Ring Size Converter

How it is calculated

You enter the inner circumference of the ring in millimetres; it must be above 30 and below 90. The tool derives everything from that one number. Inner diameter is d = C / π. The EU size is the circumference rounded to a whole number of millimetres. The US size is a straight-line formula, US = (C − 36.537) / 2.5535, shown to one decimal. The UK letter is found by converting that US value to a letter position (A to Z) with a fixed ratio of about 1.86 letters per US size. These are approximations of common conversion charts, so a jeweller's own chart may differ by a half size or a letter, and half sizes are not rounded to a ring-maker's standard stops.

Common mistakes

• Entering the finger's outside circumference measured over a thick ring instead of the inner circumference of a ring that already fits.
• Typing the diameter (about 17) instead of the circumference (about 54); values of 30 mm or less are rejected and the tool returns nothing.
• Measuring a cold or very warm finger: fingers swell with heat and shrink in cold, so measure at the end of the day at normal temperature.
• Forgetting wide bands: they fit tighter than thin ones, so many people go up a half size for a wide ring; the tool does not adjust for width.
• Treating the US decimal as an available size: 6.8 means you should pick between 6.5 and 7, not look for a 6.8.