Single transferable vote

The single transferable vote is a preferential multi-winner electoral system designed to achieve proportional representation through the use of ranked ballots. It enables voters to express ordered preferences for candidates rather than for parties, allowing their vote to move between candidates as counting proceeds. The system is widely used in countries such as Ireland, Malta, and Australia (in certain chambers), and it is also employed in a variety of non-partisan and organisational elections.

Principles and Purpose

The central purpose of the single transferable vote (STV) is to convert votes into seats in a way that reflects the diversity of voter preferences within a multimember district. Unlike plurality or majoritarian systems, where a single party or faction may dominate even with only a minority of votes, STV enables substantial groups within the electorate to secure representation in proportion to their support. Each voter effectively casts a single vote which may be transferred to later preferences if the most preferred candidate is either eliminated or elected with a surplus.
By allowing such transfers, STV reduces wasted votes. Ballots cast for candidates who are eliminated or who have already surpassed the required threshold for election may still contribute to the selection of another candidate preferred by that voter. This results in outcomes that more closely match the distribution of opinions within the electorate.

Quotas and the Counting Process

A distinctive mechanism of STV is its use of a quota, the minimum number of votes necessary to guarantee election. Common quota formulas include the Hare quota and the Droop quota. The Droop quota, which is widely used, is calculated as:
Quota = floor[(valid votes ÷ (seats + 1)) + 1]
Any candidate whose tally reaches the quota is declared elected. If a candidate receives more votes than required, the surplus—the votes exceeding the quota—is transferred to other candidates according to the voters’ next available preferences.
Surplus transfers may be conducted in different ways depending on the variant of STV. Under whole-vote methods, only a sample or last parcel of ballots is transferred. Under fractional transfer methods, all ballots held by the elected candidate are transferred at a reduced value so that the total surplus transferred equals exactly the surplus number of votes. The Gregory method used in Hare–Clark systems is a well-known example of fractional transfer and avoids randomness in the transfer process.
If vacancies remain and no candidate has surplus votes to transfer, the candidate with the lowest number of votes is eliminated. That candidate’s votes are transferred to the remaining candidates according to the next available preferences on those ballots. The cycle of surplus transfers and eliminations continues until all seats are filled, or until the number of remaining candidates equals the number of seats left to fill.

Variants and Administrative Features

Multiple variants of STV exist, differing chiefly in methods of surplus transfer, the presence or absence of ticket voting, and the requirements imposed on voters for expressing preferences.
Ticket votingSome systems allow political parties to provide group tickets that automatically determine a voter’s full sequence of preferences if the voter opts in; others, including the systems in Ireland and Malta, prohibit ticket voting so that each voter must explicitly rank candidates.
Surplus transfer methodSystems in Ireland and Malta use whole-vote transfers and may introduce some randomness. By contrast, the Hare–Clark system used in Tasmania and the Australian Capital Territory applies the Gregory method, which fractionally transfers the last parcel of votes received by an elected candidate, making the process deterministic.
District magnitudeThe number of seats in a district influences the proportionality of results. Larger multimember districts allow smaller voter groups to gain representation. In systems where district magnitude varies, each constituency is allocated an appropriate number of seats based on population or administrative rules.
Casual vacanciesVacancies arising between elections may be filled by by-elections or by recount methods such as countbacks, in which the original ballots are re-examined to determine which unelected candidate is most broadly preferred.

Comparison with Other Electoral Systems

STV differs from party-list proportional representation because voters rank individual candidates, not parties. It also differs sharply from the single non-transferable vote, in which voters cast a single vote but cannot express alternative preferences or transfer ballots. In contrast to plurality and majoritarian systems—such as first-past-the-post, instant-runoff voting in single-seat contexts, or block voting—STV minimises the likelihood that a single party will sweep all seats in a district unless its support is overwhelmingly dominant.
The proportionality produced by STV approximates the proportionality achieved by list-based systems. Because each successful candidate is typically elected with roughly the same number of votes, parties receiving twice as many votes as others tend to win roughly twice as many seats. This results in a more equitable distribution of representation.

Mechanics of Vote Transfers

The mechanics of STV transfers are central to how the system preserves voter intent. Votes are always allocated to the highest-ranked candidate on a ballot who is still in contention. If a voter’s preferred candidate is elected with a surplus, their vote may be used to help elect another candidate further down the list. If a preferred candidate is eliminated, the vote transfers to the next meaningful preference. Transfers continue until the vote either helps to elect a candidate or the ballot exhausts because all ranked candidates have been elected or eliminated.
This process can produce cross-party transfers where voters who gave early preferences to a small or under-supported party end up influencing which candidates from larger parties secure seats. Such cross-party movement helps ensure that most voters influence the outcome.

Representation and Fairness

STV is designed to ensure that substantial minority groups have the ability to elect representatives. As the number of seats in a district grows, the proportion of the electorate required to elect a candidate decreases. This allows diverse political, social, and demographic groups to achieve representation, enhancing fairness across the electoral process.
The system also reduces the effects of tactical voting because ranking candidates honestly is generally the most effective strategy. Since a voter’s subsequent preferences are only considered if their earlier preferences cannot be used to elect someone, voters are less likely to engage in strategic behaviours such as vote-splitting or vote-trading.

Non-partisan Example of STV Counting

A standard illustrative example involves a non-partisan selection process, such as choosing several options from a list of alternatives. In the well-known case of selecting three foods from seven options for a group of twenty-three participants, the Droop quota is calculated as six. When first preferences are counted, one option may reach the quota immediately and be elected, with surplus votes transferred according to preferences. Low-scoring options are eliminated in turn, with their votes redistributed. The process continues through rounds of transfer and elimination until three options satisfy the quota or until the number of remaining candidates equals the number of remaining places.
This example highlights the underlying logic of STV: that most participants ultimately contribute to selecting at least one of the winning options.

Broader Applications and Significance

STV has wide application in public elections, internal party elections, university boards, and professional associations. Its flexibility makes it suitable for both partisan and non-partisan contexts. By allowing voters to express full preference orderings and by ensuring that most votes influence the outcome, STV promotes inclusiveness and representativeness.