US: 18-20 to United Kingdom Size 12-14

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 12-14: Waist range of 70–75 $\text{cm}$ (27.6–29.5 $\text{in}$), hip range of 96–101 $\text{cm}$ (37.8–39.8 $\text{in}$), lengths (Mini/Midi/Maxi): 45/78/103 $\text{cm}$ (17.7/30.7/40.6 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.