US: Classic Regular / 53mm to United Kingdom Size Extra Small

US: Classic Regular / 53mm: 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.

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.