Partisan bias emerges when two parties with an equal number of votes are allocated a different number of seats. It is, thus, a dyadic relationship not to be confounded with the traditional pro-majoritarian bias (i.e., the typical deviation from perfect proportionality) of all electoral systems of the world. Partisan bias has been aptly studied for electoral systems with single-member districts, whereas for multi- member systems it has been largely neglected. In this article, we extend the idea of partisan bias to any kind of (single-tier) electoral system. Bias is measured as the additive result of an efficiency component (related to the heterogeneity of vote shares of main parties across districts), a vote-weight component (related mainly to malapportionment and different turnout rates across districts) and third-party effects. We also offer some descriptive information on how partisan bias operates in six established democracies nowadays.