The noun girth can be countable or uncountable. In more general, commonly used, contexts, the plural form will also be girth. However, in more specific contexts, the plural form can also be girths e.g. in reference to various types of girths or a collection of girths.

Is girth a word?

1. Size; bulk: a person of large girth. 2. The distance around something; the circumference.

What defines girth?

English Language Learners Definition of girth

: the size of someone or something measured around the middle. : a band or strap placed around the body of an animal (such as a horse) to hold something (such as a saddle) on its back.