Readability AnalyzerHow the formulas differ
The homepage tool shows five distinct readability metrics because each formula captures a different aspect of text complexity. That means different formulas can legitimately produce different scores for the same passage — this is normal and expected, not a sign the tool is broken.
Flesch Reading Ease
This score is on a 0–100 scale where higher values mean easier to read. It uses sentence length and average syllables per word. A score around 60–70 is usually comfortable for general audiences.
FRE = 206.835 − 1.015 × (words / sentences) − 84.6 × (syllables / words)
Flesch-Kincaid Grade Level
This formula translates similar inputs into a U.S. school grade level. It also uses sentence length and syllables per word, but with coefficients that map the result to a grade number. Because it is grade-based, this score is often the easiest to compare against audience targets like grade 6–8 or grade 9–11.
FKGL = 0.39 × (words / sentences) + 11.8 × (syllables / words) − 15.59
Gunning Fog Index
Gunning Fog measures the percentage of complex words (three or more syllables) along with sentence length. It is often used for business and journalism audiences. Complex words are counted by syllable count, not by dictionary difficulty, so long proper nouns or compound words can affect the score.
FOG = 0.4 × [(words / sentences) + 100 × (complex words / words)]
Coleman-Liau Index
Unlike the other formulas, Coleman-Liau uses letter counts instead of syllables. It measures average letters per 100 words and average sentences per 100 words, then converts those into a grade level. This formula is useful when syllable counting is less reliable.
CLI = 0.0588 × L − 0.296 × S − 15.8
Where L is average letters per 100 words, and S is average sentences per 100 words.
SMOG Grade
SMOG is designed to estimate the years of education needed for full comprehension. It is commonly used for health communication and regulatory text because it focuses on polysyllabic words. SMOG typically scores higher than Flesch-Kincaid on the same text, especially for formal or technical passages.
SMOG = 1.0430 × √(polysyllable_count × (30 / total_sentences)) + 3.1291
Why syllable counting matters
Many readability formulas depend on syllable counts, and syllable counting is inherently heuristic in English. Automated syllable counters work well for standard vocabulary but may have minor inaccuracies on unusual words, proper nouns, or borrowed terms. This is an inherent limitation of syllable-based readability formulas across all tools — the results are consistent enough for comparative analysis and audience targeting, but they will never be perfectly accurate on every single word.
What to do with multiple scores
- Use Flesch Reading Ease and Flesch-Kincaid for general audience guidance.
- Watch SMOG for health literacy and patient-facing text.
- Use Coleman-Liau when you want a letter-based benchmark independent of syllable counting.
- Use Gunning Fog for business and executive prose.
When formulas disagree
Most readability tools show you one score in isolation. This tool checks whether your primary formula and a secondary, audience-relevant formula agree — and tells you when they don’t, which is often the more useful signal.
For example, a legal document might score at grade 7.9 on Flesch-Kincaid (passing a grade-8 ceiling) while SMOG scores the same text at 11.7 (well above). Reporting only the passing score would be misleading. When this kind of disagreement happens, the tool surfaces a “Mixed Signal” result that names both formulas, explains why they disagree, and tells you which one to trust for your specific audience.
For practical usage, return to the homepage tool and choose the audience that best matches your target reader. For audience-specific guidance, see Writing for Investors, Plain Language Compliance, or Writing for Patients.