The number of Amazon Prime members are on the rise.

An estimated 60% of Amazon customers are Prime members, a group that is comprised of roughly 80 million members in the United States, according to research from Consumer Intelligence Research Partners (CIRP). The study is based on responses from 500 U.S. shoppers who made a purchase at Amazon.com between January and March.

Prime members spend on average about $1,300 per year, compared to about $700 per year for non-member customers. The current membership figure compares to an estimated 58 million U.S. members at the end of the March 2017 quarter — an increase of 38% in one year, according to the report.

“Looking back, Amazon Prime membership doubled in the U.S. in two years,” said Josh Lowitz, partner and co-founder of CIRP.

“While slower growth is expected as it reaches natural limits, Amazon had a surprisingly strong quarter,” Lowitz said. “Membership grew 8% in the most recent quarter, compared to 7% growth on a smaller base in the year-ago quarter ending in March 2016.”

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