Ring Size Converter (Japan, US, UK, EU)
Convert ring sizes between Japan (go/号), US/Canada, UK/Australia, EU/France, inner circumference (mm), and inner diameter (mm). Handy for sizing rings from overseas retailers or checking a size before a resize.
Ring Size Conversion Chart
| Japan (号) | US/Canada | UK/Australia | EU/France | Circumference (mm) | Diameter (mm) |
|---|---|---|---|---|---|
| 1 | 1.5 | B | 41 | 40.84 mm | 13 mm |
| 2 | 2 | C | 42 | 41.89 mm | 13.33 mm |
| 3 | 2.5 | D | 43 | 42.94 mm | 13.67 mm |
| 4 | 3 | D½ | 44 | 43.98 mm | 14 mm |
| 5 | 3.5 | E½ | 45 | 45.03 mm | 14.33 mm |
| 6 | 3.5 | F½ | 46 | 46.08 mm | 14.67 mm |
| 7 | 4 | G | 47 | 47.12 mm | 15 mm |
| 8 | 4.5 | H | 48 | 48.17 mm | 15.33 mm |
| 9 | 5 | I | 49 | 49.22 mm | 15.67 mm |
| 10 | 5.5 | I½ | 50 | 50.27 mm | 16 mm |
| 11 | 6 | J½ | 51 | 51.31 mm | 16.33 mm |
| 12 | 6 | K½ | 52 | 52.36 mm | 16.67 mm |
| 13 | 6.5 | L | 53 | 53.41 mm | 17 mm |
| 14 | 7 | M | 54 | 54.45 mm | 17.33 mm |
| 15 | 7.5 | N | 56 | 55.5 mm | 17.67 mm |
| 16 | 8 | N½ | 57 | 56.55 mm | 18 mm |
| 17 | 8.5 | O½ | 58 | 57.6 mm | 18.33 mm |
| 18 | 8.5 | P½ | 59 | 58.64 mm | 18.67 mm |
| 19 | 9 | Q | 60 | 59.69 mm | 19 mm |
| 20 | 9.5 | R | 61 | 60.74 mm | 19.33 mm |
| 21 | 10 | S | 62 | 61.78 mm | 19.67 mm |
| 22 | 10.5 | S½ | 63 | 62.83 mm | 20 mm |
| 23 | 10.5 | T½ | 64 | 63.88 mm | 20.33 mm |
| 24 | 11 | U½ | 65 | 64.93 mm | 20.67 mm |
| 25 | 11.5 | V½ | 66 | 65.97 mm | 21 mm |
| 26 | 12 | W | 67 | 67.02 mm | 21.33 mm |
| 27 | 12.5 | X | 68 | 68.07 mm | 21.67 mm |
The chart above is a general guideline based on measured inner circumference/diameter values. Exact US/UK/EU conversions can vary slightly by manufacturer, brand, and country. Measuring with an actual ring sizer or checking with the retailer before purchase is recommended whenever possible.
Tips
- Finger size fluctuates with body temperature and time of day. Measuring in the evening — rather than in the morning or in hot weather, when fingers tend to swell — gives a more reliable result.
- When buying a ring from an overseas retailer, if an actual measured circumference in millimeters is listed alongside the US/UK size, check that instead — it's usually more reliable than the size label alone.
- To check the size of a ring you already own, measure its inner diameter with calipers and enter it in the "Diameter (mm)" field to see the equivalent size in every other system at once.
- Wide band designs (such as flat or half-round bands) tend to fit more snugly than thin bands, so it's often safer to go up half a size for those styles.
- Japanese ring sizes increase in fixed 1/3 mm diameter steps per size, a measurement-based system. If you're between two sizes, check the "Circumference (mm)" field for a finer-grained reading.
Frequently Asked Questions
Side Note — why ring sizing differs so much from country to country
Japan's ring size system ("go," 号) is a measurement-based, evenly spaced scale: size 1 corresponds to a 13.0 mm inner diameter, and each subsequent size adds about 1/3 mm. It's a convention that has been used in Japan's jewelry industry for decades, and its main appeal is simplicity — once you know the size number, the diameter and circumference are fixed.
US and UK sizing have a more tangled history by comparison. UK sizes combine a letter (A through Z) with half-size steps, and developed alongside Britain's hallmarking system for precious metals. US sizes use whole numbers with quarter- or half-size increments, said to trace back to standardization efforts by American jewelry trade associations in the late 19th century.
EU/French sizing takes a more direct approach: it simply expresses the ring's inner circumference in millimeters as the size number. It's rooted in the French jewelry trade's traditional concept of "tour de doigt" (finger circumference), and unlike Japan's go numbers or the UK's letters, it uses the physical measurement itself as the size label rather than an abstract scale.
The International Organization for Standardization (ISO) has defined an international ring-size standard based on circumference in millimeters (ISO 8653), but the go, US, UK, and EU conventions that consumers in each region are already used to remain firmly entrenched, and these local labels are still what you'll typically see in stores and online retailers today.