Readability Index – what is it?

Whether blogger, marketers, or online editors, anyone who writes online texts must make sure they contain certain elements. Only then can they achieve a good ranking in the search engine results. In on page SEO, for example, keywords, headlines, links, and graphics are classic ranking factors. What SEO specialists should not forget, however, and copywriters need to take into account, is that good readability is also important. Even if this is not (yet) officially part of Google’s ranking factors, SEO analysts have found out: texts with good readability generally rank better. Therefore, it is reasonable to assume that Google’s algorithm also registers a text’s readability. And last but not least, good readability is especially important for website visitors and their user experience.

But what exactly does the term 'readability' mean? Readability refers not only to typography and font size, but also to a text’s comprehensibility.

Different mathematical methods have been developed to formally determine the readability of a text. Two criteria that can be determined quickly using appropriate analysis tools are, for example, the length of the sentence and the word length. These and other factors are related to each other by mathematical formulas and the so-called readability index is calculated from them. This readability index does not replace human judgment, but it does provide an indication as to whether the text meets the formal criteria for good readability.

The readability of texts is currently being discussed, mainly in the area of SEO. This aspect, however, has been important for a long time – as long as texts have been around – since people want them to be as easy to understand as possible. The most popular readability index, the Flesch Reading Ease Index (in short: FRE Index), is older than the internet. It was developed by the author Rudolf Flesch, who researched the topic of usability in the 1940s and 1950s. Another model for evaluating text comprehensibility is the Hamburg Concept of Comprehensibility. A Hamburg research group developed this in the 1960s and 1970s to simplify officialese. The purpose of this simplification was to reproduce information in such a way that all population groups could understand it equally.

There are many other models in addition to these two, meaning that the readability index is not the same in every case. The individual measuring methods create different criteria, are developed for different languages, and sometimes have very special target groups or text forms in their sights. When it comes to journalistic texts, however, the objectivity criteria must be weighted higher than it would be for a marketing text.

Three procedures for assessing readability

In this section, we present three readability indices: the Flesch Reading Ease Index, the Flesch Kincaid Level, and the Fog Index. All three have proven themselves in many areas. We present their formulas and criteria and give some sample analyses.

Flesch Reading Ease Index – the classic readability index

The Flesch Reading Ease Index, shortened to Flesch-Index, is a classic among the readability formulas. It takes two key figures into account when calculating readability:

  1. The average sentence length (ASL)  
  2. The average number of syllables per word (ASW)

This test was originally developed for the English language and the formulas differ slightly for different languages.


The formula is:

readability ease = 206.835 – (1.015 x ASL) – (84.6 x ASW)

The higher the readability index, the easier it is to understand the language of the text. The values are graded in a table according to the degree of comprehensibility. This way, the table shows which groups can understand the text. For the higher levels, the readability index refers to age groups: a text with a value of 90-100 is considered very easy and understandable for 11-year-old students. The lower values, on the other hand, are assigned to certain educational levels: texts with values between 0 and 30 can only be understood by academics. But even if you address this target group, easy readability is still desirable.

Flesch reading ease value Readability Understandable for
0–30 Very difficult Academics
30–50 Difficult  
50–60 Fairly difficult  
60–70 Standard 13 to 15-year-old pupils
70–80 Fairly easy  
80–90 Easy  
90–100 Very easy 11-year-old pupils

The Flesch score was calculated for all seven Harry Potter books and the average score came out as 72.83. The book with the highest score (i.e. easiest to read) was the first book, Harry Potter and the Philosopher’s Stone with 81.32.

With most marketing texts, a high value is advantageous, because marketing messages should ideally stay in the reader’s head. A comprehensible text usually has a value of between 60 and 70, however, the bar for online texts is set somewhat higher than for print media: a readability index of 60 is already considered too low for web texts. The Yoast SEO plugin for WordPress recommends formulating shorter sentences to achieve a readability value of 60.

Flesch Kincaid Grade Level

The Flesch Kincaid Grade Level (abbr: FKGL) is another readability index. The name already indicated its relationship with the Flesch Reading Ease Index: both were developed in the 1970s for the US Navy and were based on Rudolf Flesch’s research. They are also referred to as Flesch Kincaid Readability tests. They are used by Microsoft Office Packet for readability testing. In the 1990s, the US Department of Defense used the Flesch Kincaid index as a standard test for readability.

Both tests evaluate the same core criteria (ASL and ASW), but they differ when it comes to weighting: the sentence length is weighted more strongly with FKGL than with FRE: In addition, the Flesch Kincaid Index differs more between age groups, especially according to American grades. A value of 8.0, for example, means that the text in question is easy for eighth graders (Year 9) to understand, while a value of 7.0 means that even seventh graders (Year 8) can understand the text.


