Ring Size Converter — Japan, US, UK, EU

Instantly convert ring sizes between Japan (go/号), US/Canada, UK/Australia, EU/France, inner circumference (mm), and inner diameter (mm). Quickly answer "what size is a Japanese size 15 ring in the US?" — handy for overseas shopping or resizing.

Last updated: 2026-09-08

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.

Ring Size Conversion Explained — Go, US, UK, and EU Sizes

Ring sizes aren't measured on a single global scale — at least four systems coexist: Japan's "go" (号), US/Canada, UK/Australia, and EU/France. The go scale starts at a 13.0 mm inner diameter for size 1 and adds roughly 1/3 mm per step; US sizes use whole numbers with quarter-size increments; UK sizes use letters (A through Z) with half-size steps; and EU/French sizes simply state the inner circumference in millimeters. Because each system has its own starting point and step size, there's no simple formula — no single multiplier — that converts one to another.

This tool bridges all of them through the two physical measurements that actually determine fit: inner circumference (mm) and inner diameter (mm). Enter a size in any one system and instantly see the equivalent go, US/Canada, UK/Australia, and EU/France values — useful whether you're translating an overseas retailer's size chart into a familiar Japanese go number, or checking your own ring against a foreign brand's size guide. Keep in mind that converted values are general guidelines drawn from measured references, and the actual fit can vary slightly depending on manufacturer and band width (flat, half-round, and similarly wide styles fit more snugly).

How to Convert a Ring Size

Choose the unit you're converting from Select whichever system your known size is in: go (Japan), US/Canada, UK/Australia, EU/France, circumference (mm), or diameter (mm).
Enter or select the size value Type in the number from your ring or from a retailer's listing. If you're starting from a physical measurement, enter the circumference or diameter directly in millimeters.
Check the converted result All six equivalent values appear at once, so you can read off whichever system you need.
Cross-check nearby sizes on the chart If your value falls near a size boundary, scroll down to the conversion chart and compare the go sizes just above and below for extra confidence.

Tips for getting more out of it

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.

When This Comes in Handy

Buying from overseas jewelry brands

Sites for brands like Tiffany or Cartier often list only US or EU sizes. Converting your familiar go size ahead of time helps you avoid an ill-fitting purchase.

Ordering rings abroad for a wedding or engagement

Jewelers in some countries take orders in US or EU sizes rather than go. Converting in advance means you can state the right size confidently on the day, without last-minute guesswork.

Requesting a resize at a jeweler

When asking a jeweler to resize or rework a ring, stating both your current and desired size as circumference or diameter in millimeters communicates the request more precisely than a size label alone.

Buying secondhand rings from overseas marketplaces

When a seller's listed size doesn't match go, converting it beforehand with this tool reduces the risk of returns or disputes over the wrong size.

Glossary

Go (JP size): Japan's ring size system. Size 1 corresponds to a 13.0 mm inner diameter, and each subsequent size adds about 1/3 mm — a regular, measurement-based scale.
US size: The ring size system used in the US and Canada, made up of whole numbers with quarter- or half-size steps, said to trace back to standardization by American jewelry trade associations in the late 19th century.
UK size: The ring size system used in the UK and Australia, combining a letter (A through Z) with half-size steps, developed alongside Britain's hallmarking system for precious metals.
EU/French size: A ring size system widely used across continental Europe that expresses the inner circumference directly in millimeters as the size number, rather than an abstract scale.
Circumference: The length (in mm) around the inside of the ring. Wrapping a strip of paper or string around your finger and measuring its length gives this value, and it serves as the common reference for every conversion in this tool.
Diameter: The width (in mm) across the inside of the ring. Measuring an existing ring with calipers typically gives the diameter; circumference equals diameter multiplied by pi (about 3.14).

Frequently Asked Questions

Yes. Fingers swell and shrink slightly throughout the day due to body temperature, hydration, and ambient temperature, which can shift the ideal size by about half a size between morning and evening. For the most reliable measurement, try measuring in the evening when swelling tends to be more stable.

The most reliable approach is to compare actual circumference (mm) or diameter (mm) measurements rather than size labels. US and UK sizes are only marked in coarse half- or whole-size increments and vary slightly between manufacturers, so enter your measured value into the "Circumference (mm)" or "Diameter (mm)" field here and cross-check it against the retailer's own size chart.

There's no single international standard for ring size labels — Japan, the US, the UK, and the EU each define their own sizing system independently. The chart on this page is a general guideline based on measured circumference and diameter values, and actual conversions can vary slightly by manufacturer.

A simple method is to wrap a thin strip of paper or string around your finger, measure its length, and enter that as the circumference (mm). For a more precise measurement, use a physical ring sizer or have it measured at a jewelry store.

Yes, the underlying idea is the same — both convert between country-specific sizing systems using an actual physical measurement as the common reference (circumference/diameter in mm for rings, foot length in cm for shoes). Ring sizing simply works at a much finer, smaller-scale measurement.

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.

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