Tool

Ring size converter

Convert between US, UK, European, Japanese and millimetre sizes — starting from the only measurement that is not a convention.

What do you know?

Measurement

The size as it was given to you.

Inner diameter

17.32 mm

The only measurement here that is not a convention. Everything below is this number wearing a different label.

US / Canada

UK / Australia

N½

Europe (ISO)

54.4

= circumference in mm

Japan / China

A ring with an inner diameter of 17.32 mm measures 54.4 mm around the inside. That is a US 7, a UK N½, a European 54.4 and a Japanese 14.

Charts disagree, and this one is no exception

The European size is the circumference in millimetres, which is defined and unarguable. The US, UK and Japanese scales are conventions, and makers round them differently — you will find charts that put this measurement half a size either way. If a ring is being cut or resized rather than tried on, give the workshop the millimetres, not the letter.

What actually affects the fit

A finger is not one size all day

It swells in heat and late in the day, and shrinks in cold and first thing in the morning — the swing across a day is commonly close to half a size. Measure at the end of the day, at normal room temperature, and measure more than once.

A wide band needs a bigger size

A band over about 6 mm wide sits across more of the finger and feels tighter at the same measurement. Going up a quarter to a half size is the usual allowance.

The knuckle is the real constraint

If the knuckle is much wider than the base of the finger, size to get the ring over the knuckle and expect it to spin. Sizing beads or a fitted bar fix the spinning without fighting the knuckle.

String and paper measurements read small

Both compress and neither slides over a knuckle, so they tend to under-read. Treat a string measurement as a starting point and confirm with a proper sizer before anything is cut.

Not every ring can be resized

Full eternity bands, tension settings, and most titanium, tungsten and ceramic rings cannot be sized at all. On those, the measurement has to be right the first time.

The full chart

US 3 to 14, with every scale alongside. The diameter column is computed from the US standard — size 0 is 0.458 inch across and each size adds 0.032 inch — and the European column is simply the circumference, because that is what an ISO size is.

US	UK / AU	Europe	Japan	Diameter	Circumference
3	F½	44.2	4	14.07 mm	44.2 mm
3.5	G½	45.5	5	14.48 mm	45.5 mm
4	H½	46.8	7	14.88 mm	46.8 mm
4.5	I½	48	8	15.29 mm	48.0 mm
5	J½	49.3	9	15.70 mm	49.3 mm
5.5	K½	50.6	10	16.10 mm	50.6 mm
6	L½	51.9	12	16.51 mm	51.9 mm
6.5	M½	53.1	13	16.92 mm	53.1 mm
7	N½	54.4	14	17.32 mm	54.4 mm
7.5	O½	55.7	15	17.73 mm	55.7 mm
8	P½	57	16	18.14 mm	57.0 mm
8.5	Q½	58.3	18	18.54 mm	58.3 mm
9	R½	59.5	19	18.95 mm	59.5 mm
9.5	S½	60.8	20	19.35 mm	60.8 mm
10	T½	62.1	21	19.76 mm	62.1 mm
10.5	U½	63.4	23	20.17 mm	63.4 mm
11	V½	64.6	24	20.57 mm	64.6 mm
11.5	W½	65.9	25	20.98 mm	65.9 mm
12	X½	67.2	26	21.39 mm	67.2 mm
12.5	Y½	68.5	27	21.79 mm	68.5 mm
13	Z½	69.7	29	22.20 mm	69.7 mm
13.5	Z+1½	71	30	22.61 mm	71.0 mm
14	Z+2½	72.3	31	23.01 mm	72.3 mm

The only measurement that is not a convention

Inner diameter in millimetres is the real measurement. Every national ring size is a label laid over it, and the labels do not agree. The US scale defines size 0 as an inner diameter of 0.458 inch with each whole size adding 0.032 inch. The European or ISO size is simply the inner circumference in millimetres, which makes it the most sensible of the lot. Japanese sizes follow JIS: size 1 is 13.0 mm across, each size adding a third of a millimetre. British and Australian sizes use letters, anchored at US 5 equalling UK J½.

Because these are conventions rather than derivations, published charts genuinely disagree by up to half a size, and two shops can both be right. If a ring is being cut, cast or resized rather than tried on, give the workshop the millimetres.

Measuring without getting it wrong

Measure at the end of the day and at normal room temperature. Fingers swell in heat and late in the day and shrink in cold and first thing in the morning; the swing across a single day is commonly close to half a size. Measure more than once, on different days if it matters.

The knuckle is usually the real constraint. If it is much wider than the base of the finger, the ring has to be sized to clear the knuckle and will then spin — sizing beads or a fitted bar solve the spinning without fighting the knuckle. Wide bands need more room: anything over about 6 mm across sits on more of the finger and feels tighter at the same measurement, so a quarter to half size up is the usual allowance.

String and paper strips both compress and neither slides over a knuckle, so they tend to under-read. Treat them as a starting point and confirm with a proper sizer before anything is cut. And note that full eternity bands, tension settings and most titanium, tungsten and ceramic rings cannot be resized at all.

Common questions

What is a US size 7 in UK sizes?

A US 7 is a UK N½. It measures 17.32 mm across the inside and 54.4 mm around, which is also its European ISO size. In Japan it is a size 14.

How do I measure my ring size at home?

The most reliable home method is to measure a ring that already fits: lay it flat and measure straight across the inside at the widest point, in millimetres. That inner diameter converts directly to any national size. Measuring the finger itself with string or paper tends to under-read, because neither compresses the way a ring does or has to pass a knuckle.

Why do ring size charts disagree with each other?

Because most national scales are conventions rather than derived measurements, and different makers round them differently. Only the European ISO size is unambiguous — it is defined as the inner circumference in millimetres. Where precision matters, work in millimetres rather than in size labels.

What is my gold worth?

Weigh it, pick the karat, and see the value of the metal at the live gold price — then what a buyer is likely to hand over, which is a different number.

What is my silver worth?

Sterling, coin, Britannia and fine silver at the live silver price — including why small silver items are often worth less than the trouble of selling them.

What is platinum or palladium worth?

PT950, PT900 and PD950 at live prices. Both metals are denser than gold, which is why the weight surprises people.