Flesch-Kincaid grade level, explained in one short read
What the school grade means, how two averages produce it, a worked before and after, and the target most pages should hit.
By ClaudeGPT Si Newsroom · Editorial team
· 2 min read
The Flesch-Kincaid grade level turns a passage of English into a US school grade: 8.0 means a typical eighth grader can follow it. It is worked out from two averages, words per sentence and syllables per word, and for most documents Microsoft recommends landing around grade 7 to 8.
Where the number comes from
J. Peter Kincaid and his team built the formula in 1975 under a US Navy contract, reworking Rudolf Flesch's older reading ease test so it would output a grade instead of a 0 to 100 score. The Army began using it for technical manuals in 1978, and it soon became a US military standard.
The math is short:
grade = 0.39 x (words / sentences) + 11.8 x (syllables / words) - 15.59
Lower is easier. A grade of 12 matches a high school senior's reading level, and a grade of 5 matches a fifth grader's.
One sentence, before and after
Wikipedia uses this example: "The Australian platypus is seemingly a hybrid of a mammal and reptilian creature." It has 13 words and 24 syllables in one sentence.
| Original | Rewrite | |
|---|---|---|
| Sentence | The Australian platypus is seemingly a hybrid of a mammal and reptilian creature. | The platypus looks like a mix of a mammal and a reptile. |
| Words | 13 | 12 |
| Syllables | 24 | 16 |
| Grade level | 11.3 | 4.8 |
| Flesch reading ease | 37.5 | 81.9 |
The rewrite is barely shorter. The grade fell by six and a half points because "Australian", "seemingly", "hybrid" and "reptilian" became short, everyday words. Syllable counts vary a little between checkers, so another tool may give slightly different numbers for the same sentence.
Grade level vs reading ease
Both scores use the same two inputs, but they weigh them differently, so you cannot convert one into the other:
- Direction. Reading ease goes up as text gets easier; the grade goes down.
- Weighting. Long words move reading ease more than they move the grade, so two texts can tie on one score and differ on the other.
- Audience. "Grade 8" means something to a client or a manager. "Reading ease 64" usually needs explaining.
What grade should you aim for
Microsoft's Word help recommends around 7.0 to 8.0 for most documents, and seogeoaeo.ai's tool page notes that plain-language guidance for a general audience usually aims near grade 8. Pricing pages, help articles and product descriptions belong in that range. Developer docs and specialist writing can sit at a higher grade without being unclear, because their key terms are long by nature.
A few limits to keep in mind: the formula was built for school books, so it ignores layout, differences between readers, whether they know the words and whether the facts are right. In theory a passage of one-syllable, one-word sentences scores below zero.
How to check it
In Microsoft Word, the readability statistics window shows the grade level next to the reading ease score. For a web page, the Readability Analyzer from seogeoaeo.ai reads a public URL or pasted text and returns the Flesch-Kincaid grade with Flesch reading ease and the Gunning Fog index, then lists the sentences most worth rewriting. Each run costs 5 credits. It points out what to change but leaves the rewriting to you, so if you want an editor that highlights hard sentences while you type, Hemingway Editor's free checker is the better companion for drafting.