EVERYDAY CALCULATORS
Ring Size Converter — mm Diameter to US Ring Size
Convert an inside diameter in millimetres to a US ring size using standard sizing charts. Instant, in your browser, no signup.
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Turn a ring's inside diameter, measured in millimetres, into a US ring size. Measure straight across the inside of a ring that fits, and the converter maps it to the nearest standard US size.
The US scale is defined by inside circumference, but diameter is easier to measure and the two are directly related. The chart below shows the standard reference values.
US ring size by inside diameter (approximate chart values)
| Inside diameter | US size |
|---|---|
| 14.1 mm | 3 |
| 14.9 mm | 4 |
| 15.7 mm | 5 |
| 16.5 mm | 6 |
| 17.3 mm | 7 |
| 18.2 mm | 8 |
| 19.0 mm | 9 |
| 19.8 mm | 10 |
| 20.6 mm | 11 |
Values follow standard jeweller charts and are approximate. A fraction of a millimetre can shift the result by a size, so measure carefully.
Frequently asked questions
- How do I measure my ring's inside diameter?
- Take a ring that fits the right finger and measure straight across the inside, edge to edge, in millimetres. A caliper is most precise; a clear millimetre ruler also works.
- How accurate is a converted size?
- It is only as good as your measurement, and a fraction of a millimetre can move you a whole size. For an important or expensive ring, confirm with a jeweller who can size your finger directly — fingers also change size with temperature and time of day.
- Can I convert to UK or European sizes?
- This tool focuses on US sizing from a millimetre diameter. UK sizes use letters and European sizes use circumference in millimetres, so those follow different scales — check a full cross-reference chart to convert between countries.