This paper analyses the differences between L1 acquisition and L2 learning and identify four main aspects: input quality and quantity, mapping processes, cross-lingual influence, and reading experience. As a consequence of these differences, it concludes that L1 readability measures cannot be directly mapped to L2 readability. It proposes to calculate L2 readability for various dimensions and for smaller units. It is particularly important to account for the cross-lingual influence from the learner’s L1 and other previously acquired languages and for the learner’s higher experience in reading. This analysis focuses on lexical readability as it has been found to be the most influential dimension for L2 reading comprehension. This paper discusses the features frequency, lexical variation, concreteness, polysemy, and context specificity and analyses their impact on L2 readability. As a new feature specific to L2 readability, it proposes the cognateness of words with words in languages the learner already knows. A pilot study confirms our assumption that learners can deduce the meaning of new words by their cognateness to other languages.