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Why are five-member constituencies considered suitable for the single transferable vote system?

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In the context of the single transferable vote (STV) system, five-member constituencies are widely considered optimal for electoral operations.1

The effectiveness of any proportional representation system is heavily influenced by "district magnitude," which refers to the number of candidates an individual constituency elects. A higher district magnitude generally leads to more proportional outcomes, as it allows for a broader reflection of the electorate's preferences.23

By utilizing five-member districts, the STV system balances the need for accurate representation of voter preferences—for both individual candidates and political parties—with the practical requirements of the Droop quota calculation. This size provides enough seats to ensure that minority groups and diverse opinions can secure representation, while maintaining a manageable level of complexity for the transfer of surplus votes and the elimination of candidates.13