SMOG Grade and Other Readability Formulas

SMOG Grade and Other Readability Formulas Compared

Your patient education brochure scores grade 8 on Flesch-Kincaid but grade 11 on SMOG. The scores disagree because the formulas measure different things. Knowing which formula your industry trusts prevents you from optimizing the wrong number and failing a compliance review.

SMOG Grade: designed for health literacy and patient education

SMOG (Simple Measure of Gobbledygook) was developed by G. Harry McLaughlin in 1969 specifically to evaluate the readability of health education materials. Unlike Flesch-Kincaid, which samples the full text, SMOG requires exactly 30 sentences: the first 10, the last 10, and 10 from the middle. It counts all words with three or more syllables in that 30-sentence sample and takes the square root. This construction makes SMOG specifically sensitive to polysyllabic medical terminology, which is why it became the standard formula for patient education materials and health literacy compliance.¹

The US Department of Health and Human Services recommends SMOG for assessing health materials, and many state and federal agencies specify a SMOG Grade of 6–8 for patient-facing documents. Based on NIH guidance, patient education materials should not exceed a 7th-grade reading level using the SMOG formula. If your content must comply with a health literacy standard, confirm whether the specification references SMOG specifically, because other formulas produce systematically different scores and substituting one for another may not satisfy the requirement.²

SMOG thresholds in US healthcare guidelines

The American Medical Association and American Academy of Pediatrics both publish guidance recommending SMOG Grade 6 as the maximum for written patient instructions. CDC guidelines on health communication specify readability at a 6th-to-8th-grade level using a validated readability tool, with SMOG listed as a recommended option. For hospital patient education departments and pharmaceutical consumer information leaflets, these SMOG thresholds are operational requirements, not suggestions. Testing content against SMOG before publication is a compliance step in those contexts.

The thresholds exist because patient instructions fail their purpose when the reader cannot follow them. A discharge summary written above the patient's reading level is more likely to be misinterpreted, which is the specific harm these guidelines are designed to prevent. Treat the 6–8 grade band as a ceiling for any document a patient must act on without clinical support.

Coleman-Liau Index: character-based readability without syllable counting

Coleman-Liau calculates readability from characters per 100 words and sentences per 100 words, not from syllable counts. Its formula is: 0.0588 × (characters per 100 words) − 0.296 × (sentences per 100 words) − 15.8. This character-based approach eliminates the need for a syllable dictionary, making Coleman-Liau the most reproducible formula across different tools: two programs that count characters reach the same score, while two programs using different syllable dictionaries may diverge on Flesch-Kincaid. For automated content auditing systems processing thousands of documents, this consistency is a practical advantage.³

In practice, Coleman-Liau tends to score 1–2 grade levels higher than Flesch-Kincaid for the same text. Character count captures word length more precisely than syllable count: a three-syllable word with 12 characters scores higher than a three-syllable word with 8 characters. This sensitivity to character-per-syllable ratio makes Coleman-Liau more responsive to vocabulary complexity in content where words vary significantly in character length, such as compound technical terms common in engineering and legal writing.

Coleman-Liau in computational text analysis

Natural language processing and corpus linguistics researchers prefer Coleman-Liau over Flesch-Kincaid for large-scale analysis because it requires only character and sentence counts. Both are available from standard text processing libraries without additional lookup tables. If you are building a content auditing system that processes thousands of pages automatically, Coleman-Liau adds less computational overhead than any syllable-based formula. For manual readability checks on individual documents, the choice between formulas matters less; for batch processing pipelines, Coleman-Liau's calculation simplicity gives it a practical edge.

Automated Readability Index, Gunning Fog, and choosing the right formula

The Automated Readability Index (ARI) uses characters per word and words per sentence: 4.71 × (characters/words) + 0.5 × (words/sentences) − 21.43. Developed for the US Air Force in 1967, ARI tends to produce scores 1–2 grade levels higher than Flesch-Kincaid, particularly for short documents. It is most useful when you need a high-confidence score for a document too short for SMOG, which requires a minimum of 30 sentences. Gunning Fog counts only true complex words, excluding proper nouns and common inflected forms, which makes it less sensitive to named entities and more accurate for business writing dense with brand and company names.⁴

Matching each formula to its industry context

No single formula is right for every context. Flesch-Kincaid remains the most broadly recognized default. Use SMOG when content must meet a health literacy standard. Prefer Coleman-Liau for automated analysis pipelines where reproducibility matters most. For business writing training programs, Gunning Fog at 10–12 is the benchmark most corporate coaches reference. After revising content, re-score it with multiple formulas so you keep patient copy inside SMOG limits rather than improving on one formula alone.

When to use this

Use this guide when Flesch-Kincaid alone does not give you enough insight, or when your audience cares about a specific formula (health literacy standards, for example, often require SMOG).

Examples

A paragraph from a medical website scores: Flesch-Kincaid Grade 14, but you do not know what that means relative to other formulas.

The same paragraph scores: Flesch-Kincaid Grade 14 | SMOG Grade 12 | Coleman-Liau Grade 15 | ARI Grade 13 | Gunning Fog 16. Consistently high scores across formulas confirm the text genuinely requires advanced reading ability and is not a quirk of one formula's calculation method. Use these results as a baseline and rewrite to bring all scores into a target range.

Patient education materials must comply with a SMOG Grade ≤ 8 requirement.

Rewrite until SMOG drops to 8 or below. Focus on replacing multi-syllable medical terms with plain equivalents, breaking long sentences, and adding analogies. Re-score after each revision to track progress.