The Readability Guide
The six readability formulas explained — the exact equations, worked examples, how to read each score, and the edits that move them.
A readability score estimates how much effort a reader spends decoding a passage. Nearly every word processor and SEO plugin surfaces one number — usually Flesch–Kincaid — without showing the arithmetic behind it or what to change. This guide opens up all six mainstream formulas: the exact equation, a worked example on a shared sample passage, and how to interpret the result. It closes with a concrete, on-page list of edits that reliably lower a grade level without flattening your voice.
The formulas share a common instinct: text gets harder as sentences grow longer and words grow heavier. They differ only in how they measure "heavy" — some count syllables, some count characters, some count words against a known-word list. That single design choice explains why two tools can rate the same paragraph two grades apart.
The six formulas at a glance
| Formula | Output | Measures difficulty by | Best for |
|---|---|---|---|
| Flesch Reading Ease | 0–100, higher = easier | Syllables per word | General-audience web writing |
| Flesch–Kincaid Grade | US grade level | Syllables per word | The general default |
| Gunning Fog | US grade level | Share of 3+ syllable words | Business and professional prose |
| SMOG | US grade level | Count of 3+ syllable words | Healthcare and patient education |
| ARI | US grade level | Characters per word | Quick, machine-friendly estimate |
| Coleman–Liau | US grade level | Characters per word | Automated text processing |
Paste any draft into the Word Counter and all six readability scores appear live as you type, recalculating on every keystroke so you can watch an edit move the number in real time.
Articles in this guide
Flesch Reading Ease Explained: How to Score and Improve It
What the Flesch Reading Ease score actually measures, what a good score looks like, and three practical changes that move the needle.
Readability Grade Levels Explained: Flesch–Kincaid, Gunning Fog, SMOG and More
Six readability formulas, what each measures, why they sometimes disagree, and how to interpret the numbers your editor surfaces.
How to Improve Your Readability Score: 9 Practical Edits
Concrete, score-moving edits — what to change, what to ignore, and how to keep voice intact while raising Flesch Reading Ease by 10 points.
Jackpot Headline Templates That Boost Clicks, Readability, and Social Engagement
A shared worked example
To make the six formulas directly comparable, every worked example below runs on the same three-sentence passage:
"Readable writing respects the reader. Short sentences help. Long, complicated words slow everyone down."
Counting it gives the raw inputs the formulas need:
| Input | Value |
|---|---|
| Sentences | 3 |
| Words | 14 |
| Syllables | 26 |
| Complex words (3+ syllables) | 4 — readable, sentences, complicated, everyone |
| Letters (characters) | 86 |
You can reproduce these counts with the Word Counter, the Syllable Counter, and the Sentence Counter. Watch how the scores diverge: a passage this short exaggerates the "complex word" ratio, which is exactly why the word-count-based formulas below read a grade or two higher than the syllable-average ones.
Flesch Reading Ease
The oldest and most widely cited formula, published by Rudolf Flesch in 1948. It is the only mainstream metric where a higher number means easier reading.
Flesch Reading Ease = 206.835 − 1.015 × (words / sentences) − 84.6 × (syllables / words)
Worked example. Words per sentence = 14 / 3 = 4.67; syllables per word = 26 / 14 = 1.86.
206.835 − (1.015 × 4.67) − (84.6 × 1.86) = 206.835 − 4.74 − 157.13 ≈ 45
How to interpret it. The score maps onto these bands:
| Score | Reading level |
|---|---|
| 90–100 | Very easy — 5th grade |
| 80–89 | Easy — 6th grade |
| 70–79 | Fairly easy — 7th grade |
| 60–69 | Plain English — 8th–9th grade |
| 50–59 | Fairly difficult — 10th–12th grade |
| 30–49 | Difficult — college |
| 0–29 | Very difficult — graduate / professional |
Our sample lands at 45 — "difficult", dragged down by three heavy words in a tiny sample. For general web writing, aim for 60–70: readable by most adults without feeling condescending. See the Flesch Reading Ease glossary entry for the full history.
Flesch–Kincaid Grade Level
A 1975 rework of Flesch's formula, commissioned by the U.S. Navy, that reports a US school grade instead of a 0–100 index. It is the default in Microsoft Word and most SEO plugins.
