Flesh is a cross-platform, open source Java application designed to quickly analyze a document and display the difficulty associated with comprehending it. It is available for all platforms that support Java. Flesh has been released under the GPL (license for use).
After processing a document, Flesh produces two scores: the Flesch-Kincaid Grade Level and the Flesch Reading Ease Score. Each of these scores is calculated after determining the number of sentences, words and syllables a document contains. Using those numbers, the Flesch-Kincaid Grade Level and Flesch Reading Ease Score can then be calculated
Z. Yang, D. Yang, C. Dyer, X. He, A. Smola, and E. Hovy. Proceedings of the 2016 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, page 1480--1489. San Diego, California, Association for Computational Linguistics, (June 2016)