Fluency was a feature of Duolingo. It was an estimated measure of proficiency of the vocabulary, grammar, comprehension and the ability to produce the language. As of April 2018, the fluency score was only available in Spanish, German, French, Portuguese, Italian, English, Polish, Danish, and Swedish. It was replaced in April 2018 by the Crowns system.
The metrics used to estimate user's fluency included:
- progress in a course;
- accuracy of answers;
- frequency of practice;
- words learned; and
- importance of words in the language.
The fluency score would either increase through continued practice or decrease if the user doesn't regularly practice. As of April 2018 one could only reach a fluency as high as 65%, which according to Duolingo is equivalent to an advanced proficiency in the Duolingo Test Center.