What Readability Scores Actually Mean for Your Writing (And Why You Shouldn't Obsess Over Them)
Every content tool I've ever used has some kind of readability score tucked away in the corner of the interface. Sometimes it's a green badge. Sometimes it's a bar that creeps toward red when your sentences get long. Sometimes it's just a raw number sitting there, silently judging you.
Most writers do one of two things: they ignore it entirely, or they start hacking their prose to pieces trying to make the number go down. Neither approach is quite right.
The scores themselves are actually useful signals — when you understand what they're measuring. The problem is that most explanations skip straight to "aim for a Grade 8 reading level" without explaining what that even means or why Grade 8 was chosen in the first place. So let's fix that.
The Three You'll Actually Encounter
There are dozens of readability formulas out there, but three show up in almost every SEO and content tool worth mentioning: Flesch-Kincaid, Gunning Fog, and SMOG. They all take somewhat different approaches to the same underlying question: how hard is this text to decode?
Flesch-Kincaid: The Original
Rudolf Flesch developed his readability formula in the 1940s, originally to help the Associated Press write cleaner news copy. The U.S. Navy later adapted it into what we now call the Flesch-Kincaid Grade Level — and that adaptation is where the "grade level" framing comes from.
The formula looks at two things: average sentence length and average number of syllables per word. Both in the same equation, weighted together. The output is a school grade level — a score of 8.0 means a typical eighth-grader could read it comfortably.
You'll also see something called the Flesch Reading Ease score, which is the same ingredients in a slightly different formula, but inverted — higher scores mean easier reading. A score of 70 is considered easy. A score of 30 is academic territory. Most content tools use the Grade Level version because it's easier to explain.
What Flesch-Kincaid does well: it's fast, it's been validated across a huge range of texts, and it correlates reasonably well with reader comprehension in controlled studies. What it misses: it has no idea what your words actually mean. "Catastrophic" (5 syllables, sounds hard) and "beautiful" (4 syllables, sounds medium) both cost more than "good" (1 syllable) — regardless of whether your reader uses them every day.
Gunning Fog: The Jargon Detector
Robert Gunning created his Fog Index in 1952, and it takes a slightly different angle. Like Flesch-Kincaid, it cares about sentence length. But instead of counting all syllables, Gunning Fog counts "complex words" — defined as words with three or more syllables, with some exceptions carved out for proper nouns and common suffixes like -tion and -ing.
The output is also a grade level. A Fog Index of 12 corresponds to a high school senior. Above 17 is considered unreadable by most people.
The "fog" metaphor is deliberate. Gunning was writing for business communicators and journalists, and he was specifically concerned with dense professional jargon creating a fog that readers had to push through. The index was designed to identify texts where writers were leaning too hard on technical vocabulary.
Gunning Fog is particularly useful when you're writing in a field with a lot of specialized terminology — healthcare, law, finance, software. If your Fog score is high, it's often a signal that you've got too many field-specific terms that your audience might not know, even if each individual sentence isn't particularly long.
The limitation: it overcounts. Words like "understand," "implement," "important," and "marketing" all have three syllables and all get flagged as "complex" — but none of them are hard words in any meaningful sense. So Gunning Fog tends to run higher than Flesch-Kincaid for the same text, and the gap can be misleading.
SMOG: The Healthcare Standard
SMOG stands for Simple Measure of Gobbledygook, which is either charming or irritating depending on your tolerance for acronym humor. G. Harry McLaughlin developed it in 1969, and while it uses the same general approach — count polysyllabic words, account for sentence length — it was designed specifically to be more accurate at predicting the grade level needed to fully comprehend a text, rather than just decode it.
The practical upshot is that SMOG tends to produce higher grade-level estimates than Flesch-Kincaid for the same text. If Flesch-Kincaid says Grade 8 and SMOG says Grade 11, that doesn't mean one of them is wrong — they're measuring slightly different things. SMOG is asking "what level of education does someone need to genuinely understand this?" rather than just "can they get through the words?"
