Flesch–Kincaid Grade Level: the formula and what a grade means

The Flesch–Kincaid Grade Level reports difficulty as a US school grade. A score of 8.0 means the text should suit a typical eighth-grade reader. That framing is why it became so popular: “grade 8” is easier to explain to a manager than “Reading Ease 64.” It is also the source of most misunderstandings about it.

Where it came from

In 1975, J. Peter Kincaid and colleagues published a set of recalculated readability formulas for the US Navy. The Navy wanted to check whether training manuals suited the people reading them. The team tested enlisted personnel on their reading ability and on their understanding of passages from Navy training material. Then they refitted several existing formulas, including Rudolf Flesch’s 1948 Reading Ease formula, so that each one produced a grade level instead of its original scale.

The grade version of Flesch’s formula is the one that stuck. Microsoft Word and many other writing tools report it today.

The formula

Grade = 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59

It uses the same two counts as Flesch Reading Ease:

• Average sentence length. Each extra word per sentence adds 0.39 of a grade.
• Average syllables per word. Each extra tenth of a syllable per word adds 1.18 grades.

The −15.59 shifts the result onto the grade scale. Because of that constant, a text made only of very short words in very short sentences can score below zero.

A worked example

Here is a warning label written the stiff way, then rewritten.

Before: Patients should discontinue medication immediately if dizziness occurs.

8 words · 1 sentence · grade 20.0

The grade shown with each example comes from our automatic counter, so it can differ a little from this hand count. Counting syllables explains why.

Counts: 8 words, 1 sentence, and 22 syllables (pa-tients 2, should 1, dis-con-tin-ue 4, med-i-ca-tion 4, im-me-di-ate-ly 5, if 1, diz-zi-ness 3, oc-curs 2).

• 0.39 × (8 ÷ 1) = 3.12
• 11.8 × (22 ÷ 8) = 11.8 × 2.75 = 32.45
• 3.12 + 32.45 − 15.59 = 19.98, roughly grade 20

After: Stop taking the pills right away if you feel dizzy.

10 words · 1 sentence · grade 3.7

Counts: 10 words, 1 sentence, and 13 syllables (only “taking,” “away,” and “dizzy” have two).

• 0.39 × 10 = 3.9
• 11.8 × (13 ÷ 10) = 15.34
• 3.9 + 15.34 − 15.59 = 3.65

The rewrite is two words longer, yet it drops about 16 grades. Almost all of that came from shorter words. It is also clearer: it talks to the reader (“you”) and names the action (“stop taking”).

Single sentences give extreme scores like these. Longer samples average out and give steadier, more useful numbers.

What a “grade” means, and what it doesn’t

A grade score is an estimate of the schooling a reader needs to understand the text, based on how readers in the original studies performed. It is not:

• A measure of quality. Hemingway and a tax form can share a grade.
• The reader’s age or intelligence. An adult who reads at grade 8 is not “like an eighth grader.” Plenty of educated adults prefer grade 8 text, especially when they are busy, stressed, or reading on a phone.
• Precise. The difference between 8.2 and 8.6 is noise. Different syllable counters alone can move a score by a few tenths. See counting syllables.
• Meaningful above about grade 16. Scores past the end of a four-year degree simply mean “very hard.” Grade 22 is not a real school level.

This table shows how Op-Ed Insight maps grades to school levels and typical ages:

A grade score estimates the years of US schooling needed to understand the text on first reading. It is not a measure of quality.

How it relates to Flesch Reading Ease

Both formulas use the same inputs, and they usually move in opposite directions: a higher Reading Ease goes with a lower grade. But they are not a simple conversion. The grade formula gives sentence length more weight relative to word length.

You can see this with two invented texts:

Reading Ease says A is easier. Flesch–Kincaid says B is easier. Neither is wrong; they weigh the two causes of difficulty differently. That is one reason to pick one score and stick with it when you compare drafts.

How to lower a grade

The levers are the same as for Reading Ease, with slightly more payoff from sentence length:

1. Split long sentences. Breaking one 40-word sentence into two of 20 takes the average down sharply. The sentence length checker flags sentences of 25 words or more.
2. Use the plain word when it means the same thing. “Use,” not “utilize.” “About,” not “approximately.” Our plain words list has many more.
3. Turn hidden verbs back into verbs. “Reach a decision” becomes “decide.” See nominalizations.
4. Address the reader directly. “You” and plain imperatives tend to be short and concrete.

Before: Completion of the enrollment form is required for all new employees prior to the commencement of their first pay period.

20 words · 1 sentence · grade 11.1

After: New employees must fill in the enrollment form before their first payday.

12 words · 1 sentence · grade 6.8

Stop when the text is clear for your reader. Driving the grade lower by cutting needed terms or chopping sentences mid-thought makes the text worse. For targets by audience, see what readability score to aim for.