United Kingdom Size 18 to US: 18-20

United Kingdom Size 18: Waist range of 83–90 $\text{cm}$ (32.7–35.4 $\text{in}$), hip range of 109–116 $\text{cm}$ (42.9–45.7 $\text{in}$), lengths (Mini/Midi/Maxi): 49/82/107 $\text{cm}$ (19.3/32.3/42.1 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.

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.