US: 6-D to United Kingdom Size 3-AA

US: 6-D: Corresponds to a forefoot ball width of 8.9 $\text{cm}$ (3.5 $\text{in}$), ball width range of 8.7–9.1 $\text{cm}$ (3.42–3.58 $\text{in}$), reference foot length of 22.5 $\text{cm}$ (8.85 $\text{in}$), and fit classification Wide (6-D).

United Kingdom Size 3-AA: Corresponds to a forefoot ball width of 7.6 $\text{cm}$ (3 $\text{in}$), ball width range of 7.4–7.8 $\text{cm}$ (2.9–3.1 $\text{in}$), reference foot length of 21.9 $\text{cm}$ (8.6 $\text{in}$), and fit classification Extra Narrow (5-4A / Slim).