Vocabulary knowledge is one of the most important elements of reading comprehension. Text coverage is the proportion of known words in a given text. We hypothesize that text comprehension increases exponentially with text coverage due to network effects and activation of prior knowledge. In addition, the lexical threshold hypothesis states that text comprehension increases faster above a certain amount of text coverage. The exponential relationship between text coverage and text comprehension, as well as the lexical threshold, are at the heart of text comprehension theory and are of great interest for optimizing language instruction. In this study, we first used vocabulary knowledge to estimate text coverage based on test scores from N = 924 German fourth graders. Second, we compared linear with non-linear models of text coverage and vocabulary knowledge to explain text comprehension. Third, we used a broken-line regression to estimate a lexical threshold. The results showed an exponential relationship between text coverage and text comprehension. Moreover, text coverage explained text comprehension better than vocabulary knowledge, and text comprehension increased more quickly above 56% text coverage. From an instructional perspective, the results suggest that reading activities with text coverage below 56% are too difficult for readers and likely inappropriate for instructional purposes. Further applications of the results, such as for standard setting and readability analyses, are discussed.