Seats-to-votes ratio

The seats-to-votes ratio, also known as the advantage ratio, is a measure of equal representation of voters. The equation for seats-to-votes ratio for a political party i is:

${\displaystyle \mathrm {a_{i}} =s_{i}/v_{i}}$,

where ${\displaystyle v_{i}}$ is fraction of votes and ${\displaystyle s_{i}}$ is fraction of seats.

In the case both seats and votes are represented as fractions or percentages, then every voter has equal representation if the seats-to-votes ratio is 1. The principle of equal representation is expressed in slogan one man, one vote and relates to proportional representation.

Related is the votes-per-seat-won, which is inverse to the seats-to-votes ratio.