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Guide

Readability Scores Explained (and Their Limits)

·7 min read

Readability scores get quoted with more authority than they deserve. They are useful — genuinely so — but only if you understand that they measure two things and two things only: how long your sentences are and how many syllables your words have.

Everything else people credit them with is projection.

Flesch Reading Ease

The best known formula, developed by Rudolf Flesch in 1948. It outputs a score from 0 to 100 where higher is easier:

206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words)
ScoreLevelTypical of
90–100Very easyChildren's books
80–89EasyConversational writing
70–79Fairly easyPopular fiction
60–69Plain EnglishQuality journalism — aim here
50–59Fairly difficultTrade publications
30–49DifficultAcademic writing
0–29Very difficultLegal and technical documents

Flesch–Kincaid Grade Level

Same two inputs, different output scale. Instead of 0–100, it returns a US school grade, so a score of 8.0 means an average eighth-grader could follow it. It was commissioned by the US Navy in 1975 to check whether technical manuals were readable by enlisted personnel.

For general audiences, grade 8 to 10 is a sensible target. Below grade 6 tends to read as choppy; above grade 14 you are writing for specialists whether you meant to or not.

What the formulas cannot see

This is the part that gets left out of most articles about readability, and it matters.

They cannot detect meaning. A passage of confident nonsense written in short words and short sentences scores brilliantly. "The cat sat on the mat. The mat was blue. Blue is a colour." scores near 100 and communicates nothing.

They cannot detect structure. Headings, lists, and paragraph breaks affect real-world readability enormously. A 2,000-word wall of text and the same content broken into sections score identically.

They cannot detect jargon. "Cache" is one syllable and scores well. "Wonderful" is three syllables and scores badly. For a general audience, "wonderful" is far more readable.

They cannot detect audience. A cardiology paper scoring 25 is not badly written. Its readers know the terminology, and simplifying it would make it less precise and less useful.

How to raise a low score, in order of impact

  1. Split your longest sentences. This moves the number more than anything else. Find every sentence over 25 words and look for a comma or conjunction to break at. Check your average in the sentence counter.
  2. Swap long words for short ones. "Use" for "utilise", "help" for "facilitate", "about" for "approximately", "buy" for "purchase".
  3. Cut hedging phrases. "It should be noted that", "in order to", "due to the fact that" — all syllables, no meaning.
  4. Prefer active voice. "The committee approved the plan" beats "The plan was approved by the committee" on both length and clarity.
  5. Break up paragraphs. This does not change the score at all, but it changes whether people finish reading.

The gaming problem

Because the formula only sees two variables, it is trivially gameable. Replace every full stop with a semicolon and your sentence count drops, so your score plummets — despite identical text. Split every sentence at the comma and your score jumps, even if the result reads like a telegram.

This is why treating a readability score as a target rather than a signal produces worse writing. Chase the number and you get uniform, staccato prose that is technically easy and genuinely tedious. Good writing varies sentence length: a long sentence that develops an idea, then a short one that lands it.

How to actually use it

Use the score as a smoke alarm, not a thermostat.

If you write something you think is clear and the readability checker returns 35, that is worth investigating — you have probably written some 40-word monsters without noticing. If it returns 65, stop optimising and go do something useful.

The single most reliable readability test is not a formula at all: read your work aloud. Every sentence where you run out of breath or have to restart is a sentence your reader will stumble over too. No formula catches what that catches.

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