If Apple launched its Apple Watch today, between four and six out of every 10 iPhone owners would be able to use the smartwatch, according to statistics from Kantar Worldpanel Comtech and an app analytics firm.

Apple Watch will work with the new, not-yet-released iPhone 6 and iPhone 6 Plus, but the device will also link to the iPhone 5, 5C and 5S, the 2012 and 2013 series of Cupertino's flagship. Without an iPhone, the Apple Watch tells time and tracks some biometric data -- like heart rate -- but not much more.

Kantar, which regularly polls mobile users worldwide, pegged the iPhone 5, 5C and 5S share of all iPhones (just iPhone 5 from this point on) at 43% in the U.S., 38% in the top five European Union countries -- France, Germany, Italy, Spain and the U.K. -- and a much lower 26% in China.

Not surprisingly, 2012's iPhone 5, as opposed to last year's 5C and 5S, was the leader in all three regions Kantar measured. In the U.S., for example, the iPhone 5 accounted for 22% of all iPhones, with the 5S and 5C trailing at 13% and 8%, respectively.

Meanwhile, app analytics vendor Localytics said that during August, the iPhone 5 accounted for 60% of the iPhones tracked via its clients' iOS apps. At the top was the iPhone 5 with 27%, slightly ahead of the iPhone 5S at 25%, but triple last year's lower-cost model, the iPhone 5C. It accounted for 8% of the total.

In all the statistics, the iPhone 4S, which debuted in 2011, led the older iPhone 4. Kantar tagged the 4S with 35% in China, for instance, and the iPhone 4 at 32%. The remainder was mostly iPhone 4 and 4S, with a combined share of 34%. Localytics' numbers had the iPhone 4S at 21% versus 13% for the iPhone 4.

Carolina Milanesi, chief of research and head of U.S. business for Kantar, said the iPhone 4S was at the top not just because it was among the oldest models still in use. "It was a very good seller to start with, and it is still a very good phone," she said in an interview Wednesday.

While those model shares will certainly change in the months before the Apple Watch goes on sale because of the expected sales of the iPhone 6 and 6 Plus -- and a corresponding decrease of the older models as some are taken out of service -- the share statistics hint at the potential market for the watch.

"I think it will be a real revenue contributor," said Ezra Gottheil, an analyst with Technology Business Research, of the Apple Watch. "In some sense, it's not a watch."

Apple said only that it would ship the smartwatch "in early 2015," but did not narrow the window beyond that.

Another factor in potential Apple Watch sales is its support for Apple Pay, the electronic payment service and corresponding NFC-based touch-payment system Apple said would launch next month in the U.S. Although only the newest iPhones, the 6 and 6 Plus, will be able to use Apple Pay, the smartwatch provides a work-around for older models.

Apple's website states that Apple Pay works when an Apple Watch is "paired with iPhone 5, iPhone 5c, iPhone 5s, iPhone 6, or iPhone 6 Plus." Yesterday, Apple demonstrated how a customer could touch the watch to a payment terminal, or at least bring it within fractions of an inch, to pay for goods at retail -- without pulling the phone from a pocket or purse.

It's probable that Apple Watch owners will skew towards the newest models, if only because at the smartwatch's debut the most likely buyers of both it and the iPhone 6 and 6 Plus will be the early adopters and the wealthiest customers. But the inclusion of the iPhone 5 on the Apple Watch support list means that the customer pool for the smartwatch is considerably larger than new-iPhone-only.

Wall Street analysts reacted quickly to the Apple Watch revenue opportunities to update their financial models -- if they had not already taken the smartwatch into account earlier based on the strong rumors.

Brian White of Cantor Fitzgerald, for instance, assumed a 20% "conversion rate" for the Apple Watch; in other words, smartwatch sales would equal approximately 20% of iPhone sales in any given quarter. "[That] is reasonable, and less than half the 45% conversion rate for the iPad," White wrote in a note to clients today.

In White's model, that would translate into Apple Watch unit sales of almost 37 million in the first year of availability, with revenue of $12.8 billion, or about 55% of the $23.1 billion that the Mac line brought in over the last four quarters and 41% of the iPad's revenue during that same stretch.

But White's Apple Watch prediction looks a lot better when compared to, say, the iPad's first year: From Q2 2010 to Q1 2011 -- the iPad launched in early April of 2010 -- the tablet line recorded $12.4 billion in revenue.

Milanesi, who is not a sell-side financial analyst, declined to put a number to possible Apple Watch sales. 'But if I had to come up with a number, I'd look at people who upgrade every iteration," she said today. "Those are not just the early adopters, but those I'd call hard-core Apple customers. They upgrade even if they're still on a contract."

However, she expected that those people would not be the only ones upgrading to an iPhone 6 or iPhone 6 Plus, and thus adding to the Apple Watch pool. "There are a lot of iPhone 4 and 4S owners who will be upgrading this time," she said, pointing out that 43% of all iPhones were more than 24 months old and that the average time between iPhone upgrades was just 21.1 months.