This graph shows the estimated number of Amazon Prime customers, now at approximately 80 million. (Consumer Intelligence Research Partners Graphic)

A new study from Consumer Intelligence Research Partners estimates that Amazon Prime membership doubled in the last two years to more than 80 million people in the U.S.

That figure represents a membership increase of about 38 percent over a year ago, when CIRP estimated the number of Prime members at 58 million. About 60 percent of all U.S. Amazon shoppers are now Prime members, according to the study.

CIRP found that Prime members spend almost twice as much as non-Prime customers on an annual basis — $1,300 versus $700.

For years, Prime has been an annual subscription program, with a current price of $99 per year. But last year Amazon introduced a monthly subscription plan at a cost of $10.99 as well as a standalone subscription for just its Prime Video service. CIRP estimates that about 26 percent of Prime members have opted for the monthly plan.

“The monthly payment plan proved attractive, even thought it costs more than the annual plan,” said Josh Lowitz, partner and co-founder of CIRP. “We think that the monthly membership option appeals to the later Prime adopters, with a smaller, potentially temporary commitment, that ultimately yields a long term commitment. With smaller dollar, single month decisions, the new plan winds up helping with retention rates, which already average 85 percent for a member renewing after their first year.”

Prime is one of the main “pillars” of Amazon’s business. The online retail giant continuously adds new perks for members to induce more customers to sign up. The annual spending delta between Prime and non-Prime shoppers shows why Amazon is putting so much effort into adding more subscribers.

Amazon doesn’t publicly disclose the official number of Prime members, but Amazon CEO Jeff Bezos has said membership is in the “tens of millions” range. To get its estimates, CIRP did a survey of 500 Amazon shoppers and examined metrics such as subscription revenue.

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