Foursquare lets users record their location with its mobile app, which has resulted in mountains of user location data over the years. The company analyzed the foot traffic to Apple (AAPL) stores around, looking for increases year-over-year since 2012, and patterns of traffic before, during, and after the iPhone went on sale each year. Then, it looked at Apple’s iPhone sales during the same period of time, using the company’s financial disclosures, both for the total number of units, and how typical weekly sales compare with the sales on launch weekend. It then matched both sets of data.

The result: a prediction that Apple stores will see a 360% increase in foot traffic as compared to a typical Friday, and will likely break its previous iPhone sales records with 13-15 million units. Apple sold 5 million phones in 2012, about 9 million in 2013, and 10 million in 2014.

“Visit growth is clearly a strong general advance indicator for sales performance of the launch weekend,” Foursquare COO Jeff Glueck wrote in a blog post detailing the company’s findings.

Foursquare’s prediction loosely matches that of financial research firm FBR Capital Markets & Co., which foresees Apple will sell at least 12 million this weekend.

But of course, Foursquare’s method has its limitations, something Glueck acknowledges. Foot traffic is only one way to predict sales, and only accounts for one sales channel. “In the case of Apple, the Apple stores account for only a portion of sales, for instance. Phones are pre-ordered online, purchased through carriers, etc.,” adds Glueck.

Take a look at what’s happening to the iPhone lines at Apple’s Fifth Ave. store in Manhattan, as well as around the world on Friday morning.