I want to import a huge database of short English sentences into an educational software.
I need to categorize sentences into a few levels of "difficulty" (difficulty for a non-native to understand the sentence).

  • "My name is Joe and I like ponies, dogs, cats, rabbits and mushrooms." → 0.1
  • "I want to evaluate the complexity of this sentence." → 0.5
  • "Beware those whose voice supersedes this very edict." → 0.9

... or any similar system of scoring.

A good algorithm would take into account:

  • the difficulty of each term (Coelacanth is more difficult than goldfish because it is used less often in books, notably children/teen books. Will is more difficult than willingness because it has synonyms. etc)
    These would have to be retrieved from a pre-established database (this might help).
  • the difficulty of the grammar and conjunctions (that is more difficult than and).
  • ideally the complexity of in-sentence references (sentence where several expressions refer to each other, forming a complex graph with the former at first unclear but the latter later understood... or not)
  • Sentence length. This is for memorization, so a sentence twice as long might as well be considered twice as difficult.
  • other parameters might be useful too.


  • Able to process around 1,000,000 entries within a few weeks
  • Free.


  • Web service acceptable, but bonus if runnable on-premise.
  • Bonus for Open Source.
  • Bonus if supports other languages too.

OGTE asks me for a sentence and a school level, then tells me whether it fits or not. I would like a tool that asks me for a sentence, and tells me what school level it fits.

  • 2
    Are you evaluating the complexity of the sentences based on the "difficulty" of the words used, sentence length, structure, or a combination of these factors? – Qantas 94 Heavy Apr 9 '14 at 9:20
  • 1
    @Qantas94Heavy: Added in section "A good algorithm would take into account:". – Nicolas Raoul Apr 10 '14 at 1:36

You're actually asking two questions:

  • What is a good algorithm to determine what I think is the complexity of a sentence in the english language?
  • What is a program that applies this algorithm?

The first question is very hard to answer, the second one is quite easy as soon as the first one is answered.

A tool that applies various algorithms is the following. It is for free, can evaluate complete texts and you get the name of the algorithms. I suggest you read the Wikipedia pages I've linked and try to find out what suits your needs best.


My name is Joe and I like ponies, dogs, cats, rabbits and mushrooms.

I want to evaluate the complexity of this sentence.

  • Flesch-Kincaid Reading Ease: 47.3
  • Flesch-Kincaid Grade Level: 8.9
  • Gunning-Fog Score: 12.5
  • Coleman-Liau Index: 11.7
  • SMOG Index: 8.3
  • Automated Readability Index: 5.1

Beware those whose voice supersedes this very edict.

  • Flesch-Kincaid Reading Ease: 61.2
  • Flesch-Kincaid Grade Level: 6.7
  • Gunning-Fog Score: 8.2
  • Coleman-Liau Index: 16.6
  • SMOG Index: 6
  • Automated Readability Index: 8.5
  • 1
    Flesch-Kincaid and Gunning-Fog are really too primitive. Coleman-Liau, SMOG and Automated Readability Index are designed to analyze a long text/book and lose all of their value when applied to an individual sentence. I think I really need a library that uses a pre-established database of English words difficulty. Nice find though, and it is open source: github.com/DaveChild/Text-Statistics – Nicolas Raoul Apr 10 '14 at 1:48
  • OGTE seems to be using ERF graded reader scale plus feedback from users. – VicAche Apr 27 '14 at 19:35

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