The Readability Score Calculator analyzes English text with six established readability formulas at once: Flesch Reading Ease, Flesch-Kincaid Grade, Gunning Fog, SMOG, Coleman-Liau, and the Automated Readability Index, and reports the scores with the underlying statistics (words, sentences, syllables, complex words, average sentence length), reading-time estimates at three speeds, and concrete suggestions for improving the text. Paste at least ten words to begin.
How It Works
Every formula is built from the same ingredients: average sentence length (words per sentence), average syllables per word, the share of complex words (three or more syllables), and average word length in characters:
Gunning Fog is 0.4 x (ASL + complex-word percentage); SMOG is 3 + the square root of (complex words x 30 / sentences); Coleman-Liau works from characters per word and sentences per word; ARI from characters per word and ASL. The calculator's headline grade is the average of the five grade-level formulas, and its suggestions trigger on thresholds: sentence length over 25 words is critical (over 18, a warning), complex words over 20% are critical (over 12%, a warning), and an average grade above 12 is flagged for general audiences.
Worked example
A passage of 120 words in 6 sentences, with 180 syllables and 12 complex words, has an average sentence length of 20 and 1.5 syllables per word. Its scores: Flesch Reading Ease 59.6 (the "Fairly Difficult" band, just under the Standard threshold at 60), Flesch-Kincaid grade 9.9, Gunning Fog 12.0, SMOG 10.7, Coleman-Liau 12.1, ARI 12.1, an average grade of about 11.4: late high school. The spread between formulas (9.9 to 12.1 on identical text) is itself the lesson the multi-formula design teaches: readability is a family of related estimates, not a physical constant, and any workflow that treats one tool's single number as exact is measuring the tool as much as the text. The suggestions for this passage would flag its sentence length as slightly long and its grade level as above the general-web target, while passing its vocabulary.
What the scores are for, and what good looks like
Readability formulas were built for triage: matching textbooks to students (Flesch-Kincaid for US Navy manuals and school materials), insurance and government plain-language programs, and health-literacy screening. Their modern use is comparative editing: draft A against draft B for the same audience, and a release check that customer-facing text has not drifted into specialist register. Working targets most editors use: general web and consumer content reads comfortably at Reading Ease 60 to 70 and grades 7 to 9; professional and B2B content can sit a band higher; patient-facing and public-safety text should aim lower still, because its readers include everyone. The reading-time outputs (at 150, 238, and 350 words per minute, slow, average, and fast adult readers) serve the neighbouring decision: how much of a reader's day the document is asking for.
Editing with the scores, without gaming them
The honest levers the formulas respond to are the real ones: splitting a 35-word sentence into two genuinely clarifies it, and replacing a nominalization with a verb ("conduct an investigation" to "investigate") shortens words and sentences simultaneously. The dishonest levers are detectable and self-defeating: chopping every sentence to five words scores beautifully and reads like a telegram, deleting necessary technical terms scores well and misinforms, and the formulas cannot see either failure because they measure surface features, not meaning, structure, accuracy, or whether the reader learned anything. Use the suggestions as a diagnostic list (the flagged sentences are usually genuinely the tangled ones), fix the underlying tangle, and let the score follow. Syllable counting here is a heuristic algorithm, not a dictionary, so occasional words are miscounted; treat single-point score differences as noise and ten-point differences as signal.
Frequently Asked Questions
Which formula should I trust?
For US grade-level reporting, Flesch-Kincaid is the lingua franca; for UK and Commonwealth audiences, Reading Ease bands translate more naturally; SMOG is the health-literacy standard and tends to grade highest because it targets full comprehension. The calculator's averaged grade is a reasonable single figure, with the spread shown so you can see the uncertainty rather than hide it.
Why does my score differ from another tool's?
Sentence splitting (abbreviations, decimals, headings without punctuation), syllable dictionaries versus heuristics, and whether numbers and URLs count as words all vary between tools. Within one tool, scores are comparable across your drafts, which is the use that matters; across tools, compare bands, not decimals.
Can readability formulas judge whether text is well written?
No. They estimate the surface difficulty of decoding the text. Clarity also lives in structure, ordering, examples, and truth, none of which a syllable count can see. A text can be readable and wrong, or difficult and excellent; the formulas govern one variable in that larger judgement.