Apple Watch sells millions, but just how many millions?

The Apple Watch is almost certainly the biggest selling smartwatch to date - but analysts are coming up with different estimates of how many units Cupertino has sold so far.

With the launch of the Watch, Apple jumped straight into first place as the biggest vendor of wearable bands.

According to research from analysts Canalys, 4.2 million Apple Watches were shipped during the second quarter of this year - the first the devices was on sale. As a result, Apple "easily" overtook Fitbit, Xiaomi, and all other smart watch vendors by sales, despite the Watch being at the higher-priced end of the market.

The analyst noted that early sales of the Watch were hindered because supply constraints slowed shipments for the first couple months after launch. Indeed, devices were in such short supply that the only place in London where the Apple Watch could be bought and taken away on launch day was an upmarket Mayfair boutique - and only then for those who had made an appointment, although the watches are now more widely availabe.

Canalys' estimate of Watch sales is higher than some other analysts': for example, Gene Munster, analyst at Piper Jaffray, predicted three million Apple Watch sales this quarter, rising to 14 million next year and 40 million the year after, while analyst KGI predicted sales for the most recent quarter of 3.9 million.

Apple has remained tight-lipped about the true level of Watch sales and it's unlikely to provide much in the way of solid figures in its financial results due later today: the Apple Watch is included in the 'other products' category, along with the iPod and Apple TV, rather than as a category on its own.

Still, compared to iPhone sales (61 million in Apple's second quarter), sales of the Apple Watch are tiny, and come against a background of low sales in the smartwatches and wearables markets in general.

For that situation to change, app developers will need to embrace the device.

"Apple and other vendors still face important challenges to make the smartwatch a breakout hit. Improvements in performance, battery life and sensor integration are needed to make future models more attractive, but it is the quality of third-party apps that will determine whether the Apple Watch will be a long-term success," said Chris Jones, VP and principal analyst at Canalys.