SMOG Index Calculator Calculate the SMOG readability index for healthcare and public documents.

What Is SMOG Index?

The SMOG Index (Simple Measure of Gobbledygook, 1969) counts polysyllabic words (3+ syllables) and uses a square root calculation. Formula: SMOG = 3 + sqrt(polysyllabic_count * 30 / sentences). SMOG is the gold standard for healthcare literacy, validated against actual reader comprehension by the AMA, NIH, and HHS. It tends to produce slightly higher grades than Flesch-Kincaid because it validates on 100% comprehension. For public health documents, target grade 6-8.

Why Use SMOG Index?

• Gold standard for healthcare readability
• Validated against actual comprehension testing
• Shows which polysyllabic words to simplify
• Recommended by AMA, NIH, and HHS
• Essential for patient education materials

Common Use Cases

Patient Education

Ensure medication guides and consent forms meet grade 6-8 health literacy standards.

Public Health

Verify health messages reach diverse audiences.

Insurance Documents

Meet plain language requirements for benefits summaries.

Government Communications

Comply with the Plain Writing Act of 2010.

Technical Guide

SMOG = 3 + sqrt(polysyllabic_count * 30 / sentence_count). Polysyllabic words have 3+ syllables. The square root reflects that comprehension difficulty increases with the square root of complex word density. McLaughlin validated SMOG against actual comprehension tests, one of few formulas with direct validation. The tool lists polysyllabic words for targeted simplification.

Tips & Best Practices

• 1
  Aim for SMOG 6-8 for healthcare materials
• 2
  Replace polysyllabic medical terms with plain language
• 3
  Define necessary technical terms using short words
• 4
  SMOG scores 1-2 grades higher than Flesch-Kincaid by design
• 5
  Test with 30+ sentences for maximum accuracy

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Frequently Asked Questions

Q Why is SMOG preferred in healthcare?

Validated against actual reader comprehension, not estimated difficulty.

Q Why higher scores than Flesch-Kincaid?

SMOG validates on 100% comprehension vs 50-75% for FK.

Q How many sentences needed?

The original formula uses 30 sentences. The tool normalizes for other lengths.

Q What does SMOG stand for?

Simple Measure of Gobbledygook.

Q Can I use it for non-healthcare content?

Yes, its comprehension-validated methodology works for any audience-facing content.

About This Tool

SMOG Index is a free online tool by FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration or installation required.