Grade = 0.39 × (words / sentences) + 11.8 × (syllables / words) − 15.59
Worked example.
(0.39 × 4.67) + (11.8 × 1.86) − 15.59 = 1.82 + 21.91 − 15.59 ≈ 8.1
How to interpret it. The output is a grade: 8.1 means an average U.S. eighth-grader should follow the text. Because it shares Flesch's inputs, it tracks Reading Ease closely — a 45 on one scale is roughly grade 8 on the other. For a broad online audience, target grade 7–8; drop to grade 5–6 for consumer health or instructions.
Gunning Fog Index
Developed by Robert Gunning in 1952 for business writing. Instead of averaging syllables, it counts the proportion of "complex" words — words of three or more syllables, excluding proper nouns, familiar compound words, and common suffixes like -es, -ed, or -ing that merely add a syllable.
Fog = 0.4 × [ (words / sentences) + 100 × (complex words / words) ]
Worked example. Complex-word ratio = 4 / 14 = 0.286.
0.4 × [ 4.67 + (100 × 0.286) ] = 0.4 × [ 4.67 + 28.6 ] = 0.4 × 33.24 ≈ 13.3
How to interpret it. The result is a grade level: 13.3 means early college. Fog runs high here because four of fourteen words are polysyllabic — a ratio a longer passage would dilute. Gunning's own rule of thumb: aim below 12 for material meant for a wide audience, and below 8 for near-universal readability.
SMOG Index
SMOG ("Simple Measure of Gobbledygook"), devised by G. Harry McLaughlin in 1969, is the standard in healthcare and patient education because it predicts full comprehension rather than partial understanding. It counts polysyllabic words directly.
SMOG = 1.043 × √( complex words × (30 / sentences) ) + 3.1291
Worked example.
1.043 × √( 4 × (30 / 3) ) + 3.1291 = 1.043 × √40 + 3.1291 = (1.043 × 6.32) + 3.13 ≈ 9.7
How to interpret it. The output is a grade — 9.7 means a solid ninth-grade reader. One caveat: SMOG is calibrated for a 30-sentence sample, so scoring three sentences (as we do here for comparison) is only illustrative. Patient-facing material is commonly required to score at grade 6 or below. SMOG typically reads one to two grades above Flesch–Kincaid for the same text, by design.
Automated Readability Index (ARI)
ARI, from a 1967 U.S. Air Force study, was built for typewriters and teleprinters, so it avoids the hard-to-automate syllable count and works from characters instead.
ARI = 4.71 × (characters / words) + 0.5 × (words / sentences) − 21.43
Worked example. Characters per word = 86 / 14 = 6.14.
(4.71 × 6.14) + (0.5 × 4.67) − 21.43 = 28.93 + 2.33 − 21.43 ≈ 9.8
How to interpret it. The result is a grade: 9.8 ≈ tenth grade. Because ARI uses character length as its proxy for word difficulty, it is fast and language-agnostic, but it can be fooled — a short word with unusual spelling counts as easy, and a long-but-familiar word counts as hard. Treat it as a quick sanity check alongside a syllable-based score.
Coleman–Liau Index
Meri Coleman and T. L. Liau (1975) designed their index for mechanical text processing, so like ARI it counts characters, not syllables. It expresses its inputs per 100 words.
Coleman–Liau = 0.0588 × L − 0.296 × S − 15.8
where L = average letters per 100 words and S = average sentences per 100 words.
Worked example. Letters per word = 86 / 14 = 6.14, so L = 6.14 × 100 = 614.3 letters per 100 words. Sentences per word = 3 / 14 = 0.214, so S = 0.214 × 100 = 21.4 sentences per 100 words.
(0.0588 × 614.3) − (0.296 × 21.4) − 15.8 = 36.12 − 6.33 − 15.8 ≈ 14
How to interpret it. The result is a grade level — 14 ≈ college sophomore, running high here because a 14-word sample packs long words into few sentences. On a realistic full-length draft, Coleman–Liau usually lands within a grade of ARI, since both rely on character length. The index rewards short words and moderate sentence counts, and is best read as a cross-check on a syllable-based score rather than a standalone verdict.
Which formula should I use?
For most writing, lead with Flesch–Kincaid Grade Level (or its twin, Reading Ease) — it is the best-validated and the number your readers' own tools will show. Layer a second formula based on your context:
- Marketing, blogs, and general web copy — Flesch Reading Ease; aim for 60–70.
