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Morgan Stanley said most analysts are expecting Apple to sell between 10 and 30 million watches in 2015, but they think 30 million units is a reasonable - and "arguably still conservative" - estimate.
The analysts got this figure by looking at the total number of people who will own an iPhone 5 or later by the time the Watch launches next March, which is estimated to be 315 million, then estimating that 10% of those people will buy a Watch.
By way of comparison, 14% of people who owned a compatible iPhone bought an iPad within its first year, and 7% of recent iPod owners bought the iPhone in its first year.
These analysts are also pretty excited about wearables as a whole, looking at health sensors and mobile payments as the key ingredients that could drive sales to the billion unit mark. "We forecast the wearable market to hit 530M units in 2020 in our base case and 1B units in 2020 in our bull case," said Morgan Stanley's Katy Huberty.