ESL instructors tend to rely on their judgment in estimating the diffi culty level of reading passages for their learners. This common sense method, despite being drawn from experience, gut feeling and intrinsic knowledge of the learners’ ability, is neither effi cient nor objective. A more objective method is available such as using readability formulas to estimate the diffi culty level of the passages. One of the highly reliable readability formulas which have been validated to be used in an ESL/EFL context is the Flesch Reading Ease (FRE) formula. The FRE formula has a diffi culty scale that ranges from 0 (the least diffi cult) to 100 (the most diffi cult), which is rather broad and general. Therefore, it was the intention of this study to refi ne the FRE diffi culty scale for the use of a specifi c group of learners and identify additional predictors of passage diffi culty to enhance the ability of the formula in estimating the diffi culty level of reading passages. To do that, the study replicated Vogel and Washburne’s (1928) process of developing modern readability formula. Reading passages at intermediate and high intermediate levels from several ESL reading coursebooks were analyzed. Three computational tools, the Flesch Reading Ease formula, Writer’s Workbench 8.18 and WordSmith Tools 4.0 were used to extract information related to the text characteristics of the passages at text, sentence and word levels respectively. Findings of the study revealed the development of a more refi ned FRE formula at intermediate diffi culty level scale. This refi ned formula, referred as IDL Formula, used the FRE scores and the coverage of the fi rst 2000 high frequency words of English (HFW scores) of the passages as predictors of passage difficulty. This formula is able to measure 88.7% of passage difficulty, and has a high reliability with the Bormuth mean cloze scores. This formula is meant to be used by ESL instructors, test-setters, materials writers, publishers and curriculum designers to estimate the diffi culty level of reading passages at intermediate and high-intermediate levels.