Is nominal width the same as diameter?

I’m trying to figure out my condom size, but I’ve had a lot of issues figuring out condom sizes. I know that I’m about 15-16cm in girth, but when I calculate the diameter of that it ends up as as around 4.9-5cm. Which is about 50mm

When I then look up condoms, it says that I’d wear a small condom (below 52mm)

Yet, they don’t fit me in the slightest.

Then, when I put in my numbers in some calculator, it tells me my suggested condom size is anywhere between 65-73mm

Anyway. I’m just really confused.