While it's an imperfect tool, Microsoft Word's readability statistics can help you assess readability.
In Word, go to Preferences > Spelling and Grammar. Under the 'Grammar' section, check the 'Readability Statistics' box. The next time you run a spell check, a box with readability statistics will pop up.
Not all these statistics will be relevant. Generally, focus on the averages and readability.
Sentences per paragraph
For web writing, we recommend around no more than six sentences per paragraph. One or two sentence paragraphs are fine. You can read more about sentences here.
Words per sentence
Ensure it’s around 15 words per sentence or less. (Remember this is an average.) If your average is over 15 words, edit the text with aim of reducing sentence length.
Aim for less than five per cent. See verbs and voice for an explanation of the passive voice and how and why to avoid it.
Flesch-Kincaid grade level
This readability test ranks how easy or difficult content is to read. The number represents the years of education required to understand the text. This figure is a guide and will vary depending on the context and audience of the text. Aim for around eight.