The Apple Watch Solo Loop sizing effectively maps to the holes of the Sport Loop, according to John Gruber. The distance between the holes in every Apple Watch Sport Band is reportedly exactly the same as the distance between the 12 sizes of Solo Loops on Apple's measuring tool.
All Apple Watch Sport Bands have seven holes. For the 38mm or 40mm band, the S/M band corresponds to the Solo Loop sizes of one to seven, and the M/L band corresponds to the Solo Loop sizes of four to 10. For the 42mm or 44mm band, the S/M band corresponds to the Solo Loop sizes of three to nine, and the M/L band corresponds to the Solo Loop sizes of six to 12. Gruber writes:
A few people have wondered why Apple doesn't just map fluoroelastomer Sport Band sizes to Solo Loop sizes. I can see why Apple doesn't do that — they can't assume everyone already has access to an Apple Watch with a Sport Band, and even for people who do have access to one, Apple can't assume it's the right size watch (38/40mm vs. 42/44mm). And to further complicate matters, each Sport Band comes with two sizes for the side with the holes: "S/M" and "M/L." So that's four separate mappings from Sport Band holes to the new Solo Loop sizes.
The below image shows size mapping for two 40mm Apple Watch Sport Bands, with S/M and M/L sizes.
The following image shows size mapping for two 44mm Apple Watch Sport Bands, also with S/M and M/L sizes.
The news that it is possible to match Solo and Braided Loop sizing to the Sport Band will reassure many existing Apple Watch users who have been concerned about sizing issues and the potential need for an inconvenient return.
Regarding Apple's measuring tool, Gruber points out that "I think you're supposed to use the tool to precisely measure the circumference of your wrist, not to simulate the circumference of what you think would be a comfortable watch band," and he believes this may be the result of much of the discontentment about Solo Loop sizing.
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Beyond that, he's probably correct that this was intended as a wrist-measurement tool and that people are (understandably, but probably incorrectly) using this instead to interpolate what they believe a correct band size will be. In short, there's almost certainly a communication failure at work here.