US to mm Ring Size Conversion
| US | Diameter (mm) |
|---|---|
| 4 | 14.9 |
| 4.5 | 15.3 |
| 5 | 15.7 |
| 5.5 | 16.1 |
| 6 | 16.5 |
| 6.5 | 16.9 |
| 7 | 17.3 |
| 7.5 | 17.7 |
| 8 | 18.1 |
| 8.5 | 18.5 |
| 9 | 19 |
| 9.5 | 19.4 |
| 10 | 19.8 |
Sizing conventions vary by brand and by cut — this table shows the standard published conversion. Always check the garment's own size chart when one is available. See how these tables are compiled.
Converting a coded number into an actual measurement
A US ring size is an arbitrary coded number with no obvious unit attached, while a millimetre figure — either internal diameter or circumference — is the actual physical measurement a jeweler works from. Converting US to mm means decoding that arbitrary US number back into a real measurement, useful when ordering from a jeweler or platform that sizes exclusively in millimetres rather than the US letter-number system.
Be clear about which millimetre figure you need: diameter (the straight-line width across the inside of the ring) and circumference (the distance around the inside edge) are two different numbers for the same ring, related by circumference being roughly diameter multiplied by pi — a common ordering mistake is providing a diameter figure to a chart or jeweler expecting circumference, or the reverse.
Old rings and remembered sizes compound the imprecision
If your 'US size' comes from an old ring that's stretched slightly with wear, or from a size you were told years ago rather than a recent measurement, converting that number to millimetres carries the accumulated imprecision of the original sizing plus any change in your finger since — a fresh direct measurement avoids compounding these errors, and this site's at-home ring measuring guide covers getting one.
Why custom and 3D-printed jewelers specifically ask for millimetres
A growing number of made-to-order and 3D-printed ring sellers manufacture directly from a circumference or diameter figure rather than stocking pre-sized inventory in the US scale, which is why they ask for millimetres up front — converting your known US size gives them a workable starting figure, but their own printable sizing tool, if they offer one, will usually be more accurate than this chart alone.
Converting for a partner’s ring without asking them directly
If you’re converting a partner’s remembered or overheard US size into millimetres for a surprise purchase, treat the result as a reasonable estimate rather than a certainty, and favor a jeweler who offers free resizing, since a surprise ring is one of the few purchases here where you genuinely can’t double-check the fit in advance.
If the recipient already owns rings in a similar style, checking with a family member who might know their existing ring size is generally more reliable than converting a size mentioned once in passing months or years ago.
Frequently asked questions
Does my US ring size convert to one exact millimetre figure?
Approximately, not exactly — the US size scale has built-in rounding (whole, half, sometimes quarter sizes), so converting to millimetres gives a close estimate rather than a precise figure; measure directly if precision matters.
Should I provide diameter or circumference when a platform asks for millimetres?
Check which one they are asking for — diameter (straight-line width) and circumference (distance around) are different figures for the same ring, related by roughly a factor of pi, and providing the wrong one is a common ordering mistake.
Is converting an old ring's US size to millimetres reliable?
Less reliable than a fresh measurement — an old ring may have stretched slightly with wear, and a remembered size from years ago may not reflect your current finger size, both of which compound when converted to millimetres.
Why do custom and 3D-printed jewelers ask for millimetres specifically?
Many manufacture directly from a circumference or diameter figure rather than stocking pre-sized US inventory, so a millimetre figure is a more direct input to their production process than a converted US number.