- Business, finance, and professional prose — Gunning Fog; aim below 12.
- Healthcare, safety, and patient instructions — SMOG; aim for grade 6 or lower.
- Automated pipelines or non-English text — ARI or Coleman–Liau, which skip syllable counting.
Never chase a single formula in isolation. When several disagree, the spread itself is diagnostic: a high character-based score with a normal syllable-based score points to long words; a high grade across the board points to long sentences.
How to improve a readability score
Every formula rewards the same two moves — shorter sentences and lighter words — so a handful of edits will lift all six scores at once. Work through this list on any draft that scores harder than you intend:
- Split sentences longer than ~25 words. Sentence length feeds every formula; breaking one 30-word sentence into two 15-word sentences is the single fastest fix.
- Swap polysyllabic words for shorter synonyms. "Utilize" → "use", "facilitate" → "help", "approximately" → "about". This directly lowers Flesch, Fog, and SMOG.
- Cut filler and intensifiers. Delete "very", "really", "quite", "basically", and phrases like "in order to" (→ "to") and "due to the fact that" (→ "because").
- Prefer active voice. "The report was written by the team" → "The team wrote the report" — fewer words and a clearer subject.
- Break long paragraphs. Aim for two to four sentences each; white space lowers perceived effort even when the arithmetic score is unchanged.
- Convert dense prose into lists. Steps, criteria, and options read faster as bullets or a numbered list than as a comma-spliced sentence.
- Define or replace jargon. If a technical term must stay, gloss it on first use; otherwise substitute a plain-language equivalent.
- Use concrete nouns and strong verbs instead of abstract noun phrases ("make a decision" → "decide", "provide assistance" → "help").
- Front-load the point. Put the main clause first so readers grasp the sentence before the qualifiers arrive.
- Read it aloud. Any spot where you run out of breath or stumble is a sentence to shorten — the ear catches what the score only approximates.
Re-score after each pass in the Word Counter and stop when the grade lands in your target range. A two-to-four grade improvement on a first draft is typical, and none of these edits require you to dumb down the ideas — only the sentences that carry them.
Related tools
- Word Counter — six readability scores live as you type
- Syllable Counter — the syllable input behind Flesch, Fog, and SMOG
- Sentence Counter — sentence totals for every grade-level formula
- The Complete Word Count Guide
- Readability and Flesch Reading Ease in the glossary
Frequently asked questions
What is a good Flesch Reading Ease score?
Aim for 60–70 for most online writing — the "plain English" band readable by the average adult. Below 60 reads as effortful for a general audience; above 70 can feel oversimplified. Technical, legal, and academic writing legitimately scores below 50, and that is acceptable when the readers are specialists.
Why do different readability formulas give different grades?
Each formula defines "hard" differently: Flesch and SMOG count syllables, Gunning Fog counts 3+ syllable words, and ARI and Coleman–Liau count characters. A two-to-three grade spread is normal. The disagreement is useful — a high character-based score with a normal syllable score means your words are long, while uniformly high grades mean your sentences are long.
What grade level should I target?
For a broad online audience, aim for grade 7–8 (Flesch–Kincaid). Drop to grade 6 or below for healthcare, safety, or consumer instructions, where SMOG is the standard. Professional and specialist audiences tolerate grade 10–12. Lower is not always better — match the level to the reader.
Are readability scores reliable?
They are useful estimates, not verdicts. The formulas measure only sentence length and word weight — they cannot judge logic, organization, or whether jargon is defined. A passage can score "easy" and still confuse, or score "hard" and read smoothly. Use the score to flag passages for review, then trust your judgment on the fix.
How do I lower my grade level?
Two moves fix nearly everything: split sentences over 25 words and replace long words with short synonyms ("utilize" → "use"). Then cut filler like "very" and "in order to", prefer active voice, and break dense prose into lists. Work through the full ten-tactic list in the section above, and re-score after each pass in the Word Counter.
Which formula do word processors and SEO tools use?
Most default to Flesch–Kincaid Grade Level and Flesch Reading Ease — including Microsoft Word and popular SEO plugins. That makes them the safest primary metric, since they are the numbers your readers' own tools will report. Add Gunning Fog or SMOG when your content type calls for it.