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

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: 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.