The seats-to-votes ratio,^{[1]} also known as the advantage ratio,^{[2]} is a measure of equal representation of voters. The equation for seats-to-votes ratio for a political party i is:

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

where $\mathrm {v_{i))$ is fraction of votes and $s_{i))$ 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,^{[3]} which is inverse to the seats-to-votes ratio.

The Sainte-Laguë method optimizes the seats-to-votes ratio among all parties $i$ with the least squares approach. The difference of the seats-to-votes ratio and the ideal seats-to-votes ratio for each party is squared, weighted according to the vote share of each party and summed up:

The D'Hondt method approximates proportionality by minimizing the largest seats-to-votes ratio among all parties.^{[2]} The largest seats-to-votes ratio, which measures how over-represented the most over-represented party among all parties is:

$\delta =\max _{i}a_{i},$

The D'Hondt method minimizes the largest seats-to-votes ratio by assigning the seats,^{[5]}