Focusing on orthography as a primary linguistic interface in every reading activity, the central research question we address here is how orthographic intelligibility can be measured and predicted between closely related languages. This paper presents methods and findings of modeling orthographic intelligibility in a reading intercomprehension scenario from the information-theoretic perspective. The focus of the study is on two Slavic language pairs: Czech–Polish (West Slavic, using the Latin script) and Bulgarian–Russian (South Slavic and East Slavic, respectively, using the Cyrillic script). In this article, we present computational methods for measuring orthographic distance and orthographic asymmetry by means of the Levenshtein algorithm, conditional entropy and adaptation surprisal method that are expected to predict the influence of orthography on mutual intelligibility in reading.