Partisan bias is said to exist when two parties with 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 registered in all electoral systems in the world (i.e., the typical deviation from perfect proportionality). Partisan bias has been aptly studied for electoral systems with single-member districts, whereas for multi-member systems the literature has largely neglected this topic. 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 differential turnout rates across districts) and third-party effects.