Smog Index Calculator
Calculate the SMOG readability index based on polysyllabic word count. Enter values for instant results with step-by-step formulas.
Formula
SMOG Grade = 3 + sqrt(Polysyllabic Words x (30 / Total Sentences))
The SMOG formula counts words with 3+ syllables (polysyllabic words) and normalizes to a 30-sentence sample. The square root transformation and addition of 3 converts the count into an estimated US grade level required for 100% comprehension.
Worked Examples
Example 1: Health Brochure Assessment
Problem: A health brochure has 30 sentences with 20 polysyllabic words across 450 total words. What is the SMOG grade level?
Solution: SMOG Index = 3 + sqrt(polysyllabic words x (30 / sentences))\nSMOG = 3 + sqrt(20 x (30 / 30))\nSMOG = 3 + sqrt(20)\nSMOG = 3 + 4.47\nSMOG = 7.47\nPolysyllable density: 20/450 = 4.4%\nReading level: approximately 7th-8th grade
Result: SMOG Grade: 7.5 | Middle School level | Appropriate for general public
Example 2: Academic Paper Analysis
Problem: A research paper excerpt has 25 sentences with 60 polysyllabic words across 500 total words.
Solution: SMOG = 3 + sqrt(60 x (30 / 25))\nSMOG = 3 + sqrt(60 x 1.2)\nSMOG = 3 + sqrt(72)\nSMOG = 3 + 8.49\nSMOG = 11.49\nPolysyllable density: 60/500 = 12%\nReading level: approximately college level
Result: SMOG Grade: 11.5 | College level | Requires educated adult readership
Frequently Asked Questions
What is the SMOG Index and how is it calculated?
The SMOG (Simple Measure of Gobbledygook) Index is a readability formula developed by G. Harry McLaughlin in 1969. It estimates the years of education a person needs to understand a piece of writing. The formula counts polysyllabic words (words with three or more syllables) in a sample of 30 sentences, then applies the calculation: SMOG grade = 3 + square root of the polysyllabic word count. For samples that are not exactly 30 sentences, a conversion factor adjusts the count by multiplying the polysyllabic words by 30 divided by the number of sentences. The SMOG Index is considered one of the most accurate readability formulas, particularly for healthcare materials, because it correlates strongly with reader comprehension at 100 percent understanding.
How does SMOG differ from other readability formulas?
The SMOG Index differs from other readability formulas in several important ways. While the Flesch-Kincaid Grade Level uses both sentence length and syllable count per word, SMOG focuses exclusively on polysyllabic words, which research shows are the strongest predictor of text difficulty. The Gunning Fog Index is similar to SMOG but uses a different mathematical approach and tends to produce slightly lower grade level estimates. The Flesch Reading Ease Score produces a 0 to 100 scale rather than a grade level. SMOG is unique in that it predicts 100 percent comprehension rather than the 50 to 75 percent threshold other formulas target. This makes SMOG especially valuable for critical documents like medical consent forms and safety instructions.
What SMOG score should I aim for in my writing?
The target SMOG score depends entirely on your audience. For general public communications, aim for a SMOG score between 6 and 8, which corresponds to a 6th to 8th grade reading level. This is important because the average American adult reads at roughly an 8th grade level. For healthcare materials, the American Medical Association recommends writing at or below a 6th grade level (SMOG score of 6 or less), as patients under stress have reduced comprehension. Government plain language guidelines suggest targeting 6th to 8th grade levels. For business communications and marketing copy, a SMOG score of 8 to 10 is generally appropriate. Academic and professional writing typically falls between 12 and 18.
How can I lower my SMOG score to improve readability?
Reducing your SMOG score primarily means reducing polysyllabic words. Replace long words with shorter synonyms: use 'help' instead of 'facilitate,' 'end' instead of 'terminate,' 'use' instead of 'utilize,' and 'start' instead of 'commence.' Break complex sentences into shorter ones, which does not directly affect SMOG but improves overall comprehension. Avoid jargon and technical terms when simpler alternatives exist. Use active voice rather than passive voice, as passive constructions often require additional polysyllabic words. Define technical terms when they must be used. Consider using bulleted lists to break up dense paragraphs. Read your text aloud to identify unnecessarily complex phrasing, and test revised versions to track your improvement.
What is the Gunning Fog Index for text complexity?
The Gunning Fog Index estimates years of formal education needed to understand text on first reading. Formula: 0.4 × [(words/sentences) + 100 × (complex words/words)], where complex words have 3+ syllables. A score of 12 targets high school graduates; 17 targets college graduates. Most accessible writing scores between 7 and 12.
Does Smog Index Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.