US: M to United Kingdom Size 18

US: M: Corresponds to a waist/hip size of 80 $\text{cm}$ (32 $\text{in}$), waist range of 71–81 $\text{cm}$ (28–31.9 $\text{in}$), total strap length of 90 $\text{cm}$ (35.4 $\text{in}$), and fit classification Standard Waist / Hip.

United Kingdom Size 18: Corresponds to a waist/hip size of 95 $\text{cm}$ (38 $\text{in}$), waist range of 86–97 $\text{cm}$ (33.9–38.2 $\text{in}$), total strap length of 105 $\text{cm}$ (41.3 $\text{in}$), and fit classification Relaxed Waist / Hip.