United Kingdom Size Extra Small to US: Standard / 52mm

United Kingdom Size Extra Small: Corresponds to a nominal width of 45 $\text{mm}$ (1.77 $\text{in}$), length of 160 $\text{mm}$ (6.3 $\text{in}$), average structural girth of 90 $\text{mm}$ (3.54 $\text{in}$), and fit category Extra Small / Pediatric / Snug.

US: Standard / 52mm: Corresponds to a nominal width of 52 $\text{mm}$ (2.05 $\text{in}$), length of 185 $\text{mm}$ (7.28 $\text{in}$), average structural girth of 104 $\text{mm}$ (4.09 $\text{in}$), and fit category Standard / Regular.