SMOG Readability

What Is the SMOG Readability Index?

SMOG is a readability formula that maps English prose to a U.S. grade level calibrated for 100% comprehension on first reading — not the partial understanding thresholds used by many older formulas.

Why it matters: Patient leaflets, consent forms, and safety warnings need every reader to grasp the full message, not roughly half of it.

Example: A medication guide at SMOG 6 targets sixth-grade full comprehension; the same copy might read easier on Flesch–Kincaid because Flesch was built around lower comprehension cutoffs.

Common mistake: Treating SMOG as interchangeable with Flesch–Kincaid. SMOG usually scores one to two grades higher on identical text.

100% comprehension vs partial thresholds

Many older readability formulas were calibrated for 50–75% comprehension on first reading. SMOG targets 100% comprehension — every main idea understood without re-reading.

Why it matters: A passage that “passes” Flesch for a general audience may still fail a SMOG audit written for patients who must understand dosing instructions completely.

Example: Informed-consent language often needs SMOG 6–8 even when the same facts score several grades easier on Flesch–Kincaid.

Edge case: Specialist readers may tolerate higher SMOG when the audience already knows the jargon — the number describes surface vocabulary, not required expertise.

G. Harry McLaughlin and the name “SMOG”

G. Harry McLaughlin published SMOG in 1969 in the Journal of Reading as a simpler alternative to the Gunning Fog Index. The acronym expands to Simple Measure of Gobbledygook — a deliberate echo of Robert Gunning’s “fog” metaphor for opaque prose.

Why it matters: McLaughlin wanted a formula writers could score by hand with a 30-sentence sample and a square root — no syllable averages across every word.

Edge case: SMOG was normed on English. Grade numbers on other languages lack statistical validity.

Reference: Background and formulae: Wikipedia — SMOG Index (external).

What Your SMOG Score Means

A SMOG result is a grade-level number tied to years of U.S. schooling needed for complete comprehension. Lower scores mean easier reading on the SMOG scale.

Why it matters: Healthcare teams can set numeric targets — “keep patient handouts at SMOG 6 or below” — instead of debating whether copy “feels simple enough.”

Example: SMOG 8 aligns with middle-school full comprehension; many plain-language government pages aim near that band.

Common mistake: Panicking when SMOG reads higher than Flesch on the same paste. That gap is expected by design.

Score-to-grade bands

Edge case: Scores on passages with fewer than 30 sentences swing when one long word appears — treat short samples as directional.

How the SMOG Formula Works

SMOG = 1.0430 × √(P × 30 ÷ S) + 3.1291

P = count of polysyllabic words (three or more syllables). S = total sentences. The 30 ÷ S term normalizes any passage length onto McLaughlin’s 30-sentence calibration.

Why it matters: Polysyllabic count sits under a square root, so vocabulary edits move the grade steadily — replacing several three-syllable words often drops SMOG by roughly one band.

Example: With P = 35 and S = 30: SMOG = 1.0430 × √(35) + 3.1291 ≈ 9.3 → ninth-grade band.

Common mistake: Using the simplified “add 3” shortcut on a passage that is not exactly 30 sentences without scaling P and S first.

The McLaughlin precise formula

The constants 1.0430 and 3.1291 come from McLaughlin’s regression against comprehension tests at the 100% threshold. They replace the older mental-math constant +3 for any text length.

Why it matters: Automated scorers should use the precise formula when S ≠ 30. The difference between +3 and +3.1291 is usually under 0.2 grade points on typical samples.

Edge case: Hand-scored conversion tables tied to the approximate method can drift slightly from calculator output.

Simplified SMOG for 30 sentences

On exactly 30 sentences, McLaughlin’s approximate method is: count polysyllabic words, take the square root (rounding to the nearest perfect square in manual work), then add 3.

SMOG ≈ 3 + √(P) (30-sentence sample only)

Why it matters: Classroom and audit worksheets still teach this shortcut. Compare calculator output to the precise formula when stakes are high.

Example: P = 36 → √36 = 6 → simplified grade ≈ 9.

The 30-sentence sampling method

Classic SMOG instructions take 10 sentences from the start, 10 from the middle, and 10 from the end of a long document, then apply the formula to that 30-sentence slice.

Why it matters: McLaughlin normed the formula on 30-sentence samples. A dense introduction alone can misrepresent a 3,000-word chapter if you never sample the middle or close.

Common mistake: Scoring only the first screen of a long PDF. For audit-grade work on long files, sample three spaced blocks or score sections separately.

Polysyllabic Words and Counting Rules

On SMOG, a polysyllabic word has three or more syllables. Each occurrence counts — repetition of the same long token adds to P every time it appears.

Why it matters: SMOG isolates vocabulary difficulty. Sentence length does not enter the formula directly, unlike Gunning Fog.

Example: “Medication” (four syllables) counts once per use. Three mentions add three to P even if the word is familiar to clinicians.

Common mistake: Assuming common long words are excluded. SMOG counts syllables, not reader familiarity.

Manual counting conventions

• A sentence ends at a period, question mark, or exclamation point.
• Some manual methods split on semicolons between independent clauses — automated splitters may differ.
• Hyphenated compounds usually count as one word.
• Proper nouns with three or more syllables typically count in manual SMOG work.
• Numbers are pronounced to decide syllable count when rules require it.

Why it matters: Legal and medical drafts full of long names can inflate P even when the grammar is plain.

Edge case: Acronyms and chemical names syllabify differently across engines — compare scores on the same tool when auditing drafts.

What Is a Good SMOG Score?

For patient-facing health copy, many U.S. guidelines point to SMOG 6 or below. The National Institutes of Health often cites 6–8 depending on audience. For general public web prose, 7–8 is a common comfort band. Above 12, full comprehension drops sharply for average readers.

