Ring Size Converter

A ring size converter is a tool that translates a ring size from one sizing system into its equivalent in all other major systems. Ring sizes are measured by the inner circumference or inner diameter of the ring in millimeters. Different countries use different conventions — the US uses numeric sizes, the UK uses letters, the EU/ISO standard uses the inner circumference directly in millimeters, and Japan uses a separate numeric scale. Our ring size converter maps all these systems to each other using the exact formulas defined by ASTM (US), British Standard BS 6820 (UK), and ISO 8653 (EU/ISO), covering US sizes 1–16 in half-step increments.

1. 1. Select Your Input Type: Choose the sizing system you already know — US size, UK letter, EU/ISO size, Japan size, inner diameter in mm, or inner circumference in mm. The ring size converter adapts the input field to your selection.
2. 2. Enter Your Measurement: Type your ring size or measurement. For UK sizes, select the letter from the dropdown. All processing happens locally in your browser — your data never leaves your device.
3. 3. Get All Sizes Instantly: Click Convert and the ring size converter displays your size in all six systems simultaneously — US, UK/AU, EU/ISO, Japan, diameter (mm), and circumference (mm). The matching row is highlighted in the full reference chart.

• US Ring Sizes:Numeric sizes from 1 to 16 in half-step increments. Each half size = 0.4064 mm of diameter (1/64 inch). US size 7 (the most common women's size) corresponds to an inner diameter of approximately 17.35 mm. Our ring size converter uses the ASTM formula: diameter (mm) = US size × 0.8128 + 11.63.
• UK / AU Ring Sizes:Letter sizes from A to Z (and beyond for larger sizes). Each letter step = 1.25 mm of inner circumference. UK size L (a common women's size) corresponds to approximately 51.05 mm circumference. Our ring size converter uses the British Standard BS 6820 formula: circumference (mm) = 37.8 + (letter index × 1.25).
• EU / ISO Ring Sizes: The simplest system — the size IS the inner circumference in millimeters, rounded to the nearest integer. EU size 57 = 57 mm inner circumference. This is the ISO 8653 standard used across continental Europe. Our ring size converter converts directly between EU size and diameter using π.
• Japan Ring Sizes: Numeric sizes starting from 1, where each step = 0.5 mm of inner diameter. Japan size 14 corresponds to approximately 18.5 mm diameter. Our ring size converter maps Japan sizes to all other systems using the formula: diameter (mm) = 13.0 + (JP size − 1) × 0.5.

After converting your ring size, our ring size converterdisplays a printable ring sizer showing a circle with the exact inner diameter of your ring. To use it: click "Print Sizer", ensure your printer is set to 100% scale / actual size(do not use "fit to page" or any scaling), print the page, and place an existing ring over the printed circle to verify the size. If the ring fits perfectly around the circle, the size is correct. This physical verification method is the most reliable way to confirm a ring size before purchasing.