Readability Formulas Explained: What Flesch-Kincaid and Gunning Fog Actually Measure

A readability score sounds like it's measuring something precise — plug in text, get back a grade level. What the formulas actually measure is narrower than that framing suggests, and understanding the mechanics behind the number is what keeps you from over-trusting it.

What these formulas actually count

Readability Analyzer computes several classic formulas — Flesch Reading Ease, Flesch-Kincaid Grade Level, Gunning Fog, and SMOG — and every one of them is built from the same small set of countable facts about your text: average sentence length (words per sentence) and average word complexity (syllables per word, or the count of words with three or more syllables). That's it. None of these formulas understand meaning, argument structure, jargon that happens to be short, or whether your sentences are logically connected. A text made of short, choppy sentences using simple-sounding words scores as "easy," even if the ideas inside it are genuinely difficult to follow. A precise technical sentence using accurate but polysyllabic vocabulary scores as "hard," even if it's the clearest way to say exactly that thing.

This isn't a flaw specific to our implementation — it's what these formulas have always measured, going back to when they were designed decades ago for a much narrower purpose (originally, evaluating text complexity for things like technical manuals and military training materials). They're a genuinely useful proxy for surface-level reading difficulty, not a measure of quality, clarity of argument, or whether your writing is actually good.

Why there's a minimum word count

The tool won't return a score below 10 words of input — not an arbitrary limit, but a real constraint on the math: sentence- and syllable-based averages become meaningless (or wildly unstable) on a handful of words, since a single long word can swing an average that's only being computed over one or two sentences. A commonly cited recommendation is closer to 100 words for a stable score you'd actually want to act on — the 10-word floor is a technical minimum to produce a number at all, not a claim that a 10-word score is meaningful.

What the tool doesn't do

It's worth being precise here: this tool reports the scores — it doesn't suggest edits or tell you which specific sentence to rewrite. There's no "here's how to fix it" feature layered on top of the numbers; you get the readability metrics, and applying them (shortening a sentence, swapping a word) is a manual editorial decision, same as it's always been.

Related, but different: counting tokens instead of grade level

If your actual question isn't "how hard is this to read" but "will this fit in an AI model's context window," that's a different metric entirely: Token Counter estimates the token count for common LLM tokenizers (GPT-4, Claude) rather than a readability grade — words, tokens, and reading difficulty are three genuinely different measurements that happen to often get bundled into the same "text tools" category.

Cleaning up before you measure

Copy-pasted text often carries invisible formatting — Markdown syntax, stray rich-text artifacts — that can distort a word or character count before you even get to readability. Strip Formatting removes Markdown syntax and normalizes whitespace first; worth knowing it currently doesn't touch table pipes (|) or header-separator dashes (---), so a Markdown table's structural characters will pass through even after stripping the rest.

When you just need placeholder text

None of this applies, of course, if what you actually need is filler text for a layout mockup rather than something to be measured — Lorem Ipsum Generator cycles through 8 fixed paragraphs for exactly that purpose. It's not meant to be read for content, which is the entire point of lorem ipsum in the first place.

The Bottom Line

A readability score is a real, well-defined calculation based on sentence and word length — not a judgment of how good or clear your writing actually is. Treat it as one input among several (does a reader understand your point, is your argument well-organized, is your vocabulary appropriate for your audience), not as the final word on whether a piece of writing works.