Why it matters: SMOG’s 100% comprehension bar is stricter than formulas built for partial understanding — “good” on SMOG is lower than “good” on Flesch for the same audience.

Example: A benefits FAQ at SMOG 6.5 is easier than the same policy at SMOG 11, even if both pass a casual read-aloud test.

Common mistake: Forcing SMOG 6 on specialist journals where the audience expects college-level density.

Targets by content type

When to Use SMOG Readability

Choose SMOG when polysyllabic vocabulary and full comprehension matter most — especially healthcare, public health, insurance, and safety-critical instructions.

Why it matters: Research in healthcare communications often prefers SMOG over Flesch–Kincaid when measuring whether patients can understand materials completely.

Example: A hospital revises a discharge sheet from SMOG 11 to SMOG 7 before publishing to all patients.

Common mistake: Using SMOG on a very short passage. Below 30 sentences, prefer Gunning Fog or Flesch–Kincaid for a stable band.

Strong use cases

• Patient education and consent forms.
• Medication guides and pharmacy leaflets.
• Public-health alerts and vaccine instructions.
• Insurance benefits and coverage summaries.
• Plain-language audits of government health pages.
• Spot-checking jargon density vs Flesch on the same paste.
• Long passages where the 30-sentence sample protocol applies.

How to Improve Your SMOG Score

To lower SMOG (easier reading), replace three-syllable-and-up words first. Splitting sentences helps less than on Fog or Flesch because sentence length is not a direct SMOG input.

Why it matters: P drives the square-root term — halving polysyllabic count can drop SMOG by roughly 1.4 grade points on medium samples.

Example: “Utilize” → “use,” “medication” → “medicine” where accurate, “approximately” → “about” can remove several points from P in one paragraph.

Common mistake: Shortening sentences while leaving Latinate jargon untouched — SMOG may barely move.

Practical editing tips

• Define a technical term once, then use a short form.
• Score running prose only — not navigation labels, tables of codes, or SKU lists.
• Audit P with the syllable counter on disputed words.
• Re-check after edits; removing one recurring polysyllabic term cuts P multiple times.
• For long documents, sample start, middle, and end before declaring the file “done.”

SMOG vs Other Readability Formulas

SMOG complements syllable-average and vocabulary-list formulas. Use it when polysyllabic count at the 100% comprehension bar is the policy requirement.

Why it matters: A passage with few polysyllabic words but long sentences may SMOG easier than it Flesch–Kincaids.

Example: Ten monosyllabic words per sentence can yield moderate Flesch syllable averages while SMOG stays low if P is tiny.

Common mistake: Publishing only the lowest grade from whichever tool was opened last.

Quick comparison

Limitations of SMOG

SMOG measures polysyllabic density normalized to sentence count. It does not know reader expertise, topic interest, or whether a long word is everyday speech (“beautiful,” “interesting”).

Why it matters: Pair SMOG with audience testing on high-stakes documents — the index is a guide, not a veto on good style.

Example: Sports fans know “offside”; SMOG still counts it if the syllable engine tags three syllables.

Common mistake: Scoring code, JSON, or SKU lists. SMOG targets running prose.

Scorer and sample limitations

McLaughlin normed SMOG on 30-sentence English samples. Tables for fewer than 30 sentences are statistically weak. Automated tools often score the full pasted passage with the precise formula instead of three spaced 10-sentence blocks.

On this site: TextTools applies 1.0430 × √(P × 30 ÷ S) + 3.1291 to the entire paste, counting any token with three or more syllables as polysyllabic. It does not perform manual 10+10+10 sampling. Scores on texts with fewer than 30 sentences are indicative, not audit-grade.

Why it matters: Two SMOG calculators can disagree when one samples and another scores the full document, or when syllable rules differ.

Wikipedia notes that applying SMOG to languages other than English lacks statistical validity — treat cross-language numbers as non-comparable.

Token and sentence edge cases

“Dr.” and “e.g.” affect sentence counts. Bullet lists and headings may inflate S without adding readable sentences.

Example: A 150-word consent form with twelve “whereas” clauses may show unstable SMOG on a short paste even after plain-language rewrites elsewhere.

Worked Example — Polysyllabic Count to SMOG Grade

Suppose a passage has 30 sentences and 35 polysyllabic words (three or more syllables each).

Why it matters: Walking the numbers shows how P and S combine under the square root before the constants apply.

Step-by-step (precise formula)

P = 35

S = 30

Inner = P × 30 ÷ S = 35 × 30 ÷ 30 = 35

SMOG = 1.0430 × √35 + 3.1291 ≈ 1.0430 × 5.916 + 3.1291 ≈ 9.3 → ninth-grade band.

Common mistake: Forgetting the 30 ÷ S normalization when S is not 30 — the inner term scales polysyllabic density to McLaughlin’s sample size.

Vocabulary edit sketch

Replacing five polysyllabic words (P: 35 → 30) with shorter synonyms on the same 30 sentences drops the inner term to 30 and SMOG to about 8.8 — roughly half a grade band from vocabulary alone.

When S is not 30 — normalization

Suppose P = 12 and S = 15 (a shorter paste).

Inner = 12 × 30 ÷ 15 = 24

SMOG = 1.0430 × √24 + 3.1291 ≈ 8.2 → eighth-grade band.

Why it matters: The 30 ÷ S factor scales polysyllabic density to McLaughlin’s sample size so short passages still produce a grade — but scores swing more when S is low.

Common mistake: Treating a short paste as audit-grade SMOG — use it as a directional check, not a compliance sign-off.

Frequently Asked Questions