United Kingdom Size 18 to US: 16

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: 16: 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.