Lix (Swedish läsbarhetsindex, "readability index") is a readability measure developed by Swedish scholar Carl-Hugo Björnsson in 1968. It combines average sentence length with the percentage of long words (more than 6 letters) to produce a score from 20 (very easy) to 60 (very difficult). Lix is language-neutral because it uses only letter counts—no syllable analysis.

LIX = A/B + (C × 100)/A, where A is the number of words, B is the number of sentences, and C is the number of long words (more than 6 letters). Long words are counted after stripping punctuation. The formula adds average sentence length to the percentage of long words.

Scores below 25 are very easy (children's books); 25–30 is easy; 30–40 is standard (newspaper level); 40–50 is difficult (technical literature); 50+ is very difficult (academic papers). For general audiences, aim for 30 or lower. Many newspapers target 25–35.

Lix uses only letter count (long words >6 letters) and sentence length—no syllable counting. Flesch-Kincaid and SMOG rely on syllable analysis, which is English-specific. Lix is language-neutral and works well for multiple languages. It also uses a 20–60 scale instead of grade levels.

Use Lix when you need a language-neutral readability measure, when working with non-English text, or when you want a simple formula based on word length and sentence length without syllable analysis. It is popular in Scandinavian countries and effective for multilingual content.