United Kingdom Size 20 to US: XS

United Kingdom Size 20: Corresponds to a waist/hip size of 100 $\text{cm}$ (40 $\text{in}$), waist range of 91–102 $\text{cm}$ (35.8–40.2 $\text{in}$), total strap length of 110 $\text{cm}$ (43.3 $\text{in}$), and fit classification Expanded / Plus.

US: XS: Corresponds to a waist/hip size of 70 $\text{cm}$ (28 $\text{in}$), waist range of 60–70 $\text{cm}$ (23.6–27.6 $\text{in}$), total strap length of 80 $\text{cm}$ (31.5 $\text{in}$), and fit classification Waist / Snug Cinch.