Convert ring sizes across US, UK, EU/ISO and inside diameter/circumference in millimetres.. Ring Size Converter is a free online tool from the ToolsRift Special Unit Converters collection. It runs entirely in your browser — nothing to install, no signup, and your data never leaves your device.
Select your US ring size to see the matching UK letter, EU/ISO number, and inside diameter and circumference in millimetres.
Ring size is set by the inside circumference; EU/ISO size equals that circumference in millimetres.
circumference ≈ diameter × π, so a wider band means a larger circumference and size.
Half sizes and letter grades vary slightly by brand, so treat the chart as a close reference.