US: 4-6 to United Kingdom Size 16

US: 4-6: Waist range of 65–69 $\text{cm}$ (25.6–27.2 $\text{in}$), hip range of 91–95 $\text{cm}$ (35.8–37.4 $\text{in}$), lengths (Mini/Midi/Maxi): 43.5/76.5/101.5 $\text{cm}$ (17.1/30.1/40 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.

United Kingdom Size 16: Waist range of 76–82 $\text{cm}$ (29.9–32.3 $\text{in}$), hip range of 102–108 $\text{cm}$ (40.2–42.5 $\text{in}$), lengths (Mini/Midi/Maxi): 47/80/105 $\text{cm}$ (18.5/31.5/41.3 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.