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Simply install. Readability level for your posts and pages will appear on the appropriate table.

WPReadability

Displays readability level for your posts and pages.

Siteimprove

The Siteimprove CMS Plugin bridges the gap between the WordPress content management system (CMS) and the Siteimprove Intelligence Platform. Now you can scan your website for errors as soon as a page is published, allowing you to fix mistakes, optimize content, and manage your site more efficiently. 
The plugin also provides you access to Prepublish, an add-on which allows you to check your page for issues before it has been published.

Turn your most complex website challenges into manageable tasks—all from a single platform

WPReadability vs Siteimprove