Apple hasn't released official sales figures for the Apple Watch, which sold out within six hours on its first day of pre-orders last week. But at least one firm has stepped into the void, estimating sales of nearly one million units in the United States alone.

Analytics firm Slice Intelligence, which tracks the e-receipts of more than two million people, tapped into its data bank and estimated that 957,000 U.S. customers pre-ordered an Apple Watch when the device went on sale last week. It also found that several people ordered more than one -- the average order was for 1.3 watches, with a bill of $503.83.

As you may guess from that average price, the most popular model of the Watch was the Apple Watch Sport, the aluminium watch that had a starting retail price of $350. (Well, $349.)  Sport buyers spend, on average $382.83 on their watches and made up 62 percent of all buyers. Those who bought the mid-tier Apple Watch spent an average of $707.04.

More people also opted for the larger, 42 mm case rather than the smaller 38mm case. (It sounds like a little difference, but it's noticeable when it's on your wrist.)

As for the most popular bands to go with the Watch, the firm reported that the most popular was the black Sport band -- a sweat-resistant and lower-priced band -- at 28 percent. The next most popular was the Milanese Loop metal mesh band, which has earned high praise from reviewers for its aesthetic appeal.

Perhaps unsurprisingly, the report also showed the the Apple faithful were the most likely to pick up a Watch. A majority of Apple Watch buyers had also bought either an iPhone, Mac or iPad in the past two years. Just over a fifth of respondents had a track record as early adopters -- 21 percent had pre-ordered an iPhone 6 or iPhone 6 Plus.

By way of comparison, it initially took Apple 28 days to sell one million first-generation iPads and 74 days to sell one million units of the original iPhone. It took nearly two years to sell one million iPods.

Analysts have estimated that Apple will sell between 20 million and 22 million Watches within the year.