US: 16-18 to United Kingdom Size 12-14

US: 16-18: Corresponds to a relaxed bust range of 105–109 $\text{cm}$ (41.3–42.9 $\text{in}$), elastic waist range of 87–92 $\text{cm}$ (34.3–36.2 $\text{in}$), and comfort drape classification Loose Lounge Fit.

United Kingdom Size 12-14: Corresponds to a relaxed bust range of 90–94 $\text{cm}$ (35.4–37 $\text{in}$), elastic waist range of 72–76 $\text{cm}$ (28.3–29.9 $\text{in}$), and comfort drape classification Relaxed / Comfort Drape.