Harry McLaughlin’s SMOG Grading is based on just one variable — the number of polysyllables in 30 sentences (P30). Here is the formula: SMOG Grading = (square root of P30) + 3. This formula grades text on a scale of 3 to 17+. Suppose there are 64 polysyllables in a sample text, then SMOG Grading = (square root of 64) + 3 = 11. This means that the text is suitable for those with 11 years of schooling.
If we shrink the scale from both ends to measure texts from 5 to 12+, we can eliminate the square root from the formula with little expense to accuracy. And the new formula Special SMOG Grading (SSG) = (P30/10) + 5. If there are 64 polysyllables, then SSG = (64/10) + 5 = 11.4.
Let P30 take any of the following values: 4, 9, 16, 25, 36, 49, 64, 81. Here are the SMOG Grading scores with the corresponding SSG scores in brackets: 5 (5.4); 6 (5.9); 7 (6.6); 8 (7.5); 9 (8.6); 10 (9.9); 11 (11.4); 12 (12+). Even if P30 takes values that are not perfect squares, the SSG approximates SMOG Grading as long as the scale is 5 to 12+. You can easily test this for yourself.
The sample size of 30 sentences is quite large. But a smaller sample size may not guarantee accurate results. However, for those who just need an attractive formula, here is a simplification. If P3 is the number of polysyllables in three sentences, then SSG = P3 + 5.
SMOG Factor Grading (SFG): The SMOG Factor (SF) is the average number of polysyllables per sentence in a sample of 10 sentences. Here’s a simple derivation from SSG. Now, SSG = (P30/10) + 5 = [3 x (P10/10)] + 5 = (3 x SF) + 5 = SFG.
Fog Factor Grading (FFG): The Fog Factor (FF) is the average number of polysyllables (excluding personal names) per sentence in a sample of 10 sentences. Since the FF, as described in Jyoti Sanyal’s Indlish, doesn’t have a reliable scoring system, I suggest the following formula: FFG = (3xFF) + 5. Just remember that the new scoring system is: 5 to 12+.