The article presents the electoral formulae used in systems of proportional representation and it analyses the degree of proportionality in the division of mandates among political parties. Proportionality is a key factor in determining the political consequences of the formulae used and at the same time it is an indicator of the distortion of party representation. The research the article draws on was based on the Monte Carlo method. Eight indexes of proportionality were used to compare the qualities of electoral formulae. The results of their measurements differed significantly. A more detailed analysis is made of two main approaches to proportionality, represented by the two most important indexes: the Loosemore-Hanby index and the RR index. Using them electoral formulae are classified into two types of scales according to the degree of disproportionality (on a proportionality - disproportionality continuum) and the direction of disproportionality ('to the advantage of small parties - to the advantage of large parties' continuum). These scales relativise some of the conclusions formulated to date in literature in this field. The article also suggests a scale of formulae constructed on the basis of a third, compromise approach, as the weighted average of the Loosemore-Hanby and the RR indexes.