Amazon just shared new numbers that give a clue about how many Prime members it has

Amazon has finally shared more financial details around its Prime user base, giving a clue to just how popular its membership service really is.

According to its latest 10-K annual report, Amazon generated roughly $6.4 billion in sales from "retail subscription services," a newly broken out category that includes revenue from all of Amazon's subscription services, such as its Prime, audiobook, e-book, and digital video and music services.

And with a quick back-of-the-envelope calculation, that revenue would translate to about 65 million Prime members worldwide, Morgan Stanley's analyst Brian Nowak writes in a note published Monday. Amazon has never disclosed the number of its Prime members.

Nowak came up with the number based on the assumption that 90% of Amazon's retail subscription revenue came from Prime members, and by putting an average price tag of $88 per user for each subscription (Prime costs $99 a year in the US, but is cheaper in other countries like Japan and the UK).

Prime is one of the most important parts of Amazon's business because Prime members tend to spend more on Amazon's online shopping platform. A recent survey by Morgan Stanley estimated that about 40% of Amazon Prime members spend over $1,000 a year on Amazon, while only 8% of non-Prime shoppers do so. It also found that Amazon Prime members spend about 4.6-times more money on Amazon than non-Prime members.