There's a lot of discussion been going on in the Language Learning chatroom about Duolingo's fluency percentage. My fluency percentage (as well as many others') seems very inaccurate, so I was wondering what calculations actually go into creating an actual percentage. Is it knowing all of your vocabulary, keeping your skills refreshed, or something completely different?
From an official Duolingo developer,
This is exactly what it sounds like: our estimate of your fluency in the language you're learning. It is calculated based on what words you know, how important those words are, how well you know them, and how likely you are to forget them. It will increase over time as you learn more words and strengthen your skills, but it will decrease if you don't keep up your strength. Since our goal is for this estimate to be as accurate as possible, be aware that finishing your tree won't get you to 100%.
Although I do use Duolingo regularly, I would recommend against using your "fluency percentage" as an actual indicative measure of how fluent you are in a language. Not only does the fluency meter not work as it should, Duolingo doesn't realize that knowing just vocabulary doesn't make you fluent in a language.
Fluency measures your mastery of the vocabulary and grammar of a language and your ability to understand and produce the language.
Duolingo estimates your fluency based on your progress through the skill tree and the accuracy of your answers. Because learning a language requires repetition, we also take into account how regularly you practice. Without continued practice, your fluency score will decline, based on our model of how the human brain forgets over time.
With Duolingo, you can achieve a fluency as high as 50-60%, which is equivalent to Advanced proficiency.