What Are The Flesch–Kincaid Readability Tests?

If you’ve looked into improving your writing in this, our modern age, you’ve probably come across a few references to the Flesch–Kincaid readability tests. You may have heard that these tests can give you an objective score of how easy it is to understand your writing. Maybe you even think they can help you make your writing more enjoyable.

Well, you’re not wrong, but things are a little more complicated than they may first appear. Luckily, you’ve got me to straighten things out.

What are the Flesch–Kincaid readability tests?

First of all, when people talk about the ‘Flesch–Kincaid test,’ they’re actually talking about a couple of different things.

The Flesch Reading Ease test and the Flesch–Kincaid (F–K) Grade Level test use the lengths of words and sentences in a piece of writing to determine its relative ease of reading. The former is helpful in informing manual writing and the latter in choosing appropriate age-level reading material.

As you can already see, they’re not the perfect tools many authors are hoping for, but can they still be used to inform your writing project? Let’s look at the extent of their usefulness outside of the US Navy and Mrs. Cardinal’s English lessons.

How the tests work

The Flesch Reading Ease precedes the Grade Level test, which was designed to translate the Reading Ease scores into everyday terms. If I tell you a book has a Flesch score of 35, you’re unlikely to find that information useful unless I also tell you that articles in the Harvard Business Review average in the 30s; even then, you can only approximate what’s going to work for you. The Grade Level test takes the raw data and expresses it in terms of (approximately) how old the average person should be in order to understand the material.

It’s important to understand that the F–K tests are quite narrow in scope. They measure the length of words and sentences used and that’s it. People could argue for days whether Hemingway or Joyce has a better style, but there’s no doubt who would score higher on the Reading Ease scale. For this reason, while I’ll go on to discuss how to apply F–K to your writing, it’s with this limitation in mind.

Ways to apply readability tests to your writing

The main advantage of readability tests is that they are objective. Non-subjective data, while limited, can provide a unique kind of feedback that can help you tweak structure and layout – perhaps shortening your paragraphs or adjusting common sentence structures.

These metrics are particularly useful for blog posts, which ought to be punchier and physically easier on the eyes. For example, in a blog post, frequent headings help orient the reader and highlight important information, making it easier for them to skim effectively if they want to, or dive deeper into the sections that are relevant for them.

This is totally different in a long piece of fiction, where there’s a different relationship between the reader’s ongoing appreciation of the story and their comfort with how it’s being told. Here, an author might not be bound by the aforementioned tests, but the results might clarify the age range of the ideal reader (which can help with everything from marketing your own work to pitching it to a publisher.)

Nonfiction works, on the other hand, may benefit considerably from these metrics; in fact, that’s why they were created. Military manuals were put through readability test to check that their contents would be readily accessible to a wide range of readers. High readability can be a valuable way to make sure your message can be understood by your intended audience. This might include more succinct paragraphs, subtitles used as micro-summaries, sentences that are clear-cut and free of jargon, and vocabulary that is less erudite.

In grad school, I encountered a lot of writing whose purpose seemed to be to impress the reader with how smart the author was, rather than to convey an important message using lucid and empathetic language. If your nonfiction message is really important then accessibility should trump style. That isn’t to say that style has no place in nonfiction, but that readability metrics might be more informative with respect to a piece whose purpose is communication.

Another interesting way to apply readability tests is to compare your score to that of a well-known example from literature. If I’ve got a score that’s comparable to that of the Twilight series, and the readers I want to attract have shelves full of Proust and Henry James, I’m going to need to revamp my vocabulary and add a lot of conjunctions and semicolons.

On the other end of the spectrum, children’s lit authors can definitely benefit from readability scales. Adults who are accustomed to sophisticated forms of communication, both in conversation and in writing, may have subconscious habits of using words and sentences that are oversized for little people. Even in sports-bar-level conversation, adults have a tendency to speak in run-on sentences, joining clauses willy-nilly through conjunctions and non-verbal connectors. We have no trouble following each other (unless the topic is boring and our minds wander), but children would. This is why kindergarten-level reading says, ‘The cat is sad. She can’t find her Mommy Cat’ instead of ‘The cat is sad because she can’t find her Mommy Cat.’ It’s also why you may want readability formulae to tell you your kindergarten book is reading at a 2^nd grade level, or that your YA fantasy book does, in fact, parallel the readability of the Harry Potter series. Grade level 8, corresponding to Harry Potter and Jurassic Park, implies that 80% of US readers will be comfortable with your work, but you might be aiming higher (Flaubert and Tolstoy) or lower (3^rd graders.)

‘Quantifying subconscious biases’ might be a good way to describe the overall usefulness of F–K and other readability measures. Even if you are writing a 200k-word novel, running it through a test might show you that 80% of your sentences are 20+ words, and 90% of your paragraphs are longer than the recommended length. While the rubric’s advice is not necessarily applicable – that is, a fiction writer can get away with much longer sentences and paragraphs – you can use the data to fashion your own advice.

For instance, you might decide that 90% of a book’s paragraphs shouldn’t be the same length, short or long. You then look for some poignant moments in the story to accent by way of a much shorter paragraph and you’ve built on the raw F–K data in a creative way. If your sentences tend to run long, you might look for ways to tighten them without losing meaning or tone. For instance, almost any time you say someone ‘started’ doing something, you can do away with the ‘started’ unless you need that person to be interrupted. ‘She stood up and starting walking across the room to the door’ can become ‘She walked across the room to the door,’ making room for the longer sentences that need to be longer.

Finally, in marketing content, you can directly apply readability metrics and expect consistently positive results. Email communications, blurbs, promotion posts, etc. should be extremely concise. Knowing this, make sure every word has value and every sentence is tight. When your messages are to the point, people are more likely to engage, and follow-through rates will rise in sync with your readability rates.

The limitations of Flesch–Kincaid

The Flesch–Kincaid grade level and reading ease scores are based primarily on words, syllables, and sentences – not good grammar, punctuation, or syntactical flow. The internet-popular sentence ‘While Bob ate an apple was in the basket’ illustrates the limited capability of the tests. The sentence and words are short and will yield a good readability score, but without a pertinent comma, the sentence is awkward and confusing.

As with any data-generating tool, it’s unusual that the results can be uniformly applied. Using a construct that was designed for manual improvement to tweak art means straightforward application is unlikely. But taking an objective lens to your subjective work, you can put a number on your habits and then decide what to do about them. We’ve said it before, and we’ll say it again: quantifying your writing habits helps you improve.

Have you ever run your writing through a readability test? I’d love to hear about your experiences in the comments below, and for more insight on this topic, check out Here’s How To Vary Your Sentence Structure, 5 Ways Your Paragraphs Are Broken (That You Can Fix), and How Long Should Your Book Be? The Complete Guide.