The formula of the Flesch Kincaid Grade Level is:

readability index FKGL = 0.39 x ASL + 11.8 x ASW – 15.59

This test was originally developed for the English language. If you calculate the readability index using the Flesch Kincaid formula for the first Harry Potter book in the series: Harry Potter and the Philosopher’s Stone, you will get a score of 5.75, meaning this formula calculates it a little lower than the Flesch Reading Ease Index at 81.32 (mentioned above) due to the different weighting factors. Here are some examples of average scores of famous people:

Stephen King – 6.1

J.R.R. Tolkien – 6.5

John Grisham – 7.8

Hilary Clinton – 8.1

Mitt Romney – 9.5

Susan Cain – 11.1

Gunning Fog Index

This test was also developed for the English language – the classification is based on American High School grades. It was developed in 1952 by businessman Robert Gunning, and is particularly popular in the management sector. In this respect, it isn’t surprising that it is frequently used to evaluate annual reports.

The Gunning Fox Index evaluates different key figures to the previously explained analysis metrics:

  1. Number of words in a text (W)
  2. Number of sentences in a text (S)
  3. Number of 'complex words' with three or more syllables (D)

The readability index is calculated from these figures.


The formula of the Gunning Fox Index is:

readability index GFI: 0.4 x [(W/S) + 100 x (D/W)]

The values are graded in a table, ordered according to the education level needed to understand the text.

Gunning Fog Index Comprehensibility according to educational level
17 College graduate
16 College senior (4th year uni/college)
15 College junior (3rd year uni/college)
14 College sophomore (2nd year uni/college)
13 College freshman (1st year uni/college)
12 High school senior (6th Form)
11 High school junior (6th Form)
10 High school sophomore (Year 11)
9 High school freshman (Year 10)
8 Eighth grade (Year 9)
7 Seventh grade (Year 8)
6 Sixth grade (Year 7)

This text also sets out some guidelines that must be taken into account when accessing the text. You should test a text passage with only 100 words. In addition, some words will be excluded to determine the value D, e.g. proper nouns or word compositions and verbs that only have more than three syllables due to an ending being added.

The readability index on page SEO

Most texts on the internet are intended to provide quick and clear information. Readability is therefore essential. Only texts with a high degree of legibility are easy to understand.

Experience from SEO experts shows that texts with a readability of between 30 and 60 (FRE) are better. For this reason, it is assumed that readability in some way also influences the search engine ranking. It is therefore advisable to use a readability index as part of search engine optimisation and to check the text content accordingly.

Tests and tools to verify and improve readability

Here’s a list of some online tests with which you can check your own content for legibility:

  1. An online test that uses the Flesch Index can be found on the site,
  2. The Flesch Kincaid Grade Level is used by the text verification tool, PerryMarshall.
  3. The Gunning Fog Index has its own website where you can test your text for free with the readability index.

The above tests are prone to errors since they do not always succeed in determining the correct number of syllables. However, these free online tools allow at least a rough initial assessment of the readability of your own texts.

If the tests yield very high values, you should try to delete unnecessary technical terms, foreign words, and abbreviations from the text.


For better accessibility you should write texts in plain language. There are corresponding tools available to check texts for the characteristics of plain language. Simplish can be used to simplify your texts to make them more understandable.

Tips for SEO: criteria of the Hamburg Comprehensibility model

If you want to ensure good legibility of your texts when writing them, you can use the Hamburg Comprehensibility model as a guide. This model uses four quality criteria to evaluate texts: simplicity, structure, concision, and stimulation.

  1. Simplicity
    • Favour shorter sentences (orientation value: approx. 9 to 13 words)
    • Favour shorter words
    • Include the most important information at the start of the sentence
    • Express only one thought per sentence
  2. Structure
    • Favour shorter paragraphs (orientation value: up to 90 words)
    • Appropriate length of chapters under the subheadings (orientation value: up to 300 words)
    • Include the most important information at the beginning of the text
  3. Concision
    • Use many verbs
    • Formulate actively
    • Use figurative language
    • Work with transitional words (orientation value: approx. 30 % of the sentences should contain transitional words)
  4. Stimulation
    • Include images and graphics to supplement the text
    • In online marketing, stimulus and signal words such as 'great', 'easy', or 'cheap' can have a stimulating effect. For other kinds of texts, you should use these words carefully. 

When considering the criteria of the Hamburg Comprehensibility model, there are some things you should make sure to avoid when writing:

  • Long sentences
  • Technical terms and abbreviations
  • Passive constructions
  • Fillers
  • Modal verbs (could, might, would, should)
  • Nominal style

If you take these guidelines into account, your texts will be easier for readers to understand in the future and your website visitors will be more satisfied.