A readability score estimates how hard a text is to read, expressed as a school grade or a 0-100 ease score. Different formulas measure different things, which is why one text can score grade 8 on one scale and grade 11 on another. Knowing what each formula counts explains the disagreements.
The four scores
Flesch Reading Ease scores 0 to 100, higher is easier. 90-100 is very easy, 60-70 is standard, 0-30 is very difficult. Flesch-Kincaid Grade converts the same idea into a US school grade. Gunning Fog also gives a grade level, weighting long sentences and complex words heavily. SMOG gives a grade level from polysyllabic words (3+ syllables) in a 30-sentence sample.
The formulas share two ingredients: sentence length and word length. Flesch-Kincaid Grade = 0.39 x (words/sentences) + 11.8 x (syllables/words) – 15.59. Reading Ease = 206.835 – 1.015 x (words/sentences) – 84.6 x (syllables/words). Fog and SMOG use complex-word counts instead of raw syllable averages.
Worked example
Take a 100-word passage with 6 sentences and 150 syllables. Average sentence length = 100/6 = 16.67 words. Average syllables per word = 150/100 = 1.5. Flesch-Kincaid Grade = 0.39 x 16.67 + 11.8 x 1.5 – 15.59 = 6.50 + 17.70 – 15.59 = 8.61, about grade 8.6. Reading Ease = 206.835 – 1.015 x 16.67 – 84.6 x 1.5 = 206.835 – 16.92 – 126.90 = 63.0, in the standard range.
SMOG on the same passage would need a 30-sentence sample, so it cannot be computed from 100 words alone. That is a real difference between the formulas: SMOG needs a longer sample and suits longer documents, while Flesch works on short passages too.
How to read the numbers
For general web content, aim for grade 8 or below on Flesch-Kincaid, which most adults read comfortably. Technical or legal content for specialists can score higher legitimately. A grade above 12 means a university-level text, which will lose most general readers regardless of topic.
When formulas disagree by a grade or two, that is normal: they weight sentence length and word length differently. When they disagree by four or more grades, suspect a counting problem, such as bullet lists, headings, or numbers inflating the syllable count. Clean the sample before trusting the score.
How to fix a high score
Shorten sentences first: splitting a 30-word sentence into two 15-word ones usually drops the grade by one or two levels on its own. Then replace long words with shorter ones where meaning survives: “utilise” becomes “use”, “commence” becomes “start”. Break up dense paragraphs and cut filler phrases. Re-score after each pass, and stop when the text reads clearly rather than chasing a magic number. Readability formulas measure length, not clarity of thought, so a short confusing text can still score well.
Readability, accessibility, and search
Plain language helps more than readability scores suggest. Clear text helps readers with low literacy, readers in a second language, and readers on small screens, all of whom are a large share of any audience. Accessibility guidance for public content commonly recommends plain language around a reading age of 12 to 14, which maps roughly to grade 6 to 8.
Do not game the scores by chopping every sentence into fragments. Readers notice, and the text becomes harder to follow even as the grade falls. Write naturally, then use the scores as a check: if a passage scores far above your target, rewrite it for clarity, not for the formula. The formula follows good writing; it cannot replace it.
Try it faster with the tool
You can follow every step above by hand, but the free Readability Checker — Free Flesch, Fog, SMOG & Grade-Level Scores does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Flesch Ease: higher is easier; grade scores: lower is easier.
- Aim for grade 8 or below for general readers.
- Shorten sentences first, then swap long words.