I feel like a big jerk and made a mistake when measuring my chain. I took it to a jeweler this morning and it is indeed 4.5mm as advertised. Changing my review to five stars and I will be using Frost again to upgrade my chain hopefully within a year. My recommendation for other buyers is that please do not measure your chains MM yourself. Take it to a local jeweler for accurate measurement. Frost is legit, the Jeweler also tested the Karat with one of those guns instead of the scratch test and it is indeed 14-karat. If you're in the market for a chain they are legitimate. Sorry for the original review FrostNYC.