Graded readers can be an enjoyable way for learners to improve their English. Levels range from beginner to upper intermediate (B2). Oxford University Press provide a ‘test’ to help learners choose the right level.
Of course what an appropriate level is depends on what the learner wants to get from the reading so the OUP test needs to be used with this in mind. Naturally, for fluency practice a relatively easy level will be appropriate but for more intensive vocabulary work, a higher level might may well be better.
You can go to the OUP test here
Doing the tests could be an interesting non-lesson independent afternoon activity for your students and will help them with reading speed, vocabulary, grammar, syntax and punctuation.