Syllable Counting Errors in Readability Scores

Software cannot hear a word. It has to guess the syllables from the spelling, and English spelling makes that hard. This post explains how the guess is made, where it fails, and how much it matters.

Which formulas use syllables

• Flesch Reading Ease and Flesch-Kincaid count every syllable.
• Gunning Fog and SMOG count words of three or more syllables.
• Coleman-Liau and the Automated Readability Index count letters. They use no syllables at all.

How much one error matters

The Flesch-Kincaid formula adds 11.8 × (syllables per word). So one miscounted syllable moves the grade by 11.8 divided by the number of words in the text.

Text length	Effect of one wrong syllable
10-word sentence	About 1.2 grades
100 words	About 0.12
500 words	About 0.02

On a full page, a few miscounts barely register. On a single sentence, one miscount can move the score by a whole grade. This is one reason one-sentence scores are unreliable.

For SMOG and Gunning Fog, a miscount matters only when it crosses the line between two and three syllables.

How tools count

Most simple counters look for groups of vowels in the spelling. The authors of the Coleman-Liau formula noted in 1975 that programs estimate syllables "usually by counting vowels." Other tools look words up in a pronunciation dictionary and fall back on spelling rules for words they do not know.

Vowel counting fails in predictable ways:

• Vowels that are written but not spoken. "Every" and "business" are said with two syllables but spelled as if they had three.
• Vowel pairs that are two sounds. In "being" and "diabetes," letters that look like one vowel group are spoken as two.
• Words with no ordinary vowel. "Rhythm" has two syllables and no a, e, i, o or u.
• Abbreviations. "API" is three spoken syllables. A spelling rule sees one short word.

This site's calculator used a plain vowel-group counter until October 2026, and it made exactly these mistakes. It now uses spelling rules plus a list of common exceptions, and it is checked against a list of about 200 words. It will still miss some words, especially rare ones and names.

Other counting choices that differ between tools

• Hyphenated words. Some tools count "well-being" as one word, some as two. This site counts one, which is the rule Flesch used. The syllable total stays the same, but the word count changes.
• Numbers. "25" may be counted as one syllable, or as "twenty-five."
• Words with more than one pronunciation. Dictionaries list both a two- and a three-syllable form of "general."

These choices are why two tools can give the same text slightly different grades.

How to check a count

Say the word slowly and count the beats. Or look it up and count the parts of the pronunciation.

One caution about Merriam-Webster. The dots in a headword are not syllable breaks. The dictionary says so itself: the dots "do not necessarily show syllable division" and are "simply potential points of end-of-line division." Count the hyphen-separated parts of the pronunciation instead.

What to do about it

1. Do not chase decimals. A grade of 8.2 and a grade of 8.0 are the same result.
2. Use one tool for before and after. Its errors stay the same in both versions, so the comparison is fair.
3. Cross-check with a letter-based formula. If Flesch-Kincaid and Coleman-Liau broadly agree, syllable errors are not distorting your score.
4. Watch your key terms. If one product or drug name appears many times, check how your tool counts it.
5. Score more text. The longer the sample, the less any single word matters.

How we checked

The formula arithmetic is shown above. The examples of miscounted words come from tests of the vowel-group counter this site used until October 2026. We did not test other tools, so this post makes no claim about how any named product counts a specific word. An earlier version listed such claims without a source, and they have been removed.