United Kingdom Size 50 to US: 38

United Kingdom Size 50: Corresponds to a waist size of 125 $\text{cm}$ (50 $\text{in}$), total strap length of 140 $\text{cm}$ (55.1 $\text{in}$), and recommended fit classification Extended / Plus.

US: 38: Corresponds to a waist size of 95 $\text{cm}$ (38 $\text{in}$), total strap length of 110 $\text{cm}$ (43.3 $\text{in}$), and recommended fit classification Standard / Regular.