US: 16-18 to United Kingdom Size 4-6

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 4-6: Corresponds to a relaxed bust range of 78–82 $\text{cm}$ (30.7–32.3 $\text{in}$), elastic waist range of 60–64 $\text{cm}$ (23.6–25.2 $\text{in}$), and comfort drape classification Relaxed / Comfort Drape.