ReadablePro’s letter grades are our own unique readability score. We factor in all of the scores from the other readability algorithms we use and create an overall score. One that is quick and easy to understand.
Ratings are from A to E. A rated text is very easy to read for the general public. E rated text is very hard to read.
We are in the process of tweaking and improving these ratings, so we are not publishing the calculations we use for them at the moment. We do plan to publish our criteria in the future, once we are reasonably confident we will not make any substantial changes to the rating system.