Reading is an indispensable skill for learners who desire success throughout their academic lives, and vocabulary knowledge is a sine qua non companion of reading comprehension. Despite being inextricably related entities, very little has been written about the necessary vocabulary coverage to understand an expository text and its equivalent in terms of vocabulary size in Turkish EFL context. Therefore, with this study, we focused on the relationship between the vocabulary coverage and reading comprehension of a group of foreign language learners. For this study, 178 university students completed a vocabulary checklist based on the vocabulary items of two different expository texts, and their reading comprehension levels were measured through two piloted reading comprehension tests for each text. The descriptive statistics, Pearson’s correlation value and regression analysis were employed to analyze the data. The results revealed that the text-based vocabulary knowledge moderately correlated with reading comprehension, and there was a relatively linear relationship between them. It was also concluded that the 98% vocabulary coverage is needed for foreign language learners to comprehend academic texts, and this coverage, in fact, refers to approximately the most frequent 8000 word-families based on the related studies.