Flesch-Kincaid Readability Score: What It Measures

The Flesch-Kincaid grade level is 0.39 × words per sentence + 11.8 × syllables per word - 15.59, so it only sees sentence length and word length. A grade of around 8 is a common target for general readers, and no score can tell you whether a page is clear or accurate, or who wrote it.

Short answer: the grade is built from two ratios, and you can change it a lot by changing only one of them. Sentence length is the easier one to move, and it is the half of the formula this site counts.

What the Flesch-Kincaid grade level actually calculates

The formula contains two ratios. The first is words divided by sentences, which is average sentence length. The second is syllables divided by words, a rough stand-in for how long the words are. Multiply the first by 0.39 and the second by 11.8, add them, then subtract 15.59.

The result corresponds to a US school grade. J. Peter Kincaid and his team developed it under contract to the US Navy in 1975. Above about 10, the number can also be read as the years of education someone would generally need to follow the text.

The formula has no view of your argument, your tone or your jargon, unless the jargon happens to have a lot of syllables. All it measures is how long your sentences are and how many syllables your words have.

Grade level versus Reading Ease: the two formulas

Flesch Reading Ease is the older sibling, devised by Rudolf Flesch. It uses the same two inputs:

Reading Ease = 206.835 - 1.015 × (words ÷ sentences) - 84.6 × (syllables ÷ words)

The two scales run in opposite directions. On Reading Ease, a higher score means easier text. On the grade level, a higher number means harder text. When a calculator gives you both, they come from the same measurements and are not contradicting each other. The commonly cited bands map Reading Ease onto US grades:

90 to 100: 5th grade, very easy to read
70 to 80: 7th grade, fairly easy to read
60 to 70: 8th and 9th grade, plain English
50 to 60: 10th to 12th grade, fairly difficult
30 to 50: college, difficult

What is a good score? Why around 8 is a convention

Readable suggests that text for the general public should aim for a grade of around 8, which it puts at schooling age 13 to 14. That is also roughly where the 60 to 70 "plain English" band of Reading Ease sits.

Treat it as a starting point. Nobody tested your audience and found that 8 is the right number for them. A specialist looking for a technical answer may be happy with denser text, and someone reading in a hurry on a phone may want less. If your readers follow you comfortably at grade 12, the formula has no way to know that.

Worked example: how much one longer sentence moves the grade

This part is plain arithmetic. Hold syllables per word fixed, and sentence length is the only thing that can move the grade. Each extra word per sentence adds 0.39:

1 extra word per sentence: +0.39
2 extra words: +0.78
10 extra words: +3.9, nearly four school grades

If a page averages 14 words per sentence and a rewrite takes it to 15, the grade rises by 0.39. Nobody would notice that by eye. Make every sentence ten words longer, though, and the grade jumps 3.9 without a single change of word choice.

This is why so much "improve your score" advice comes down to splitting sentences. It does lower the number. Whether the writing gets better is a separate question.

What sentence length looks like across sectors

The SGR-010 study measured words per sentence on UK brands' own blog posts and press releases. These figures are brand-weighted: each brand's pages are averaged first, then the brands are averaged. That way one prolific brand cannot set its sector's figure.

| Sector | Words per sentence | Pages | Brands | |---|---|---|---| | Energy | 14.8 | 1,949 | 11 | | Supermarkets | 14.7 | 620 | 10 | | Banks | 14.2 | 2,669 | 18 | | Insurance | 13.6 | 2,049 | 10 | | Software | 13.3 | 3,422 | 16 | | Food and drink | 13.2 | 751 | 8 | | Telecoms | 13.0 | 1,816 | 15 | | Charities | 12.8 | 1,994 | 10 | | Travel | 11.6 | 459 | 8 | | Fashion | 10.0 | 221 | 10 |

Source: SGR-010, blog posts and press releases together.

The gap between energy (14.8 words, 1,949 pages) and fashion (10.0 words, 221 pages) is 4.8 words per sentence. If syllables per word were the same in both sectors, the formula would put that gap at 4.8 × 0.39, about 1.9 grades. That is an illustration of the arithmetic. It is not a grade for either sector, for two reasons. The formula is designed for one text's total words and sentences, while these are averages of averages. And we do not know the syllable half. Each sector also rests on 8 to 18 brands, so read the figures as a description of those brands' pages, not a ranking of writing quality.

The banks and supermarkets sector pages show the same measure brand by brand, with the page count beside each.

What we count, and what we do not

brandsovertime counts two things that map onto the formula's halves:

Words per sentence. This is the sentence-length half, the same ratio Flesch-Kincaid uses.
Long-word share (7+ letters). This is the share of words with seven or more letters. It stands in for word length because code can count letters exactly, whereas syllable counts are estimates. In SGR-010, the median brand's long-word share is 24% on blog posts and 28% on press releases.

We do not count syllables, and we do not calculate a grade level or a Reading Ease score. So you will not find a Flesch-Kincaid score for any brand here. That is a deliberate choice: long-word share is a different measure, and turning it into a grade would mean inventing a number the data does not hold.

Every figure is a code count from ScriptGrain's instruments, with no AI judging. The pages come from each brand's own blog and press pages. You can browse every brand at /brands/.

What a readability score cannot tell you

It cannot tell you a page is clear. Short sentences full of vague words score well and say very little.

It cannot tell you a page is accurate. A wrong claim in short, plain words can still get a low, "easy" grade.

It cannot tell you who wrote a page, or whether a tool was used. The inputs are counts of words, sentences and syllables, and none of them carries authorship. Two pages with identical numbers could have been written in completely different ways.

UK note: grade levels versus school years

The Flesch-Kincaid grade is an American scale, tied to US school grades. We have not found a reliable source for converting it into UK school years, so we do not give a conversion. For a UK audience, treat the grade as a rough difficulty marker on an American scale.

Questions people ask

What is a good Flesch-Kincaid grade score?

Readable suggests around grade 8 for text aimed at the general public, which it puts at schooling age 13 to 14. It is a common target and nothing more. Your own readers may cope with more or need less, and the formula cannot tell which.

What is the formula for the Flesch-Kincaid grade level?

Grade level = 0.39 × (total words ÷ total sentences) + 11.8 × (total syllables ÷ total words) - 15.59. The result corresponds to a US school grade. Kincaid and his team developed it for the US Navy in 1975.

What is the UK equivalent of a Flesch-Kincaid grade level?

We have not found a reliable source for converting the grade into UK school years, so we do not give one. The scale is American. Use it as a rough marker of difficulty.

Does a lower Flesch-Kincaid score mean better writing?

No. A lower grade only means shorter sentences, words with fewer syllables, or both. It tells you nothing about whether the writing is clear, accurate or useful.

Can a readability score tell you whether a page was written by AI?

No. A readability score is built from counts of words, sentences and syllables. Those counts cannot show who wrote a page or whether a tool was used. Two pages with identical numbers could have been written in completely different ways.

Flesch-Kincaid readability score: what it measures, and what sentence length contributes