Apple CEO Tim Cook speaks at the WSJD Live conference in Laguna Beach, California October 27, 2014. REUTERS/Lucy Nicholson Apple's market capitalization has dropped by roughly $90 billion since July 20, as CNBC first noticed.

That $90 billion figure is based on Apple's opening stock price of $130.97 on July 20 and its current number of outstanding shares, which is 5.77 billion.

Apple stock has been dropping since July 20, the day before it reported earnings. As of Tuesday, for example, stocks were down by more than 14% since that date. Apple stock is currently up 0.38% at $115.07 per share.

At its highest, Apple's market cap surpassed $765 billion earlier this year in February. Apple's current market cap now sits at $656.21 billion, according to Yahoo Finance.

It's unclear exactly what caused the slip in Apple's stock, but Bank of America Merrill Lynch recently downgraded Apple stock from "Buy" to "Neutral" with a new stock-price target of $130 from $142.

Analysts have been concerned about Apple's reliance on the iPhone, whose sales are slowing, and think it will be hard for Apple to grow its smartphone market share in China — last quarter, Apple's share slipped into third place behind Xiaomi and Huawei, according to Canalys.