NP STUDIO / JOURNAL / RING SIZE
International Ring Size Calculator
Convert between US, EU, UK, JP, inner diameter and circumference without searching through conversion tables.
Convert a ring size
Convert a ring size
The calculator uses inside circumference in millimeters as its single internal reference. Diameter and circumference are linked through π, while US, UK and JP values come from one coherent lookup table rather than mixed formulas.
What US, EU, UK and JP ring sizes mean
RU is shown as inner diameter in millimeters. EU follows an inner-circumference-in-millimeters logic. US is numerical, UK uses letters, and JP uses a numerical Japanese sizing scale.
How to use the calculator
- 1. Choose what you already know: a ring size, inner diameter or circumference.
- 2. If you know a size, select RU / US / EU / UK / JP.
- 3. Enter or choose the value; the tool converts it to the other systems.
- 4. If your measurement sits between standard rows, compare the smaller and larger neighboring sizes.
If you know the inner diameter
Enter the inner diameter in millimeters. The calculator uses diameter × π to obtain circumference and then finds the nearest international lookup row.
If you still need to measure it, use How to Measure Ring Size →.
If you know the circumference
Enter the circumference in millimeters. The calculator derives the inner diameter and returns the nearest US / EU / UK / JP equivalents.
This page does not repeat the full measuring method; see the measurement guide →.
Why international charts can differ slightly
International systems do not line up perfectly to tenths of a millimeter, and manufacturers may round differently. This tool therefore reports the nearest standard equivalent rather than claiming one mathematically universal conversion for every brand.
The canonical dataset on this page is based on one coherent Cartier international table covering 44–71 mm and was cross-checked against GIA's published international chart. Values from different charts are not averaged.
Wide bands and actual fit
A wide band often feels tighter because more metal is in contact with the finger. The calculator therefore does not apply an automatic “+0.5 size” rule; that would be too crude for every hand and every design.
Use the conversion as a starting point and compare a nearby size. See How Should a Ring Fit? →.
Common sizes — quick reference
| Circumference, mm | Diameter, mm | US | EU | UK | JP |
|---|---|---|---|---|---|
| 44 | 14.0 | 3 | 44 | F 1/2 | 4 |
| 47 | 15.0 | 4 | 47 | H 1/2 | 7 |
| 50 | 15.9 | 5 1/4 | 50 | K | 10 |
| 53 | 16.9 | 6 1/4 | 53 | M 1/2 | 13 |
| 56 | 17.8 | 7 1/2 | 56 | P | 16 |
| 57 | 18.1 | 8 | 57 | P 1/2 | 17 |
| 60 | 19.1 | 9 | 60 | S | 20 |
| 63 | 20.1 | 10 1/4 | 63 | U 1/2 | 23 |
| 66 | 21.0 | 11 1/2 | 66 | W 1/2 | 26 |
| 69 | 22.0 | 12 1/2 | 69 | Z 1/2 | 29 |
This is only a compact sample, not a duplicate of the full reference page. For manual lookup, use the Ring Size Chart →.
International conversion charts can differ slightly between manufacturers. This calculator shows the nearest standard equivalent and does not replace an actual fit check.
Choosing a size for an NP Studio ring
Sculptural, wide and top-heavy rings can feel different from a thin conventional band. When discussing an order, it helps to provide a known size or the inner diameter of a ring that already fits.
Remote conversion cannot guarantee fit with absolute certainty, so I treat the number as the starting geometry rather than the only decision.
For object selection and form, see How to Choose a Unique Silver Ring → and Jewelry as Wearable Sculpture →.
Frequently asked questions
More ring-size guides
If you do not know the starting size yet, measure your ring size first. For a manual reference, use the ring size chart, and before ordering a wide or sculptural piece, check how a ring should fit. You can explore real pieces in NP Studio Works.
