Paste the same paragraph into two tools. Flesch-Kincaid says Grade 8. Gunning Fog says Grade 10. Both are "correct." Here's what's going on.
Flesch-Kincaid Grade Level uses sentence length + syllable count. It's the US Navy standard for training manuals, and it's what Microsoft Word shows by default. It's tuned for technical writing.
Gunning Fog uses sentence length + "complex word" count, where complex means 3+ syllables (excluding proper nouns, compound words, and common suffixes like -ed, -es, -ing). It's tuned for business writing.
The practical difference: Fog punishes jargon harder. A sentence with "organizational" and "infrastructure" will score roughly the same on Flesch-Kincaid as one with "running" and "eating" (all are 3-syllable). Fog counts the first as complex but excludes the second.
Which to use:
- Technical documentation, fiction → Flesch-Kincaid - Business writing, corporate comms → Gunning Fog - Academic papers → both, report the average (SMOG also works) - Marketing copy → Flesch Reading Ease instead of either grade score
Our free grader shows all six at once. If they all agree within 1 grade, your text is stylistically consistent. If they spread across 4+ grades, your writing is uneven — mix of simple sentences with dense vocabulary, or vice versa.