Book Readability Checker

How each score is worked out

The formula behind every number is written out below and cited to the paper it came from, and the two figures no paper backs — the age band and the syllable count — say plainly that they are our own assumptions, because a reading grade with no working shown is a guess wearing a lab coat.

Every formula below is fed from the same four counts — words, sentences, syllables and letters — which the tool prints above so you can check them. Take a six-word sentence where every word has one syllable, like The cat sat on the mat. That is 6 words per sentence and 1 syllable per word, so Flesch Reading Ease is 206.835 − 1.015 × 6 − 84.6 × 1, which is 116.145, and the Flesch-Kincaid grade is 0.39 × 6 + 11.8 × 1 − 15.59, which is −1.45. Paste that sentence in and you will see them rounded to one decimal. Grades below zero are real, and for a picture book they are the point.

• Flesch Reading Ease

  Reports a 0–100 score, where higher is easier — the band labels on this page are Flesch's own, from that paper.

  Flesch, R. (1948). A new readability yardstick. Journal of Applied Psychology, 32(3), 221–233.

• Flesch-Kincaid Grade Level

  Reports a US school grade, recalculated from Flesch's own inputs so the two numbers always agree with each other.

  Kincaid, J. P., Fishburne, R. P., Rogers, R. L., & Chissom, B. S. (1975). Derivation of New Readability Formulas (Automated Readability Index, Fog Count and Flesch Reading Ease Formula) for Navy Enlisted Personnel. Research Branch Report 8-75. Naval Air Station Memphis.

• Gunning Fog Index

  Reports a US school grade, weighted towards the share of long words rather than their average length.

  Gunning, R. (1952). The Technique of Clear Writing. McGraw-Hill.

• SMOG Grade

  Reports a US school grade from polysyllable count alone — defined on a 30-sentence sample, and flagged here when the text is shorter.

  McLaughlin, G. H. (1969). SMOG grading: a new readability formula. Journal of Reading, 12(8), 639–646.

• Coleman-Liau Index

  Reports a US school grade from letters per word rather than syllables, so it does not depend on our syllable counter at all.

  Coleman, M., & Liau, T. L. (1975). A computer readability formula designed for machine scoring. Journal of Applied Psychology, 60(2), 283–284.

• Automated Readability Index

  Reports a US school grade from characters per word — the second of the two cross-checks that never sees a syllable count.

  Senter, R. J., & Smith, E. A. (1967). Automated Readability Index. Technical Report AMRL-TR-66-220. Aerospace Medical Research Laboratories, Wright-Patterson Air Force Base.

Formula references checked against the literature verified 30 August 2026.

Where the age band comes from

Ours, not anybody's published table. It has two steps. First, a US school grade G is ordinarily reached at ages G+5 to G+6 — that part is a convention about school years, not a finding about readers. Second, we widen the band by a grade in each direction, because the grade it starts from is itself an estimate from two averages and quoting a single school year of it would claim a precision the number does not have. Amazon publishes no reading-age standard for KDP, and this band is not one.

So the chain is: your text gives the counts, the counts give the Flesch-Kincaid grade from the 1975 Navy report cited above, and we turn that grade into an age range using the two steps described here. The first link is published work. The last step is ours, and it is labelled as ours everywhere it appears, because a reading age that borrows somebody else's authority is worse than no reading age at all.

Mapping last reviewed verified 30 August 2026.

Readability for children's and low-content books

A low-content book is mostly not prose, which is exactly why a readability score is easy to misuse on one. A word search grid, a maze, or a page of blank lines has no sentences, so pasting it in tells you nothing. What a child actually reads in a puzzle book is the instructions, the puzzle titles, the theme words and the front matter — and those are usually written by an adult in a hurry, three grades above the age band printed on the cover.

Paste those. If your cover says ages 6 to 8 and your instructions come back at grade 6, that gap is a real problem a reviewer will find before you do. The same goes for the back-cover blurb, which is often the hardest text in the whole book.

Readability for your Amazon description

Switch to the description tab and paste your listing copy. The arithmetic does not change between the two modes — only the guidance does — so the difference between the two grades is a real comparison rather than an artefact of how each was measured. A description read on a phone, in a grid, by somebody deciding in seconds should read easier than the book it sells.

Two things this mode is careful about. It scores the reading level of that description, not of your book — they are different pieces of text and they usually score differently. And it counts your plain words: KDP's description field counts the HTML too, which is what the Amazon book description HTML formatter is for.

What this does not measure

Six formulas run over the same counts, and each is cited on this page with the paper it comes from. They agree with each other because they are all built from the same two inputs: average sentence length and average word length. None of them reads your vocabulary, your subject, or whether a sentence holds together — a formula cannot tell a clear short sentence from a nonsensical one. The age band goes one step further and is ours rather than anybody's published table: it maps the Flesch-Kincaid grade onto the ages that grade is usually reached at, then widens it, because a grade estimated from two averages does not deserve to be quoted as a single school year.

• Whether the writing is any good. These formulas reward short sentences and short words, and a page of disconnected three-word fragments scores beautifully. Use the grade to find the paragraph you already suspected, not to decide whether a draft works.
• Vocabulary and subject matter. "Cat" and "quark" are the same word to every formula here — one syllable, four letters — so a physics book written in short sentences can score as easier than a picture book.
• The syllable count, which is an estimate from spelling. English spelling does not reliably encode how a word is said, so the counter carries a hand-maintained list of the words it would otherwise get wrong, and it is still an estimate on the words that list has never seen.
• Anything on the page that is not running prose. Puzzle grids, word lists, answer keys, recipes, exercises and verse do not obey a sentence-length formula, so paste the instructions and the front matter of a low-content book rather than a page of the puzzles themselves.
• Any Amazon or KDP requirement. Amazon publishes no reading-level standard for books or for descriptions, does not ask for one, and does not check one — nothing here is a compliance test, and passing it guarantees nothing about how your listing performs.