Mechanical royalties are generated whenever a copy of a song it made. Some examples of this are when a label produces CDs, someone digitally downloads your song, or when your song is streamed. These royalties must be paid by a third-party (normally a record label) for the use of the song.
 The current statutory mechanical royalty rate for physical recordings (such as CDs) and permanent digital downloads is 9.1¢ for recordings of songs 5 minutes or less and 1.75¢ per minute or fraction thereof for those over 5 minutes. The total mechanical royalties paid will then be those rates multiplied by the number of copies made.
Streaming rates are not statutory, so each streaming service pays out a bit differently and each one has their own calculation to determine the rate along in addition to their own multiple factors that would effect each song's rate. 

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