Why a grade-level score isn't the same as a reading level
A readability grade is a prediction based on the readers in the original study, not on your readers. Here is what the number means and how to use it.
Every readability formula returns a grade level. Most people read that number as "the school grade of the person who can read this." That is close, but it hides three things worth knowing.
A grade is a prediction
Harry McLaughlin, who created the SMOG formula, put it plainly in his 1969 paper: "A readability formula is simply a mathematical equation derived by regression analysis."
A researcher takes sample passages, measures how well test readers understand them, and fits an equation that predicts the result from things that are easy to count. The things counted are sentence length and word length. Nothing else goes in.
The grade depends on who was tested
Each formula was fitted to a different group and a different standard.
Flesch-Kincaid comes from a 1975 US Navy study. The researchers tested 531 Navy enlisted personnel on 18 passages from Navy training manuals. A passage was assigned a grade when half the readers at that level scored 35 percent or better on a fill-in-the-blank test. The report notes that this is comparable to about 75 percent on a multiple-choice test.
SMOG was fitted to 390 passages from a school reading test. McLaughlin used a stricter standard: "the grade of subjects showing complete comprehension."
So the two grades are not the same kind of number. SMOG asks what grade a reader needs to understand everything. Flesch-Kincaid asks for less. On the same text, SMOG will usually be higher, and neither formula is wrong.
The readers were not your readers
The grade reflects the people in the original study. It knows nothing about the people you are writing for.
A formula cannot tell whether your reader knows the subject. A nurse reads "hypertension" without effort. A new patient may not. The score is the same for both.
National surveys of adult literacy do not report a reading grade for adults at all. They report skill levels. The OECD's Survey of Adult Skills found that about one in six US adults, roughly 36 million people, have weak literacy skills. A school-grade number does not capture that picture.
The number has a wide margin
McLaughlin reported that SMOG predicts a passage's grade "within one and a half grades in 68 percent of cases." A 2013 review found that formulas varied by up to five grade levels on the same text.
A draft at grade 8.4 and a draft at grade 8.0 are the same result.
How to use a grade well
- Compare drafts. The score is most useful for showing that version two is easier than version one. Use the same formula both times. The comparison tool does this side by side.
- Name the formula. "Grade 8 on Flesch-Kincaid" is a claim someone can check. "Grade 8" alone is not.
- Set the target from your audience. See what counts as a good readability score for common targets.
- Look at more than one formula. If they agree, you can be more confident. If one is far off, find out what it counts. Our formula guide explains each one.
- Test with real readers when the text matters. Ask a few people from your audience to read it and tell you what it says.
A note on other languages
These formulas were built on English. A grade from an English formula does not carry over to Spanish or German text. Other languages have their own formulas, such as Fernández Huerta's for Spanish.
How we checked
The calibration details come from the 1969 SMOG paper and the 1975 Navy report, both linked above. An earlier version of this post gave population figures we could not source. They have been removed.
Frequently asked questions
What does a grade-level score actually mean?
It is a prediction from a formula. The formula was fitted to how a group of test readers did on sample passages decades ago. It tells you how long your sentences and words are, expressed as a US school grade.
Why do different formulas give different grades for the same text?
They were calibrated against different standards. SMOG is set to full comprehension. Flesch-Kincaid used a lower bar. So SMOG usually reads a grade or more higher on the same text, by design.
Does a grade 8 score mean an 8th grader can read my text?
Roughly, and only as an estimate. Formulas can be off by a grade or more, and they ignore whether the reader knows the subject. Test important text with real readers.