A Plain-English Guide to Text Readability Scores

Why readability scores exist

Readability formulas were originally developed in the mid-20th century for two practical purposes: helping publishers pitch books at the right grade level for school curricula, and helping the US military rewrite technical manuals that soldiers were failing to understand.

Today they are used by UX writers, content marketers, journalists, and plain-language advocates to audit writing before it reaches an audience. No formula captures every nuance of writing quality — a long word can be simple ("elephant") and a short sentence can be confusing — but they are useful as a consistency check at scale.

Flesch Reading Ease

The Flesch Reading Ease score runs on a 0–100 scale. Higher is easier. The formula counts average sentence length and average syllables per word:

• 90–100 — Very easy. Understood by an average 11-year-old. Think children's books and simple instructions.
• 70–90 — Easy. Plain conversational English. Most web copy and blog posts aim here.
• 60–70 — Standard. Considered appropriate for the average adult. Newspapers typically score in this range.
• 50–60 — Fairly difficult. Some professional writing, corporate reports.
• 30–50 — Difficult. Academic writing and professional journals.
• 0–30 — Very difficult. Legal and technical documents.

For most general-audience content, aim for 60–70. For consumer-facing web copy, aim for 70 or above.

Flesch-Kincaid Grade Level

The Flesch-Kincaid Grade Level translates the same underlying calculation into a US school grade (1–12+). A score of 8 means an 8th grader can read it; a score of 12 means a high-school senior; above 16 approximates graduate-level text.

The Guardian and BBC News typically publish at grade 8–10. Most successful email newsletters are grade 6–8. If your content scores above 12 and it is not intended for specialists, that is a signal to simplify.

Gunning Fog Index

The Gunning Fog Index, developed by Robert Gunning in 1952, specifically targets complex words — words of three syllables or more. The formula adds average sentence length to the percentage of polysyllabic words, then applies a multiplier.

The result maps to a grade level similarly to Flesch-Kincaid. A Fog score above 12 is considered hard to read; above 17 is classified as graduate-level difficulty.

Gunning Fog is particularly useful for technical writing reviews, because it directly penalises jargon and long terminology that can often be replaced with simpler synonyms without loss of precision.

SMOG Index

The SMOG (Simple Measure of Gobbledygook) Index, created by G. Harry McLaughlin in 1969, was designed specifically for healthcare documents. Unlike other formulas, it counts only polysyllabic words across a fixed sample of 30 sentences and produces a grade level.

SMOG is widely used by health communicators because research shows it better predicts comprehension in medical contexts than Flesch-Kincaid. If you are writing patient instructions, consent forms, or public health guidance, SMOG is the score to prioritise — and a score of 6 or below is the target for plain-language health content.

How to improve your score

All readability formulas respond to the same two levers: shorter sentences and shorter words. Here are the most effective techniques:

• Break long sentences in two. Any sentence over 25 words is a candidate. Find the natural pause — often a conjunction like "and", "but", or "which" — and split there.
• Prefer the common word. Use "use" instead of "utilise". Use "show" instead of "demonstrate". Use "end" instead of "terminate". Native English speakers often default to Latinate words in professional writing — reversing that habit is the fastest route to a lower Fog score.
• Eliminate filler phrases. "In order to" → "to". "Due to the fact that" → "because". "At this point in time" → "now". These pad length without adding meaning.
• Vary sentence length intentionally. A string of very short sentences reads as choppy and childlike. The goal is an average length of 15–20 words, achieved by mixing short punchy sentences with medium-length ones — not by making everything the same.
• Read it aloud. Sentences that trip you up when spoken are almost always too long or structurally awkward in ways a formula won't catch.

A note on limitations

Readability scores are a signal, not a verdict. They measure surface complexity — syllable count and sentence length — and have no understanding of meaning, coherence, or relevance. A text can score 65 on Flesch and still be confusing because it uses vague language, buries the main point, or assumes context the reader doesn't have.

Use readability scores as a quick audit tool, not a replacement for editing and user testing. A score of 60 is a good starting point, not a finish line.