Amazon has 85 million Prime customers in the U.S., up from 63 million a year ago, according to Consumer Intelligence Research Partners estimates.

The number of Amazon Prime customers in the U.S. has grown by nearly 35% in the past year, and membership could be in for a boost when Amazon holds its third annual Prime Day on July 11.

Consumer Intelligence Research Partners (CIRP) says Amazon.com Inc., No. 1 in the Internet Retailer 2017 Top 500, now has about 85 million U.S. members of its Prime loyalty program, up 34.9% from 63 million this time last year. Amazon does not disclose how many consumers are Prime customers. CIRP’s data is extrapolated from a survey of 500 shoppers in the U.S. who bought something on Amazon from April-June.

In April, the securities research firm estimated Amazon had 80 million Prime customers in the United States, which was a 37.9% increase from its April 2016 estimate of 58 million.

Here’s a look at Prime’s growth trajectory in the U.S. since March 2016:

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[infogram id=”number_of_amazon_prime_members_in_the_usfigures_in_millions” prefix=”5mT” format=”interactive” title=”Number of Amazon Prime members in the U.S.
(Figures in millions)”]

Prime is Amazon’s loyalty program, which costs $99 per year or $10.99 per month. It offers access to benefits such as free expedited shipping and streaming video. CIRP’s data shows that more than a quarter—28%—of all Prime customers have a monthly membership.

“Amazon Prime membership encourages much more frequent shopping, likely because the free shipping benefit knocks down a key barrier to buying online often and makes Amazon their first stop for online purchases,” said CIRP co-founder and partner Mike Levin. “Otherwise, Amazon Prime members buy the same number of items per visit, and spend roughly the same amount per visit as non-Prime members.”

CIRP’s data shows that Prime members, on average, tend to spend nearly twice as much per year ($1,300) on Amazon as non-Prime members ($700). Prime members also shop on Amazon nearly twice as frequently per year (25 times) compared with non-Prime members (14 times).

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CIRP’s latest estimate lands just a few days before Amazon kicks off Prime Day at 6 p.m. Pacific on Monday. The 30 hours of deals will end at midnight Pacific on July 11. Prime Day was initially held on July 15, 2015, to celebrate Amazon’s 20th anniversary and was repeated last July.

The day has become sort of summer holiday for the retail industry as other retailers look to capitalize on the online shopping interest generated by Amazon’s promotion. Last year, 24 of the 50 largest retailers in the U.S., including Amazon, ran some kind of special on Prime Day. Amazon, which has not disclosed its Prime Day sales numbers, said last year that Prime Day was its biggest sales day ever.

Other retailers have started to to unveil plans for Prime Day, even if they don’t sell on Amazon.

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Consumer electronics e-retailer Newegg Inc. (No. 21) yesterday announced that it will hold its third annual FantasTech sale, which will launch Tuesday and run for three days. QVC Group (No. 7) is planning a 12 Days of Christmas sale, offering online deals on holiday-themed products through Wednesday.

A Kohl’s Corp. (No. 18) spokeswoman confirms the retailer will be running its own 30 hour online sale starting at 6 p.m. Central on July 10, offering 30% off summer gear including apparel, swimwear, beach towels, accessories, and patio furniture.

1-800-Flowers.com Inc. (No. 59) is promoting what it calls Passport Day on Tuesday to coincide with Prime Day, seeking to generate interest in its Celebrations Passport rewards program. Celebrations Passport is similar to Amazon Prime in that shoppers pay a $29.99 annual membership fee in exchange for such perks as same-day delivery, rewards points across all 1-800-Flowers brands and free unlimited standard shipping.

The retailer also will offer deals on Monday specifically for Celebrations Passport members. There is financial incentive for 1-800-Flowers to promote the loyalty program. CEO Chris McCann told analysts on the retailer’s fiscal third quarter 2017 earnings call in May that Passport members spend up to four times more with 1-800-Flowers than non-Passport members.

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Last week, J.C. Penney Co. Inc. (No. 33) confirmed that it would be bringing back its online-only Penney Palooza sale, which, like Prime Day, will run Monday and Tuesday. Multichannel kitchen supply retailer Sur La Table Inc. (No. 245), meanwhile, will hold its first-ever “Lime Day” sale on Monday.

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