Ring Size Chart in MM

The full conversion table. Every ring size is a label for one of two millimetre measurements — find the value you know and read across.

By Eleanor Whitfield Reviewed 22 August 2026

Ring size calculator

Or enter a measurement and let the calculator find the row for you.

Your size

— Enter a measurement in millimetres

The calculator needs JavaScript. You can still find your size by matching your measurement against the US, UK and EU size chart.

Every ring size in the world is a label attached to one of two physical measurements: the inside diameter of the band, or the inside circumference. Those two millimetre figures never change. Only the names do.

That is what makes this chart work in every direction. Find any value you already know — a US number, a UK letter, an EU size, or a measurement you took yourself — and read across the row.

Complete ring size chart. US and Canada sizes, UK/Australia/Ireland letters, EU/ISO sizes, and the inside diameter and circumference in millimetres.
US / Canada	UK / AU / IE	EU / ISO	Inside diameter	Inside circumference
3	F	44	14.1 mm	44.2 mm
3.5	G	45–46	14.5 mm	45.5 mm
4	H	47	14.9 mm	46.8 mm
4.5	I	48	15.3 mm	48.0 mm
5	J½	49	15.7 mm	49.3 mm
5.5	K½	50–51	16.1 mm	50.6 mm
6	L½	52	16.5 mm	51.9 mm
6.5	M½	53	16.9 mm	53.1 mm
7	N½	54	17.3 mm	54.4 mm
7.5	O½	56	17.7 mm	55.7 mm
8	P½	57	18.1 mm	57.0 mm
8.5	Q½	58	18.5 mm	58.2 mm
9	R½	59–60	18.9 mm	59.5 mm
9.5	S½	61	19.4 mm	60.8 mm
10	T½	62	19.8 mm	62.1 mm
10.5	U½	63	20.2 mm	63.4 mm
11	V½	64–65	20.6 mm	64.6 mm
11.5	W½	66	21.0 mm	65.9 mm
12	X½	67	21.4 mm	67.2 mm
12.5	Y½	68–69	21.8 mm	68.5 mm
13	Z½	70	22.2 mm	69.7 mm

If you would rather enter a measurement than scan a table, the calculator does the same lookup and flags readings that fall between two sizes.

How to read the chart

US / Canada: A numeric scale, normally 3 to 13 for adults, in half sizes. The numbers are arbitrary labels — a size 7 is not seven of anything.
UK / Australia / Ireland / New Zealand: A letter scale running from A upward, with half sizes written as N½. One letter step is about 1.25 mm of circumference, so roughly half a US size. The letter scale explained
EU / ISO: Not a code at all — the ISO size is the inside circumference in millimetres. An EU 54 ring measures 54 mm around the inside. Why EU sizing is the simplest system
Inside diameter: Straight across the inside of the band, through the centre. What you measure with calipers or a ruler on a ring you own. How to measure inside diameter
Inside circumference: The distance around the inside of the band, which is what your finger actually measures. Diameter multiplied by pi. How to measure finger circumference

Where the EU column shows a range

Some circumferences fall on a half millimetre — US 9 is 59.5 mm, for instance. Retailers genuinely stock either 59 or 60 in those cases, so both are listed. When you have the choice, go by the millimetre figure rather than the label.

Sizes below US 3 exist for children and follow the same scale downward. Children's ring sizes in millimetres

Women's ring sizes

Ring sizes are not gendered — the scale measures a finger, not a person. But most women's rings are ordered within this band, so it is a useful slice of the chart to work from. More on typical women's sizes.

The range most women's rings are ordered in, US 4 to 9.
US / Canada	UK / AU / IE	EU / ISO	Inside diameter	Inside circumference
4	H	47	14.9 mm	46.8 mm
4.5	I	48	15.3 mm	48.0 mm
5	J½	49	15.7 mm	49.3 mm
5.5	K½	50–51	16.1 mm	50.6 mm
6	L½	52	16.5 mm	51.9 mm
6.5	M½	53	16.9 mm	53.1 mm
7	N½	54	17.3 mm	54.4 mm
7.5	O½	56	17.7 mm	55.7 mm
8	P½	57	18.1 mm	57.0 mm
8.5	Q½	58	18.5 mm	58.2 mm
9	R½	59–60	18.9 mm	59.5 mm

Men's ring sizes

The same scale, a different part of it. Note that men's bands are often wider, which changes the size you need independently of your finger measurement, and thumb rings sit higher again. More on men's sizing and band width.

