the effectiveness of every vote to ensure that as many voters as possible will help elect a candidate they rank highly. It minimizes wasted votes and the impact of tactical voting, allows voters to have more choices, and encourages positive campaigning and coalition-building. It upholds both minority representation and the principle of majority rule. Because of its proven history, its emphasis on candidates rather than parties, and its ability to allow voters to express their full, honest preferences on their ballots, ranked choice voting is the form of fair representation voting best suited for use in U.S. elections.
Under ranked choice voting, voters rank candidates in order of choice. They mark their favorite candidate as first choice and then indicate their second and additional back-up choices in order of choice. Voters may rank as many candidates as they want, knowing that indicating a later choice candidate will never hurt a more preferred candidate.
To find out who wins, we first need to know how many votes are enough to guarantee victory, which we call the election threshold. That threshold is the number of votes that mathematically guarantees that the candidate cannot lose. For example, if three candidates will be elected, the threshold is 25% of votes. That's because if one candidate has more than 25% of the vote, it is impossible for three other candidates to get more votes than them (because that would add up to more than 100% of votes). If four candidates will be elected, the threshold is 20% of votes. If five candidates will be elected, it is about 17% of votes.
After counting first choices, candidates with more votes than the election threshold are elected. Then, each elected candidate's "surplus" are added to the totals of their voters' next choices. For example, if a candidate has 10% more votes than the election threshold, every one of their voters will have 10% of their vote count for their second choice. That way, voters aren't punished for honestly ranking a very popular candidate first.
After the surplus has been counted, the candidate with the fewest votes is eliminated. When a voter's top choice is eliminated, their vote instantly counts for their next choice. That way, voters aren't punished for honestly ranking their favorite candidate first, even if that candidate cannot win.
The process of counting surplus votes and eliminating last-place candidates repeats until all seats are filled. This method of counting is performed by hand in many places, though it can also be quickly administered using existing voting machines.
The chart below shows the results of a partisan race using ranked choice voting. Six candidates are running for three seats in a hypothetical district with 1,000 voters. Candidates Perez, Chan, and Jackson are Democrats, while candidates Lorenzo, Murphy, and Smith are Republicans. The district is majority Democratic; the Democratic candidates collectively earn 60% of first choices. However, there are a substantial number of voters who prefer the Republicans.
In this simulation, Jackson is the most mainstream Democratic candidate, while Perez and Chan have support among Democrats, Independents, and even some Republicans. Similarly, Murphy and Smith are both mainstream Republicans, while Lorenzo has support among Republicans, Independents, and some Democrats.
With 1,000 voters, the election threshold is 250 votes (25% of 1,000).
A count of first choices elects the most popular Democratic candidate, Perez. Perez has 20 more votes than the threshold, so every voter who ranked Perez highest will have 7.4% of their vote (20 divided by 270) count for their next choice, so 20 votes will be added to other candidates in the next round. More than half of Perez voters ranked Chan second, with a smaller number ranking Jackson or one of the Republicans next.
Rounds two through four resolve vote splitting among the three Republicans and the two remaining Democrats. Round two eliminates the weakest Republican candidate, while round three eliminates the weakest Democrat. When the one remaining Democrat passes the threshold and is elected, most of her surplus goes to Lorenzo, who comfortably wins the third seat. Note that in the final round, 45 ballots are "exhausted" because some of Chan's voters were indifferent to the two remaining Republican candidates and so did not rank either of them.
The winners are Perez (D), Chan (D), and Lorenzo (R). If this were a single winner election, the most mainstream Democrat (Perez) easily would have won, leaving all others unrepresented. Instead, two additional candidates are elected, both of whom are rewarded by coalition-building among the district's remaining center-left and center-right populations. In the end, 96% of voters can point to a candidate who they supported and helped elect.
Had these three seats been elected by bloc voting, in which every voter casts three votes for the three candidates they support, the Democrats would have almost certainly swept all three seats, because each Democratic voter could vote for all three Democratic candidates. In fact, even if the three seats were elected by the single vote system, a weaker form of fair representation voting, Democrats still would have swept all three seats due to the Republican vote being split among three candidates.
Had these three seats been filled by dividing the district into three single-winner districts, the outcome would depend on how the district lines were drawn. The districts could have been drawn to elect two Democrats and one Republican, or they could have been gerrymandered to over-represent either party. Regardless, the district elections would probably not be competitive, and each district's primary election would likely weed out the candidates who won by coalition-building.
By using ranked choice voting to elect three seats, the election results fairly represent the district's diversity after a competitive election without any opportunity for partisan gerrymandering.