United Kingdom Size Classic to US: Small / 51mm

United Kingdom Size Classic: Corresponds to a nominal width of 53 $\text{mm}$ (2.09 $\text{in}$), length of 190 $\text{mm}$ (7.48 $\text{in}$), average structural girth of 106 $\text{mm}$ (4.17 $\text{in}$), and fit category Standard / Original.

US: Small / 51mm: Corresponds to a nominal width of 51 $\text{mm}$ (2.01 $\text{in}$), length of 180 $\text{mm}$ (7.09 $\text{in}$), average structural girth of 102 $\text{mm}$ (4.02 $\text{in}$), and fit category Small / Regular-Narrow.