This is why SMOG is the dominant standard in health literacy research. When you're writing patient instructions for post-surgical care, it matters whether someone can truly understand and act on what they're reading — not just whether they can technically decode the sentences. The U.S. Centers for Disease Control and the National Institutes of Health both reference SMOG guidelines for health communication.
What All Three Get Wrong
Here's the core problem with every readability formula: they're measuring the surface features of language, not language itself.
Take these two sentences:
The cat sat on the mat.
The boy bought the pen.
Identical Flesch-Kincaid scores. One is a nursery rhyme. The other is a plausible sentence you might find in an English language textbook. They're both trivially easy, sure — but now imagine the same formula applied to a passage where every sentence is technically short and simple, but the concepts being discussed require significant background knowledge to follow. The score would look great. The text would be incomprehensible to a newcomer.
Readability formulas can't measure:
- How logically your ideas flow from one sentence to the next
- Whether your examples actually clarify or confuse
- The assumed background knowledge your reader brings
- Whether your structure matches how people actually process information
- Tone, voice, and whether someone wants to keep reading
A piece of content can score beautifully on every metric and still be a slog to get through. I've read plenty of them.
So When Do the Scores Actually Help?
They're genuinely useful in a few specific situations.
Spotting problem passages. If you run your text through a readability checker and one paragraph comes back significantly harder than the rest of the piece, that's worth a second look. Not because the score is automatically right, but because it's flagging something worth reviewing. Sometimes you'll find a sentence that ran away from you, or a paragraph where you stacked too many technical terms without explanation.
Calibrating for your audience. If you're writing consumer-facing content for a general audience, a Flesch-Kincaid Grade Level in the 7–9 range is a reasonable target. Not because anyone has proven that Grade 8 readers are the platonic ideal web reader, but because it's a rough proxy for "I'm not making this unnecessarily hard." If you're writing for specialists who expect and understand technical language, those targets shift significantly — and that's fine.
Catching genuine structural issues. Very high sentence complexity scores often do point to real problems: sentences with multiple embedded clauses, passive constructions stacked on top of each other, or paragraphs where you've never stopped to let the reader breathe. The score doesn't tell you how to fix it, but it can tell you that something structural is worth examining.
Healthcare, government, and legal writing. In these contexts, readability guidelines have actual teeth — and for good reason. Plain language standards exist because lives and livelihoods can depend on whether people understand what they're reading. Here, SMOG in particular earns its keep.
The Number Isn't the Goal
The trap I see writers fall into — especially in content marketing — is treating the readability score as the goal rather than as a rough proxy for something else.
When you write toward a score, you end up making decisions based on syllable counts instead of communication. Long words that your audience uses every day become suspect. Genuinely complex ideas get oversimplified not because your reader needs you to simplify them, but because simplifying will bring your grade level down. Sentences get chopped up into short fragments that technically score well but read like instructions for assembling furniture.
The score is a lagging indicator. It's measuring characteristics that tend to correlate with clarity — shorter sentences, simpler words — but those characteristics aren't what makes writing clear. Clarity comes from having something worth saying and saying it in a way that respects your reader's intelligence and time. The readability score, at its best, is a sanity check on the output of that process. It's not a substitute for it.
If your writing is genuinely clear — if you're using concrete examples, if your ideas follow logically from one to the next, if you're not hiding behind jargon, if you've actually thought about what your reader needs to know — your readability scores will usually be fine. Not perfect, not optimized, but fine. And fine is enough.
A More Useful Way to Think About It
Rather than targeting a specific score, try this instead: after you finish a draft, run it through a readability tool and look for outliers. Paragraphs or sentences that score significantly harder than the rest of your piece are worth revisiting. Ask yourself honestly whether those passages are complex because they need to be — because the ideas genuinely require careful handling — or because you were tired, or rushing, or trying to sound authoritative.
Most of the time, the answer is somewhere in the middle. You'll find one sentence that genuinely needs to be complex, and two that could be simplified without losing anything. Fix the two. Leave the one alone.
That's what readability scores are actually for: not as a target to hit, but as a lens to help you see your own writing a little more clearly.