A spike in Amazon Prime customers, thanks in part to its Prime Day promotion, could lead to a spike in business for Amazon.com Inc.

That’s according to just-released data from market research firm Consumer Intelligence Research Partners LLC, which estimates there are 47 million Amazon Prime subscribers as of Sept. 30. That’s up 6.8% from 44 million members at the end of the second quarter, June 30. Amazon, No. 1 in the Internet Retailer 2015 Top 500 Guide, does not say how many Prime members it has.

“Prime Day looks like it was a big success,” CIRP co-founder Mike Levin said. “In the same quarter last year, Amazon Prime membership grew by 3%, adding 1 million net members. In the most recent quarter, Prime grew by 7% and approximately 25% of those new members reported joining to take advantage of Amazon Prime Day.”

Amazon Prime gives members free two-day shipping on 20 million items for $99 a year and includes other services such as free streaming video and e-books.

With Amazon set to report its third quarter earnings Thursday, more details may emerge about the impact of Prime Day. CIRP’s data shows that Amazon Prime members accounted for nearly half (46%) of the online retailer’s customer base during the third quarter. Those members spend twice as much annually ($1,200) on Amazon as non-Prime members ($600) and convert at a higher rate. Data released in June by website traffic measurement firm Millward Brown shows that Prime members have a 74% conversion rate.