SMOG Index Readability Test


You might have come across the SMOG Index before, and think that it has something to do with weather events. It’s actually a test that assesses the reading level of a variety of texts. But what does SMOG stand for?

Before answering that, the SMOG Index was formulated in 1969 by Henry McLaughlin, a psycholinguistic graduate, and psychologist. At that time, there have been many readability tests available. Still, McLaughlin wanted to develop one that is ‘laughably simple’ and calls attention to polysyllabic words (or words with more than two syllables).

MacLaughlin named this formula the Simple Measure of Gobbledygook. Some would say SMOG is a nod to another readability test called the FOG Index by Robert Gunning. Regardless, the SMOG Index was met with doubt and criticism during its inception due to its simplicity. But further study and use have shown that it is an accurate measure of readability. It is still widely used today.


In a time where computers weren’t as regular in day-to-day life, readability tests were bothersome to conduct. That’s because you manually have to cover the entire text. With the SMOG Reading Level Formula, all you have to do is get 30 sentences and base your results from there. You will be able to determine the grade level a person must have to understand the text.

When using the SMOG formula, you will have to remember a few things:

  • One sentence is a group of words ending in a period, exclamation point, or question mark.
  • Long sentences with a semi-colon are considered two sentences.
  • Hyphenated words are considered one word.
  • Polysyllabic proper nouns are counted.
  • Polysyllabic numbers (regardless of how they are written) are counted.
  • Abbreviations should be avoided unless necessary. If not, it should be read as unabbreviated and counted.

SMOG Formula

The SMOG readability formula, like MacLaughlin said, is quite simple. You just need to follow a few simple steps:

Step 1: Count 10 sentences in a row at the beginning of your text, another 10 from the middle, and the last 10 from the end to get a total of 30 sentences.

Step 2: Count all the words with three or more syllables.

Step 3: Get the square root of the total number from Step 2 and round to the nearest 10.

Step 4: Add three to the total from Step 3.

The number you get in Step 4 is your SMOG Grade. It’s fairly easy to understand the SMOG Index results as they compute for the grade level. So if you get a 4 in the SMOG Index, your text can be understood by 4th graders and above.

As a general guide, it is a good idea to aim for a reading level equivalent to 6th to 8th graders, as this is what the average American can most easily comprehend. Anything higher than that would be somewhat challenging to read.

Grade Conversion

This is only a quick overview of the conversion table of the SMOG Index. But it will help clarify your results.

Total Number of Polysyllabic Words School Level Comprehension
0-2 4th Grade Very easy to read
3-6 5th Grade Very easy to read
7-12 6th Grade Easy to read
13-20 7th Grade Fairly easy to read
21-42 8th & 9th Grade Conversational English
43-56 10th Grade Fairly difficult to read
57-72 11th Grade Fairly difficult to read
73-90 12th Grade Fairly difficult to read
91-110 College Freshman Difficult to read
111-132 College Sophomore Difficult to read
133-156 College Junior Difficult to read
157-182 College Senior Difficult to read
183-210 College Graduate Very difficult to read
211+ Professional Extremely difficult to read


The SMOG Index is widely used today due to its simplistic approach. But it is most notably preferred by the healthcare sector. Doctors and medical practitioners are notorious for using highly-specialized jargon that non-specialists cannot understand.

With the SMOG Index, medical literature can be assessed so that healthcare professionals are ensured that the typical person can comprehend their message. As such, more people can access the internet to search for consumer-related healthcare information and not be confused by it.

How to Improve Score

Improving your score in the SMOG Index is relatively simple. Lessen the big words you write and stick to the jargon that your target audience uses on the daily. Remember to write for your audience first. If you are writing for the everyday person, use a common language. If you are writing for a specialized audience, that’s the time you can use the terms that all of you understand.

If all else fails, you can easily use a modern SMOG Index calculator to assess your text and then edit according to the result.

Further Readings


SMOG Index

The SMOG Index is:

See Grade Conversion Table to interpret the score.