United Kingdom Size Slim Fit to US: Small / 51mm

United Kingdom Size Slim Fit: Corresponds to a nominal width of 47 $\text{mm}$ (1.85 $\text{in}$), length of 170 $\text{mm}$ (6.69 $\text{in}$), average structural girth of 94 $\text{mm}$ (3.7 $\text{in}$), and fit category Snug / Slim.

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.