Default Belly Bar Sizes
The default size for belly bars is 10 mm length and 1.6 mm thickness, which
should be suitable for most belly button piercings. However, if you’re not sure what size you need you can measure the one you’re currently wearing.
Despite the default thickness being 1.6 mm, some people prefer a 1.2 mm belly bar. Be careful when switching from 1.6 mm to 1.2 mm, as your belly button piercing might heal back to 1.2 mm. It is only recommended to do this if you know what you are doing.