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%. Keep in mind that the Duolingo fluency score is solely based on your Duolingo activity, and does not take into account any external activities you may engage in to improve your foreign language skills.
Fluency score on the website: