Vocabulary size is an easy number to track, which is exactly why it gets used as a proxy for overall progress far more often than it deserves to be. Fluency involves several dimensions that a raw word count does not capture at all, and paying attention to those other dimensions gives a much more accurate picture of where you actually stand.
Why Word Count Alone Is Misleading
Knowing a large number of words does not guarantee you can understand them at natural reading or speaking speed, use them correctly in a sentence, or recognize them in spoken form rather than only in writing. A large but shallow vocabulary can coexist with real difficulty following an actual conversation or reading a genuinely challenging text - word count measures exposure, not integration.
Reading Speed as a More Revealing Metric
How quickly you can read a passage of a given difficulty, without needing to stop for translations, is a strong indicator of genuine comprehension progress that word count does not directly capture. Timing yourself on a similar type of text every month or two - a news article, a book chapter - and comparing reading speed over time reveals fluency growth that a vocabulary count alone would miss entirely.
Tracking How Often You Need to Look Words Up
A declining rate of tap-to-translate lookups per page, while reading material of consistent difficulty, is a direct measure of how much of that vocabulary has actually become usable rather than just previously encountered. This is arguably a better fluency metric than total saved words, since it reflects active, working knowledge rather than cumulative exposure.
Comprehension Without Translation Support
Periodically trying to read a short passage without any translation tool available, and gauging how much you genuinely understand versus how much you are guessing, gives a rawer, less tool-assisted measure of where your actual comprehension currently stands, separate from how well-supported your reading normally is.
Combining Multiple Metrics for an Honest Picture
Word count, reading speed, lookup frequency, and unsupported comprehension each capture a different facet of fluency, and no single one tells the whole story on its own. Checking all of them periodically - rather than fixating on vocabulary count as the sole measure of progress - gives a far more honest and complete picture of how your language ability is actually developing.