US: 18-20 to United Kingdom Size 16

US: 18-20: Waist range of 91–99 $\text{cm}$ (35.8–39 $\text{in}$), hip range of 117–125 $\text{cm}$ (46.1–49.2 $\text{in}$), lengths (Mini/Midi/Maxi): 51/84/109 $\text{cm}$ (20.1/33.1/42.9 $\text{in}$), 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.