It has long been shown that Korean EFL students have not practiced extensive reading in school curriculum. Their vocabulary knowledge is limited and a tool for measuring text readability for them has hardly been developed and used. The study aimed to check the current situations on the above issues and develop a reading framework appropriate for extensive reading within the national English curriculum, including the Readability Formula in Korean EFL contexts (KRF). The study consists of four steps. The study does a survey to check the current Korean EFL situations on the issues, suggests a new vocabulary learning size, develops a readability formula as a tool for measuring text difficulty based on the newly suggested vocabulary size, and finally validates the formula with various kinds of English books, including the primary and secondary school English textbooks. It is expected that this study will provide a model for the vocabulary size and the readability index for extensive reading in EFL contexts.