Instant-Runoff Voting is a preferential voting method designed to determine a single winner through successive elimination and transfer of ranked preferences without requiring voters to return for a separate second election. Voters rank candidates in order of preference, and if no candidate receives the required majority of first preferences, the lowest placed candidate is eliminated. Ballots assigned to that candidate are then examined for their next available preference and transferred accordingly. The process continues until a candidate reaches the required winning threshold. International IDEA describes the Alternative Vote using essentially this mechanism and classifies it as a preferential plurality or majority system used in single member districts.
The term Instant-Runoff Voting emphasises the fact that the successive rounds of counting are performed from preferences recorded on one ballot rather than through separate polling events. A hypothetical example illustrates the mechanism. If candidate A has 45 percent of first preferences, candidate B has 35 percent, and candidate C has 20 percent, no candidate has an absolute majority. Candidate C is eliminated, and the ballots that ranked C first are examined for their next valid preference. If enough of those ballots transfer to B, B can reach more than half of the continuing valid votes and win. The actual result depends on the preferences expressed and the rules governing exhausted ballots.
Instant-Runoff Voting is closely associated with the Alternative Vote and is often used as a descriptive name for the same basic single winner mechanism. The terminology can vary between jurisdictions and academic contexts, so the exact legal definition should always be checked. The essential features are a single member contest, ranked candidate preferences, elimination of the lowest candidate when necessary, transfer of ballots to later preferences, and a majority based final determination. The mechanism should not be confused with Single Transferable Vote, which also uses preference transfers but normally elects multiple representatives from multi member districts and can therefore produce proportional representation.
A technically important issue in Instant-Runoff Voting is the treatment of ballots that do not contain a usable later preference after the candidates to whom they were assigned are eliminated. Such ballots can become exhausted under applicable rules and therefore cease to count toward the continuing total used to establish a majority. This means that the phrase majority must be interpreted according to the particular counting rules rather than assumed to mean more than half of every ballot originally cast. Ballot design and voter instructions are consequently important because the number and placement of valid preferences can affect whether a ballot remains transferable during later rounds.
Instant-Runoff Voting differs from ordinary plurality voting because the winner is not necessarily the candidate with the largest number of first preferences. A candidate who initially ranks second can win after receiving transferred preferences from eliminated candidates. This makes later preferences electorally relevant and means that the final winner can depend on the complete ranking structure rather than only the first choice. It differs from a Two Round System because the elimination rounds occur through counting the original ballots rather than requiring voters to attend a second election. It differs from proportional representation because a conventional single winner Instant Runoff election does not allocate multiple seats proportionally.
The counting process can be represented algorithmically. First preferences are counted and the winning threshold is checked. If no candidate reaches the threshold, the candidate with the lowest relevant total is eliminated. The ballots belonging to that candidate are then reassigned according to their highest ranked continuing preference. Totals are recalculated and the process repeats. Depending on the jurisdiction, rules can govern ties, withdrawn candidates, incomplete rankings, invalid marks, exhausted ballots, and recounts. Because these rules can differ, a technically accurate explanation of Instant Runoff Voting must specify the jurisdiction when discussing implementation details rather than assuming that every version uses identical procedures.
Instant-Runoff Voting can alter campaign incentives because candidates may seek not only first choice support but also acceptability among voters whose first preference lies elsewhere. The ballot structure permits voters to express an ordered set of preferences, allowing those preferences to become relevant if earlier choices are eliminated. However, the behavioural consequences depend on the number of candidates, voter information, campaign competition, party organisation, ballot completion rules, and the political environment. Claims about strategic voting or campaign moderation therefore describe possible effects rather than universal characteristics of every election using the method.
Instant-Runoff Voting is best understood as a single winner preferential counting procedure that simulates successive runoff stages within one election. Its defining mechanism is the elimination and transfer process, not merely the existence of ranked ballots. It is closely related to the Alternative Vote and belongs to the plurality or majority family identified by International IDEA. Understanding the system requires attention to first preferences, transfer order, continuing candidates, winning thresholds, exhausted ballots, tie rules, and the distinction between an internal counting round and a separate electoral round.
