When Apple Inc. opens the doors on Apple Watch sales later this month, it could come away with at least a million units sold in the first weekend, according to Piper Jaffray senior analyst Gene Munster. That's three times the first-day haul of Apple's last all-new product, the iPad.

The estimate is based on an attach rate of nearly half a percent of total iPhone users, Munster said. In unit terms, that would put the Apple Watch behind Apple's first iPad, which sold 3 million in its first weekend in 2010, but ahead of the first iPhone, which sold 700,000 units when it debuted in 2007.

It's a fraction of the 10 million iPhone 6 and 6 Pluses sold in the same timespan.

Once preorders begin, Munster predicts, the  Apple could generate 300,000 in the first 24 hours of its availability, or 8 percent of 4 million iPhone 6 and 6 Plus preorders obtained in the same timeframe. Overall he expects Apple to sell 8 million units of the Apple Watch through 2015, bringing in $4.4 billion in revenue. That estimate remains on the conservative side compared to estimates by some investment firms such as JPMorgan Chase, which peg the smartwatch’s first-year sales closer to 26 million units and Credit Suisse, which estimates sales as high as 35 million.

Of those sales, the bulk is expected to come from the aluminum Apple Watch Sport, which starts at $349, and the stainless steel Apple Watch, which starts at $549, according to most analysts. As for the 18-karat gold Apple Watch Edition -- which costs between $10,000 and $17,000 -- the company could sell as many as 8,000 units, according to a previous research note from Munster. Those models are expected to be highly limited and initially available only at Apple’s flagship stores.

While Apple may reveal some performance metrics of the Apple Watch after preorders and its first weekend, specific details on those numbers may not be revealed when it reports its fiscal 2015 second-quarter earnings April 27. “I’m not very anxious in reporting a lot of numbers on Apple Watch and giving a lot of details on it because our competitors are looking for it,” Apple CEO Tim Cook said during an October earnings call.

As for future earnings reports, the company for now will report Apple Watch revenue in its “other products” category, which includes the iPod, Apple TV, Beats headphones and other accessories. This differs from how Apple handled the iPhone and iPad, which had their results immediately broken out on the company’s earnings reports.

Try-ons and preorders for the Apple Watch start on April 10, followed by its official release on April 24 in Apple Stores in Australia, Canada, China, France, Germany, Hong Kong, Japan, the U.K. and the U.S. Apple Watch models will also be available to preview and try on at three dedicated Watch shops at Galeries Lafayette in Paris, Isetan in Tokyo and Selfridges in London.

Last year 89 companies sold 6.8 million smartwatches accounting for $1.3 billion in revenue, according to the Smartwatch Group, an independent research firm. But with the Apple Watch debuting this month, that number is expected to explode to $8.7 billion this year.