The Dale-Chall Readability Score is a readability test that provides a numeric gauge of the comprehension difficulty that readers come upon when reading a text. It uses a list of 3000 words that groups of fourth-grade American students could reliably understand, considering any word not on that list to be difficult.
Disclaimer: The Dale-Chall Readability Score is designed for English text only. It may not provide accurate results for other languages.