United Kingdom Size Extra Large to US: Small / 51mm

United Kingdom Size Extra Large: Corresponds to a nominal width of 60 $\text{mm}$ (2.36 $\text{in}$), length of 205 $\text{mm}$ (8.07 $\text{in}$), average structural girth of 120 $\text{mm}$ (4.72 $\text{in}$), and fit category Extra Large / Magnum Custom.

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.