When reading researchers compare different languages, they generally focus on the effect of orthographic depth on reading ability, acquisition, and cognitive processing. For extending cross-linguistic reading research to multiple languages, we need to be able to objectively quantify the level of orthographic depth. Some measures have already been proposed (e.g., onset entropy, Borgwaldt et al., 2005; complexity and unpredictability, Schmalz et al., 2015). However, previous work has suggested that different dimensions underlie orthographic depth, and it is not always clear how the quantifications map onto the different underlying constructs. Here, we first examine how different measures relate to underlying theoretical constructs. Then, we quantify the relative depth of 8 European orthographies. We use both existing methods, and present two approaches which have not been previously used to quantify orthographic depth: Distance-based measures relying on the closeness of the phonology of orthographically similar words, and mutual information, as a theory-neutral approach. An exploration of the relationship between the different measures shows that they map on two separate dimensions, in line with previous theoretical work. The measures derived based on different theoretical assumptions largely show agreement, suggesting that, for our languages of study, all theoretical approaches are approximately equally suitable to understanding orthographic depth.