US to UK Ring Size Conversion

US/Canada uses a numeric scale (e.g. 6, 6.5, 7) where each whole-size step is a fixed increase in inner diameter. UK/Australia (also used in Ireland, New Zealand, and South Africa) uses a letter scale (e.g. N, N½, O) where each letter step is a fixed increase in inner circumference. Because the two scales are built from different base measurements and different step sizes, there's no simple arithmetic shortcut between them — the reliable way to convert is via the shared millimeter measurement both systems are ultimately anchored to, which is exactly what the table below and the calculator above do.