US: 4-6 to United Kingdom Size 18

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 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.