The LIX readability index: the formula that does not care about syllables

Every popular English readability formula — Flesch, Gunning Fog, SMOG, Flesch–Kincaid — leans on counting syllables. That works beautifully for English and falls apart the moment you cross a border. Syllable-counting rules are language-specific and genuinely hard to automate well; what counts as a syllable in German, Swedish, or Dutch follows different patterns, and a formula tuned to English vowel clusters produces nonsense on them. So if you write or edit in more than one European language, most of the famous formulas are quietly useless to you.

LIX was built to solve exactly that problem. It scores readability from two things you can count in any alphabetic language without a pronunciation dictionary: how long your sentences are, and how many of your words are long. The LIX readability calculator puts that to work. This piece explains where LIX comes from, the precise arithmetic behind the score, how to read the band it lands in, and the one limitation you should keep in mind.

Where LIX comes from

LIX — short for Läsbarhetsindex, Swedish for "readability index" — was devised by the Swedish scholar Carl-Hugo Björnsson in the 1960s. His goal was a measure that would travel: a single formula that ranks the difficulty of a text the same way whether it is written in Swedish, German, English, or Danish. Because it never tries to count syllables or match words against a curated list, nothing in it is bolted to one language's quirks. That portability is why LIX is still widely used across Scandinavia and continental Europe, and why this calculator accepts English and the other Germanic languages rather than English alone.

The two numbers it counts

LIX rests on a simple bet: difficult text tends to have longer sentences and more long words, and you can measure both by counting letters and full stops — no linguistics required. Concretely, the formula combines:

• Average sentence length — the total number of words divided by the number of sentences.
• The percentage of long words — the share of words that are more than six letters long. In this calculator a "long word" is any word of seven letters or more once punctuation is stripped off, so "sentence" (8 letters) counts but "simple" (6 letters) does not.

Those two signals are deliberately crude proxies, and that crudeness is the point: both are trivial to compute identically in any language that uses an alphabet and spaces.

How the score is calculated

The formula adds the two numbers directly:

LIX = (words / sentences) + (long words × 100 / words)

In words: take your average sentence length, then add the percentage of your words that run past six letters. If you write 100 words across 5 sentences (20 words per sentence) and 30 of those words are long (30%), your LIX is 20 + 30 = 50. There are no weighting constants, no square roots, no regression coefficients — just an average added to a percentage. That transparency is part of LIX's appeal: you can usually predict which way an edit will move the score before you run it.

Notice that both inputs pull in the same direction. Splitting a long sentence lowers the first term and often nudges the second if you swap a heavy word for a plain one at the same time. Unlike SMOG, which is dominated by word length, LIX gives sentence length and word length equal billing — they are simply summed.

Reading the result

LIX is not a school-grade number; it is its own scale, and the calculator maps it onto interpretation bands drawn from Björnsson's research:

• Under 30 — very easy, the register of children's literature.
• 30 to 40 — easy, typical of fiction and accessible prose.
• 40 to 50 — average, the level of ordinary newspaper writing and popular non-fiction. This is the healthy target band for most general-audience copy.
• 50 to 60 — difficult, the territory of factual and technical writing.
• 60 and above — very difficult, specialist and academic prose.

So a result in the low 40s is roughly "a competent newspaper article" — readable by most adults. If your text climbs past 50, the calculator's advice is the obvious lever: shorten sentences and cut the proportion of words longer than six letters. Because the long-word term is a straight percentage, replacing even a handful of seven-letter-plus words with shorter synonyms can move the needle noticeably on a short passage.

The limitation to keep in mind

LIX measures word difficulty by letter count, and letter count is only a proxy for how hard a word actually is. A short, common word like "through" is seven letters and counts as long, even though every reader knows it; a genuinely tricky word like "area" is short and does not count at all. Over a paragraph these quirks mostly wash out — long words really do correlate with difficulty in aggregate — but on a tiny sample the score can mislead, and LIX will never notice that "leverage" used as a verb is harder than "elephant". It is a structural measure of surface complexity, not a measure of meaning.

That is also precisely the trade LIX makes on purpose. By refusing to judge meaning or count syllables, it buys language independence and total transparency. For deciding whether a translation reads at the same level as its source, or whether your Swedish and English versions match in difficulty, that trade is exactly the right one.

Putting it to use

If you write for an international audience, or simply want a readability number whose every input you can verify by hand, paste your text into the LIX readability calculator and read the band it lands in. Aim for the 40–50 newspaper range for general copy, and treat anything above 50 as a prompt to shorten sentences and prefer plainer words. Because no single formula is the last word, it is worth checking LIX against a couple of others — the automated readability index, which is also letter-based, makes a natural companion, and a readability consensus averages several formulas at once so one measure's quirk does not steer you wrong.