The quickest way to calculate your reading level is to take an online reading test. There are several types of free tests available to calculate different types of reading levels. Mindsprinting.com offers a grade level reading assessment, while Oxford and Macmillan Readers use their own level system based on their graded readers.
These assessments are primarily timed reading comprehension tests in which you are given a short passage to read followed by several questions. The grading systems vary from site to site, but give an approximate reading level. The levels usually correspond to the difficulty of the company's "graded readers," which are books written with a specific complexity in mind. For example, the Oxford Bookworms system is made up of seven levels with two tests each. For each level they recommend that a person with higher than 95% comprehension move up a level, and that a person with under 80% comprehension move down a level..
In addition to reading level tests there are reading speed tests. These tests ask you to read a passage normally and test comprehension, but are also timed and report the words you read per minute. Your speed reading level is your words read per minute multiplied by the percentage of words that you comprehend. Top readers can read 1,000 words per minute with an 85% comprehension rating, or 850 efficient words per minute. The average reader can read 200 words per minute with a 60% comprehension rating, or 120 efficient words per minute.