When purchasing hardware I always prefer the use of the "Metric" system which is expressed in millimeters. Millimeters typically provide you with a more accurate representation of the size of the hardware. I say "typically" because some USA sellers round things off.
In the USA many sellers express the size of their hardware in inches because it's what American buyers are used to. However the issue with this is that when the seller gets the hardware from their Chinese supplier, they've probably purchased it based on millimeters. Then they list the item size based on rounding to the nearest number in fractional inches. Or if they list it in mm, they may also sometimes round to the neared whole millimeter number. Perhaps they think they are "simplifying" things by doing this, but in my view they only make it more complicated.
This is why sometimes when you buy a similarly stated size from one seller, it can actually be different than from another. This may not mater as much for a rivet cap that is listed as a 9mm but is actually 9.2m. However, it will matter more with items such as purse locks and especially grommets. It can impact the hole you need to cut, and why you may need to start with a smaller hole, then proceed to cut it larger.
Here is an example. The USA seller may list their grommet ID as 12mm or 1/2" However the actual ID is 12.7. This may be ok when the actual number is slightly larger and you can fit your cord thru the hole easily. But when the reverse happens it can cause problems, and it can be frustrating.
I always prefer it when the seller lists their item in millimeters and is exact with their listing. Below are conversion charts you can use for quick look up to convert your numbers in either direction. But whenever the listing is only in inches, it's a good idea to email the seller and ask them for the number in millimeters that their supplier provided.
These are two different PDF documents I created for looking up conversions. The first one is a quick reference that you can print out either as a one-page 8.5x11 or as a folded-two-sided 4x6 reference card.
Download chart that shows Inches to MM PDF Conversion Charts