Lix Readability Formula

The Lix Reading Formula has become extremely useful for application in several non-English languages, being used for both English and non-English texts. You might have heard of the Rix Reading Formula before. The Lix is much the same as the Rix and assesses reading skills and readability of texts. 

What does the Lix do?

It measures readability levels by counting the number of letters in a text. It doesn’t count syllables like other formulas do because syllable counting can be inaccurate if you are using non-English languages. Instead, both the Lix Reading Formula and the Rix use the letter counting method. 

Where did the Lix Formula come from?

The Lix Reading Formula comes from Sweden. A man by the name of Carl-Hugo Björnsson created it in 1968. He published the results in a book called Läsbarhetsindex. If you translate that into English, it means ‘readability.’ Carl-Hugo Björnsson called it ‘Lix’ for short.

What Björnsson did to ensure accuracy in his reading formula

Björnsson kept 162 books which he used to rate readability features. Teachers and student groups used his readability content. These 162 texts consisted of fiction, textbooks, and technical literature. You can see he had a large range of literature available to the teachers and students.

He would compare text features to measure the readability of a text

He would look at the complexity of the text, the length of sentences, unusual words, and different words to assess its level of readability. He would eliminate anything that wasn’t consistently accurate with all readability formulas. There were other readability assessments that counted syllables and unfamiliar words.

Lix is used to calculate the percentage of words with seven or more letters. It is quite similar to other formulas such as the Flesch-Kincaid Readability Score Formula, Spache Formula, and Fry Formula. It calculates the average number of words in a sentence.

Check out a description of the Lix Formula here:

Lix is the percentage of long words plus the average number of words in a sentence.

It was shown that Lix had a high level of accuracy

Jonathan Anderson created the Rix Formula ten years after the Lix Reading Formula was developed. Anderson wanted to take the Lix formula to a grade level because he realized that many librarians and teachers used the grade level. He modified the formula, creating the Rix Reading Formula.

The criteria he used were revealed in a reading study by RM Thorndike in 1973. Anderson tested Lix, using the same texts from the study by RM Thorndike. In a second study, he discovered that the Lix Reading Formula showed a high level of accuracy as a readability test.

Because Anderson was satisfied with his results after testing the accuracy of Lix’s readability assessments, he made a conversion table. Here it is, available in Readable.

The usefulness of Lix

The formula is a simple measure that is useful for determining the readability score of a text. It's used for educational purposes and has been found reliable for detecting reading levels for many ages, from young kids to adults. The Lix is also a valuable formula for categorizing books.

For readability in a few non-English languages, Lix was also studied. One of these studies focused on Lix being used for languages such as German, French, English, and Greek.

To compute Lix scores, these are the steps

  1. Count the total number of words as well as the number of long words that have more than six letters. Also, count the number of sentences in the text.
  2. Then calculate the percentage of long words in the text.
  3. Next, calculate the average words in the sentence.
  4. Add the amounts you come to in Steps 2 and 3 and round them to the closest whole number.

The scores obtained will range from around 20, which is very easy, to around 60, which is very difficult.

The Lix Formula is easy to use for calculation and for examining text at different levels, from small children’s materials to secondary levels and adults’ text.

Lix proved an excellent solution for foreign language readability

Lix used to be a little-known readability index that was only popular in Scandinavia, but now it’s popular all over the world.

  1. Nelson commented on the computation of the Lix like this:

"Lix is not only simple to calculate, avoiding as it does, mathematical formulae, it differs from the English readability measures in two important ways. Firstly, it bypasses the problem of whether to count monosyllabic words, polysyllabic words or total syllables by including only words beyond a certain length; that it is a measure which ignores the linguistic rules of syllabification suggests that it is potentially useful across languages. Secondly, Lix calculates the percentage of long words from 100 word samples while sentence length is computed from separate 10- sentence samples."

The comparisons of French and English Texts with Lix

The French Lix indices correlated 0.87 with Lix indices in English. Flesch indices across languages correlated 0.90. Both of these varying correlations suggest that the French and English formulas were very similar. Using Lix and Flesch for French books, the correlation was -0.80. For the English books, it was -0.78. The results are negative because difficulty is rated high on Lix, and low on the Flesch.

Factors that contribute to making reading difficult

o Factors concerning the reader.

o The purpose of why the text was written.

o How readers ‘read’ the text and interact with it.

Lix, like the other readability measures, only provides incomplete and inexact estimates of reading difficulty with its readability scores.

Björnsson provided this table of readability categories to interpret readability test scores for Lix. The norms are for the Swedish language. You can only interpret Lix scores in the Greek, French, and German languages.


"Lasbarhetsindex" or Lix is a readability formula that was developed in Sweden. It appears to be able to assess text difficulty in other languages too, including English.

For documents in Western European languages, the Lix formula is useful as a readability analysis tool. It bases itself on sentence length and the number of long words. Long words are words that have over six characters in them.

Jonathan Gillham

Founder / CEO of Originality.AI I have been involved in the SEO and Content Marketing world for over a decade. My career started with a portfolio of content sites, recently I sold 2 content marketing agencies and I am the Co-Founder of, the leading place to buy and sell content websites. Through these experiences I understand what web publishers need when it comes to verifying content is original. I am not For or Against AI content, I think it has a place in everyones content strategy. However, I believe you as the publisher should be the one making the decision on when to use AI content. Our Originality checking tool has been built with serious web publishers in mind!

More From The Blog

AI Content Detector & Plagiarism Checker for Serious Content Publishers

Improve your content quality by accurately detecting duplicate content and artificially generated text.