How does proportional representation match a party’s share of legislative seats to its share of the vote?

Proportional representation aims to ensure that a political party’s share of legislative seats broadly reflects its share of the popular vote. Because perfect proportionality is rarely achieved, systems rely on specific electoral formulas and district magnitudes to translate votes into seats.12

Key Mechanisms

• District Magnitude: This refers to the number of seats assigned to a constituency. Generally, the higher the number of seats per district, the more proportional the final outcome.12
• Electoral Formulas: These mathematical rules determine how votes are converted into seats.13
  • Party-List Systems: Parties submit lists of candidates, and seats are awarded based on party vote totals. Common methods include the largest-average formula (such as the d’Hondt rule), which assigns seats one at a time to parties with the highest average votes, and the greatest-remainder method, which uses a quota system to allocate seats based on vote remainders.23
  • Single Transferable Vote (STV): Voters rank candidates by preference. A quota is used to determine the number of votes required to win a seat, with surplus votes transferred to other candidates based on voter preferences.2

Proportionality can be further influenced by electoral thresholds, which require parties to reach a minimum percentage of the vote to gain representation.3