By G. Harry McLaughlin

My readability formula SMOG estimates the years of education needed to understand a piece of writing. SMOG is widely used, particularly for checking health messages. Because I dropped reading research to become a clinical neuropsychologist decades ago, I did not know how popular SMOG had become until I recently Googled SMOG formula and got half a million hits!

SMOG was published in 1969 BC [Before Computers] so I made calculating a text’s readability easy by offering an approximate formula — count the words of 3 or more syllables in 3 10-sentence samples, estimate the count’s square root, and add 3.

To save you even that counting and calculating, I have collaborated with JAVA programming expert Alain Trottier to provide a free SMOG Calculator which instantly assesses an entire text, not just a sample. Try the service using this link:

You can even paste a link like the one here into your own html webpage: copy it from here.

Alternatively you can put a link to this page in your browser list of favorites or into a pdf file. More information is on Alain Trottier's SMOG Calculator webpage.

SMOG Calculator uses my precise formula which yields an outstandingly high 0.985 correlation with the grades of readers who had 100% comprehension of test materials. Here is the formula generalized for more than 30 sentences:

The standard error of the estimated grade level is 1.5159 grades, comparable to that of other readability formulae. For a pdf file of the original paper click G. Harry McLaughlin (1969) SMOG grading: A new readability formula. Journal of Reading, 12 (8) 639-646.

SMOG Calculator incorporates Trottier's Syllable Counter, which uses a dictionary to look up the syllable length of words. Because of this, SMOG Calculator counts syllables more accurately than any other readability program.

You may have seen SMOG conversion tables compiled by one Harold C. McGraw. They are slightly inaccurate because they are based on the approximate formula I offered for ease of calculation. Tables for texts of fewer than 30 sentences are statistically invalid as well, because the formula was normed on 30-sentence samples.

A sketch of how SMOG came to be devised was published in a recent Plain Language at Work Newsletter.

Trottier and I are developing a Readability Checker for the British Standards Institute. The Checker is a computer program that will help anyone to write plain English. It offers far more than any readability formula, by highlighting what needs improvement in order to increase readability.

The Checker estimates the years of education that an adult needs to easily understand a submitted text, by giving it a level between 1 and 20. In addition the Checker estimates the difficulty of every word and every sentence and returns the text color-coded to mark parts that may benefit from revision. The Checker also suggests familiar substitutes for many difficult words.

The Checker will estimate a text’s difficulty for adult British readers more accurately than any extant readability formula because it will be the only one
• validated on a broad section of the adult population,
• validated on contemporary reading matter for adults,
• validated on British readers, and
• validated on British reading material.

The Checker consists of two integrated parts: the program which calculates the readability level of an input text and outputs the text colour-coded to indicate the more difficult words and sentences; and a dictionary giving the estimated difficulty of at least 150,000 orthographic words.

The Checker will be made available free online, with no advertising, hosted by Mr. Trottier’s company www.wordscount.info. It could be placed on any webpage at no charge by copying a free snippet of HTML, like that for SMOG.

Users will submit a document via the web page; the remote engine at Words Count will immediately process it and return the readability level and the colour-coded text to the user.

The Checker will be normed on the educational levels of millions of readers and linguistic data derived from a multi-million word corpus, as I proposed decades ago in Temptations of the Flesch and Proposals for British Readability Measures.

Please enjoy the rest of this site by clicking

Copyright (c) 2008 G. Harry McLaughlin. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.