可読測定 (Readability Measurement) is a free Chrome add-on developed by ika.monster. This add-on allows you to measure the readability of selected Japanese text. Simply select the text and right-click to access the "可読性を測定" (Measure Readability) option. The result will be displayed as a badge on the extension icon.

The measurement method is based on the research paper "12学年を難易尺度とする日本語リーダビリティー判定式" (2010 Shibasaki, Hara). The result value represents the grade level suitable for reading the target text, ranging from 1st to 12th grade, which corresponds to elementary school 1st grade to high school 3rd grade.

The add-on utilizes "kuromoji.js" for text analysis and determines clauses and predicates independently. Due to differences in the accuracy of this determination, the measurement results may differ from other applications based on the same research paper.

It is worth noting that the add-on tends to provide more meaningful results for well-structured and formal texts, while the difficulty of kanji is not reflected in the results.