Amazon Prime is growing like crazy: 54 million members, up 35% from last year, says estimate

Amazon Prime, the company's $99-per-year loyalty program, now has 54 million US members, according to a$4. That could mean good news for Amazon ahead of its Q4 2015 earnings report this week.

That's up 35% from this time last year - again, based on CIRP estimates; Amazon itself doesn't reveal anything but the hazy "tens of millions" when talking about Prime subscribers.

Of the 14 million members Amazon added in 2015, 7 million joined during the fourth quarter holiday season.

Interestingly, CIRP points out that growth was spread more evenly this year in large part because of "Prime Day," the $4.

The loyalty service gives users free, two-day shipping on millions of items, as well as access to of thousands of free TV shows, movies, music, and books and unlimited cloud storage for photos.

Amazon can afford all those "free" perks and $4 because research from CIRP $4 has long shown that Prime members spend much more on the site than non-members do.

This latest analystis shows that members spend ~$1,100 per year, compared to ~$600 per year for non-members.

That's actually down from the beginning of 2015 though, when $4, on average, compared to $625 per year, for non-members.

CIRP bases its insights on a survey of 500 US subjects who made a purchase on Amazon between October and December.

Amazon reports Q4 earnings this Thursday, January 28.

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.