The range most men's rings are ordered in, US 8 to 13.
US / Canada	UK / AU / IE	EU / ISO	Inside diameter	Inside circumference
8	P½	57	18.1 mm	57.0 mm
8.5	Q½	58	18.5 mm	58.2 mm
9	R½	59–60	18.9 mm	59.5 mm
9.5	S½	61	19.4 mm	60.8 mm
10	T½	62	19.8 mm	62.1 mm
10.5	U½	63	20.2 mm	63.4 mm
11	V½	64–65	20.6 mm	64.6 mm
11.5	W½	66	21.0 mm	65.9 mm
12	X½	67	21.4 mm	67.2 mm
12.5	Y½	68–69	21.8 mm	68.5 mm
13	Z½	70	22.2 mm	69.7 mm

Converting between measurements yourself

If your figure falls between rows, these relationships let you work it out directly.

The formulas

Circumference = diameter × 3.1416
Diameter = circumference ÷ 3.1416
EU / ISO size = inside circumference in mm

Each full US size is about 2.5 mm of circumference; each half size about 1.25 mm. Each UK letter step is also about 1.25 mm, which is why one US size covers roughly two UK letters.

Common questions about reading and using this chart are answered in the sizing FAQ.

Printing this chart

This page is set up to print cleanly — navigation and footer links are stripped, and the table prints in full rather than scrolling. Use your browser's print function.

If you plan to measure against the printout

This chart is a conversion table, not a printable ring sizer, so print scaling does not affect the numbers. But if you use any printed sheet with circles to measure a ring against, check the scale first: print at 100% with page scaling off, and verify against a bank card, which is 85.60 mm wide under ISO/IEC 7810. Since a full US size is about 2.5 mm out of roughly 54 mm, a 4% print error is about a whole size.

How reliable are these conversions?

The millimetre figures are consistent and checked, but the size labels attached to them are convention rather than enforced standard. Published charts vary between manufacturers by up to roughly a quarter of a size. When precision matters, quote the inside circumference in millimetres and use the retailer's own chart rather than a general one.

Questions about the size chart

What size is a 54 mm ring?

A 54 mm inside circumference is a US 7, a UK N½ and an EU 54 — the EU size and the circumference are the same number by definition. Its inside diameter is 17.3 mm.

What ring size is 17 mm in diameter?

17.3 mm is a US 7. A reading of exactly 17 mm falls just below that, between US 6.5 (16.9 mm) and US 7, so it is close to a US 6.75. Given that half a size is only about 0.4 mm of diameter, take a second reading before deciding.

How many millimetres is one ring size?

One full US size is about 2.5 mm of inside circumference, or about 0.8 mm of diameter. A half size is about 1.25 mm of circumference and 0.4 mm of diameter. One UK letter step is about 1.25 mm, so roughly half a US size.

Is there one official international ring size standard?

ISO 8653 defines size as the inside circumference in millimetres, and most of continental Europe follows it. The US and UK systems predate it and remain in general use, so in practice you are converting between conventions rather than reading one standard. Published charts also differ slightly between manufacturers.

Do ring sizes differ between jewellers?

Slightly, yes. There is no enforced global standard, and published charts vary between brands by up to roughly a quarter of a size. For most purchases that is immaterial; for an expensive or hard-to-resize ring, confirm against the specific retailer's own chart.

Where to go next

Why the systems differ

The logic behind US numbers, UK letters and ISO circumference sizing.

US to UK letters in depth

Why US whole sizes land on UK half-letters rather than whole ones.

US to EU sizes in depth

The system where the size and the measurement are the same number.

Get a measurement first

The chart is only as good as the millimetres you bring to it.