Originally Posted by
Jon Szanto
Nib numbers are almost meaningless, with the small exception of the last 10-20 years. For all the decades before, many pen companies, if not most, made their own nibs and used various numbering systems. As would be obvious, the smaller the number, the smaller the nib. There are pens from 100 years ago that even sported #0 nibs.
However, in recent years fewer pen makers make their own, and there was a great reduction - in Western pens - down to nibs from two main makers: Bock and JoWo. Of these, they pumped out primarily two main sizes, #5 and #6. If you go to any number of online pen / hobbyist sites, you'll find a variety of nibs in those sizes, often branded (such as Goulet) but made by one of those two makers. Proprietary nibs vary in both size and nomenclature, and if you start going backward in history, it's all over the place.
The biggest takeaway? There is very little standardization, and while a fair number of lower-end pens will come with a basic nib that you can pull out and plop in another ("Hey gaiz, I found that the #6 nib from Bonzo's Nibs will fit a PenCCT 488 pen perfectly!"), nothing is certain and a certain degree of trial and error are needed.. The thing is, nibs have length and width that must be matched, as well as shape and curvature, so that they mate well with the feed.
Others can and will expand and correct on that, but it gives you a bit of the lay of the land. It's a jungle out there!